SGM Beamline @Jan 28, 2010 4:34:56 PM

General Beamline Information

Safety Information

Before an experiment can be performed on the SGM beamline a valid experimental permit must be issued by the CLS User Office (CLSUO) and signed off by one of the beamline staff. In addition, all participants must have performed a Beamline Specific Orientation (BSO), which is provided by the beamline staff. This will provide users with information regarding the hazards involved with a synchrotron experiment and brief them on the safety features found at the SGM. If there is no valid experimental permit present or no one with a completed BSO present, operation of the beamline if forbidden.

Several hazards exist for people working around the SGM these include:

  • High voltage electrical systems
  • Cryogenics
  • Pressurized gases
  • Toxic and corrosive samples
  • Automated mechanical systems / robotics
  • Heavy components

People working on the beamline should be aware of the possibility of injury from any of these systems at any time. Any unsafe apparatus or practices should be reported to the Heath, Safety and Environment (HSE) department.

SGM Beamline Components

Insertion Device

Photons are provided to the SGM beamline by a pure permanent magnet undulator with a period length of 45 mm and a peak field of > 0.84 T at 12.5 mm gap. This undulator is the first of two undulators in straight 11. The other is the 185 mm period undulator for the Planar Grating Monochromator (PGM) beamline. The SGM undulator is comprised of 252 permanent NdFeB magnets, mounted on a jaw like structure that can open to 293 mm and close to 12.5 mm. The intensity and energy distribution of photons from the insertion device is controlled by setting the gap to produce the desired magnetic field between the poles of upper and lower magnets. The relationship between the undulator gap and the photon energy is shown below in Figure 3.

Optical Layout

<div align="center">Figure 1: The Optical Layout of the SGM Beamline</div>
Figure 1: The Optical Layout of the SGM Beamline

The SGM beamline is comprised of 9 optical elements. These are the variable aperture mask (VAM), first deflecting mirror (M1), second deflecting mirror (M2), vertical focusing mirror (M3), entrance slit (ENS), diffration grating (LEG, MEG and HEG), exit slit (EXS), first refocusing mirror (M4) and second refocusing mirror (M5). The layout of these components is shown in Figure 1. The VAM is used to define an aperture for the beamline optical system. It serves to block the intense unusable radiation from the first mirrors in the beamline. The VAM position is fixed by the beamline staff and shouldn't be moved by beamline users. M1 and M2 are identical plane mirrors that serve several purposes. Their primary purpose is to deflect the beam by a total of 3 degrees. In doing this, these mirrors also act as a filter for the higher energy xrays and gamma rays coming down the beamline.


While the soft xrays are deflected down the beamline, the higher energy radiation is either absorbed by the mirrors themselves, the mirror tanks and associated hardware, a large lead block (called a pig) or the walls of the primary optical enclosure (POE). In addition, these mirror have two different coatings – silicon on the top half and carbon on the bottom half – so they have to be moved vertically depending on which stripe is desired – silicon stripe for doing carbon analysis and carbon stripe for doing silicon analysis. See the appropriate section in the Operation Procedures chapter for information on changing the stripe.

Table 1: SGM grating properties
Grating Line Spacing (/mm) Energy Range Coating
LEG 600 250-700 Ni
MEG 1100 450-1250 Au
HEG 1700 750-2000 Au

The M3 mirror is used to collect the light from the first two mirrors and focus it vertically on the entrance slit. The beam is deflected vertically by 2 degrees by this mirror. The entrance slit, gratings and exit slit comprise a dragon type monochromator [1]. Light coming through the entrance slit illuminates one of the three spherical gratings. The grating angle can be controlled with great precision to pass the desired photon energy through the exit slits. This energy is determined from the grating equation,

Image:eq1.jpg

where m is the diffraction order, λ is the desired wavelength, d is the grating spacing and θi and θd are the angle of incidence and angle of diffraction of the light.

Table 2: SGM mirror properties
Mirror Profile Surface
M1 Planar Si/C
M2 Planar Si/C
M3 Cylindrical (R=224.0 m) Si/Rh
M4 Toroidal (M=118.7 m, S=0.070 m) Au
M5 Toroidal (M=118.7 m, S=0.070 m) Aukk

Specifications for the three gratings used on the SGM are given in Table 1. The efficiencies of the gratings are inversely related to their line spacings so the total flux at the endstation is greatest for the LEG and decreases for the MEG and HEG.

The exit slit gap is the adjustable from 0 to 500 microns and is the primary instrument used for controlling the beamline energy resolution. By decreasing the slit size the range of photon energies that can pass through the exit slit is also reduced, improving the resolution but also reducing the total flux of photons passing through the slit to the endstations. The exit slit also has to translate along the beamline as the photon energy changes. This is due to the spherical shape of the grating, which focuses the beam at a different distance from the grating depending on the angle of incidence/energy.

The monochromatic light passing through the exit slit is refocused by M4 onto the first endstation area (EA1) and then refocused again onto the second endstation area (EA2) by M5. Table 2 shows gives more information on the mirrors used in the beamline.

Diagnostics

<div align="center">Figure 2: Still image taken from the video camera on the exit slit. The light is being produced by 500 eV photons exciting phosphor on the slits.</div>
Figure 2: Still image taken from the video camera on the exit slit. The light is being produced by 500 eV photons exciting phosphor on the slits.
The xray beam can be monitored at several points along the beamline. Feedback on the beam position, intensity and size is generated by electrically isolated blades, two Io ladders and video cameras. These diagnostic systems were used in the initial alignment of the beamline and are monitored continuously to ensure that the beamline is operating properly.

Electrically isolated blades at the leading edge of each of the five mirrors produce a photoelectric current that is proportional to the flux of photons hitting these blades. The feedback from these systems was critical in the alignment of the beamline. The current produced in these blades is monitored as a function of mirror position to determine the optimal location of the mirrors.

Io mesh ladders before each of the endstations provide flux information on the xray beam at the endstations. These devices are critical for normalization of the signals k measured at the samples. They are also the best systems to quickly check on the status of the beamline. Typically, the meshes in these ladders allow 85% of the beam to pass and are made from gold because it resists contamination better than most other materials.

Contamination on these meshes is a serious problem because it produces misleading current increases at the photon energies corresponding to the absorption edges of the contaminants. The signals from the two meshes are referred to as EA1 Io and EA2 Io, for the meshes before the first and second endstations, respectively.

A video system that monitors the visible portion of the beam or the visible light produced as a result of xray stimulation. Cameras are located at the variable aperture mask, exit slits and endstations. Additional cameras can be added at the entrance slit and monochromator chamber if necessary. Video feedback is extremely valuable as qualitative check on the beam. A light phosphor coating on the exit slits glows brightly and indicates the relative intensity of the beam as shown in Figure 2. Video is also useful in the alignment of the sample with the photon beam.

Control System

Control System Architecture


Control of the SGM is provided through the use of the Experimental Physics and Industrial Control System (EPICS)[3]. EPICS is a distributed control system meaning that there are a number of different computers, connected together over a high speed network, that work in concert to monitor and control the various components of the beamline. More specific information on the CLS control system can be found in the CLS Control System Overview and Technical Specifications document[2]. An individual parameter of the control system is called a process variable (PV). An example of a PV is 'BL1611ID1:Energy', which is the PV that represents the beamline energy. Changing this PV invokes a process that moves the motors on the undulator, grating and exit slit to the proper positions to produce light of the desired energy at the endstation.

Table 3: IOCs used in the SGM control system
IOC Applications Function
IOC1611-433 picoammeter Picoammeters control
IOC2408-303 idMotors Insertion device control
IOC2400-106 machProtection Machine protection, vacuum, water flow, water temperature, and valve control
IOC1611-427 Energy Beamline energy calibration and coordination
smMotors Low level VME motor control
beamline Optical element motor coordination
HVCAENx527 High voltage power supply controls
softIoc Beamline status flags, picoammeter, coordination, valve coordination
IOC1611-415,416 NA Ion pump interface
IOC1611-417,418,419,420 NA Cold cathode gauge interface
IOC1611-407,408,423,424,425,426 NA Motor driver interface
Another PV, 'BL1611ID1:Energy:fbk', is used to monitor the energy to which the beamline is currently set. This value for this PV is determined through a calculation based on the grating equation and the feedback provided by an encoder that measures the angle that the grating is currently set to. All parameters of the SGM control system such as the undulator gap, grating angle, exit slit position, mirror positions and detector outputs are represented by such PV's. A list of commonly used PV's related to the SGM beamline can be found in the appendix of this document.

Process variables are generated and handled by software running on computers called inputoutput controllers (IOC). For the beamline to work properly, several different applications must be simultaneously running on multiple IOCs. Table 3 outlines the different IOCs and the applications that are required for beamline operation.

Machine Protection

Vacuum

Photons in the soft xray region of the electromagnetic spectrum are easily absorbed by air. This means that the photons need an evacuated path to travel down from the undulator to the endstation and all optical elements have to be under vacuum. The result is the that vacuum at the beamline endstation is connected to the vacuum system in the storage ring, booster and linear accelerator. Thus, a sophisticated machine protections system is required to ensure that a vacuum accident at the endstation does not affect the storage ring and interrupt all experimental operations. The vacuum system on the SGM is divided into 17 separate sections by pneumatic valves. Each of the valves is interlocked to the pressure sensors in the two sections upstream and downstream of it's position. The pressure sensors in each section consist of a thermoconductive gauge (TCG), for pressures between atmospheric pressure and 10−3 Torr, and a cold cathode gauge (CCG) for pressures between 10−3 and 10−10 Torr. When the pressure in any section exceeds a threshold value, the valve is automatically closed.

Photon Shutters

A network of three photon shutters, in conjunction with a single safety shutter, are used to ensure that <NEEDS TO BE COMPLETED>

Coordinated Beamline Control

thumbnail| <div align="center">Figure 3: Plot of the gap set point versus photon energy for the SGM undulator</div>
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Figure 3: Plot of the gap set point versus photon energy for the SGM undulator
Coordinated control of the SGM undulator gap, grating angle and exit slit position is required to keep the beamline optimized. This is accomplished using EPICS, which takes a desired energy and determines the proper set points for the beamline components based on previously calibrated functions. Any combination of the three active components can be selected to participate in the calibration by turning it's tracking on from the beamline interface. Typically, all three elements are tracked during a scan, but it is sometimes desirable to turn the tracking off on the exit slit or undulator.

Calibration of the undulator gap was initially based on the measurements made at the CLS magnetic mapping room. Coordination of the undulator is required so that it will produce the greatest intensity of the desired photon energy. This is done by changing the gap between the upper and lower jaws of the undulator. The function relating the gap setting, g, in mm to the photon energy, hν, of the peak of the undulator output as provided by the CLS undulator group is

Here, l is the desired harmonic and A1, A2, and A3 are parametric fit parameters. This function is shown in Figure 3 for the first and third harmonics with values of 0.1430, 36.01 and 1776 for A1, A2, and A3.

The grating calibration, relating the angle of incidence of the beam onto the gratings, θi, to the desired photon energy, is determined from Equation 1 to be

In this equation, d is the grating spacing (space between two consecutive rulings) and θm is the included angle of the monochromator. The included angle is defined by the positions of the entrance slit and exit slit and is equivalent to the angle between the incident beam and the diffracted beam. For the SGM, θm is 175o and the values for d are 1/600, 1/1100 and 1/1700 mm.

thumbnail| <div align="center">Figure 4: The grating encoder position as a function of photon energy for the low, medium, and high energy gratings.</div>
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Figure 4: The grating encoder position as a function of photon energy for the low, medium, and high energy gratings.
thumbnail| <div align="center">Figure 5: The exit slit position as a function of photon energy for the three SGM gratings.</div>
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Figure 5: The exit slit position as a function of photon energy for the three SGM gratings.

The grating encoder value, xg , is related to θi through a sine arm linkage with an arm length of S, so

where Cg is the number of encoder steps per millimeter. Using this relationship the position of the grating encoder, xg, can be found for any desired photon wavelength, λ, as is shown in Figure 4. On the SGM, S is 510 mm and Cg is 2.45e5 /mm. Note that for xg = 0, λ = 0, or the photon energy is infinite, indicating that this is the zero order position. Also note that

has been used to convert wavelength into photon energy.

To relate the distance between the exit slit and the grating, rd, commonly referred to as the exit arm length, to the photon energy is given by

where ri is the distance from the entrance slit to the grating and R is the radius of curvature of the grating. This formula is derived from the Rowland circle geometry[1] which restrains the exit slit, grating and exit slits to points on a circle with radius R. Then the position of the exit slit encoder, xexs, is then given by

with roffset being the distance between the grating and the zero position of the encoder , and Cexs being the number of encoder steps per millimeter. This relationship is plotted for the SGM energy range and each of the SGM gratings in Figure 5. A value of 70.48 m is used for R and 1.5 m is used for ri.

Calibration

Calibration of the SGM was performed using a photoionization chamber and a Scienta SES100 photoemission endstation. Several absolute energies were determined through the comparison of gas phase photoionization spectra for Ar, CO, N2 and Ne with previously published spectra. With the beamline set to these known energies, photoemission spectra of a gold sample were obtained using the Scienta. Then the beamline photon energy was scanned through the entire range (250 to 2000 eV) in 10 eV steps and a photoemission spectrum was taken at each point. Through a comparison of the energy of the gold Fermi level, between the photoemission spectra taken at the known photon energies with the spectra taken at the intermediate photon energies, the absolute energy of the beamline was determined at 10 eV intervals. At each intermediate energy the grating encoder feedback and optimized values for the undulator gap and exit slit position were recorded.

A theoretical calibration was done by applying equations (2), (4) and (6) with parameters determined from the specifications of the beamline[*1]. This provided a rough calibration that was used for much of the commissioning process. However, this initial calibration lacked accuracy so an experimental calibration was produced by fitting equations (2), (4) and (6) to the Scienta data using the least squares method. A closed loop feedback system is used to set the beamline components to the precise locations indicated by the calibration relationships. This system operates by driving a motor towards a target encoder position, then checking to see if it was successful in reaching that position. If it is not, it will try again. Parameters such as the deadband and maximum number of retries can be adjusted to optimize the control loop. This system is implemented in the motor driver software and improves the precision and repeatability with which the motors are controlled.

Beamline Energy Scanning and Timing

Energy scanning is accomplished using the CLS dataacquisition program. Dataacquisition makes requests to set the beamline energy PV, 'BL1611ID1:Energy', waits until the beamline is ready, then records the value of a number of PV's or other information in the data file. Dataacquisition has the capability to perform nested scans of any variable on the beamline. For example, the high voltage on a detector could be scanned at each energy setpoint during a XAFS scan. More information on dataacquisition can be found in the software manual.

When a call is made to change the beamline energy, from either the beamline interface program or from dataacquisition, a calculation of the required setpoints of the undulator, grating and exit slit is made and the setpoints are applied. At this point the beamline status PV, 'BL1611ID1:ready', is set to 'MOVING'. The status of the motors driving the undulator, grating and exit slit are polled at a frequency of 100 Hz to determine when they have stopped. When all three are stopped and report that the movement was successful,'BL1611ID1:ready' is set to 'STOPPED'.

The time it takes to acquire a single data point in a scan can be broken down in to the dead time, τDead, and the dwell time, τDwell. The dead time can them be broken down further into the network overhead, τNet, the computing time, τComp, and the mechanical movement time, τMove. Thus, the time it takes to measure a single point, τTotal, can be represented as

with

The parameters τNet and τComp are nearly constant, varying only slightly with the computer processor usage and network traffic. The SGM beamline shares a dedicated 1 Gb virtual local area network (VLAN) with the PGM beamline, so it is unlikely that there could be too much variance in the τNet time. It may be possible to affect τNet if there was a very high bandwidth application running on a computer in the VLAN or if there was a errant process or virus that was flooding the network. τComp could be increased if any one of the control system computers became overloaded. The primary beamline IOC runs the Linux operating system which is not a real time system so it is impossible to guarantee that the beamline operations will be executed with a given priority. Therefore, if the computer becomes overloaded, calls to update the beamline energy or get feedback on the beamline status, or any other operation requiring the IOC, could be delayed. During typical beamline operations the beamline IOC is operation at less than %1 of it's full capacity, and even when the system is stressed by executing multiple simultaneous requests it never exceeds %5.

thumbnail| <div align="center">Figure 6: Plot of the moving status for the exit slit, undulator, and grating of the SGM. The overall beamline status and the read status are also shown. Here, a high status indicates the elements is moving, or the beamline is not ready. The read status goes high during the dwell period. This data was acquired with a 1/10 th of a second resolution.</div>
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Figure 6: Plot of the moving status for the exit slit, undulator, and grating of the SGM. The overall beamline status and the read status are also shown. Here, a high status indicates the elements is moving, or the beamline is not ready. The read status goes high during the dwell period. This data was acquired with a 1/10 th of a second resolution.
The dominant source of dead time is τMove, which is the time required to move the undulator, grating and exit slit into the requested positions. When a new energy is requested all three elements begin to move simultaneously. Since it is unlikely they will all finish at the same time, τMove will be determined by the element that takes the longest amount of time to finish moving. Figure 6 shows the status flags of the beamline, undulator, grating and exit slit during a scan. It can be seen that the last component to complete it's movement is the exit slit. This is not always the case. From Figure 5 it is apparent that the exit slit travels different distances for the same energy step size depending on the energy of the beamline. For instance, to go from 250 eV to 255 eV the exit slit has to move farther that it does to go from 390 eV to 395 eV.


To improve the beamline scanning rate τMove would have to be decreased. This could be done by increasing the speed of the motors driving the exit slit and grating. This would, however, have an effect on the accuracies of these mechanical systems and may not be possible. For most applications increasing the scanning rate would not be advantageous due to the length of time required to change samples. Most users of the SGM are limited to a maximum number of samples, not by the scanning rate of the beamline, but by the amount of time it takes them to change and align the sample. Therefore, improving the current scanning rate would not help the overall throughput of the beamline until a better sample handling system is in place.

Operations and Procedures

Permits

Every experiment requires a valid permit. If no valid permit is found at the beamline health and information centre, the floor coordinator has the right to shut down the beamline. The permits are generated through the user office proposal system and contain, among other things, a list of people involved in the experiment and a list of samples. The beamline staff should bring the permit to the beamline at the start of the experiment and verify that the list of samples on the permit is accurate and that all safety related issues are in order. The beamline staff will also verify that all of the members of the experimental team have received the Beamline Specific Orientation (BSO). Once the beamline staff are satisfied that the permit is accurate, they will sign it and ask a member of the experimental team to sign it. The user should only sign it if they feel that the experiment is ready to proceed safely.

Beamline User Accounts

Individual user computer accounts are generated for all valid permits. The accounts allow users to operate the beamline and store their data in a secure, externally accessible location. The accounts are named according to their proposal. For instance, if your permit is number 7-1501, then you will log in with the username 7-1501. The first time you log in you will use the password 'Welcome1'. The system will then prompt you to enter a new password. The password has to be at least 8 characters and must contain at least one number and upper case letter. Be sure to remember your password for future visits. If there are any problems with accessing your account, you can e-mail the ITC help desk (helpdesk email yet to be determined). When you log in to your account for the first time it is populated with links, icons, directories and files required for your experiment. To access your data remotely, type the following into any web browser:

ftp://ex.lightsource.ca

When promted for the username, enter

'experiment_number'@ex.clsi.ca

For example, 2-112@ex.clsi.ca, then enter your password.

X-ray Absorption Spectroscopy

Overview

Performing a standard XAS experiment on the SGM involves the use of the SGM Solid Sample Analysis End-station (SSA). This end-station can be configured with a fluorescence yield (FLY)detector, a X-ray Excited Optical Luminescence (XEOL) lens, a Total Electron Yield (TEY) monitor, a Residual Gas Analyzer (RGA) as well as other temporary instruments. The chamber has a side chamber, or load lock, used to transfer samples into measurement position. Moderate to high vacuum is maintained on the chamber at all times by a turbo pump and an ion pump. The load lock has it's own turbo pumping system and another turbo is found just upstream of the main chamber on the diagnostic chamber. The end-station is separated from the rest of the beam line by a pneumatic vacuum valve that is interlocked to the pressure gauge in diagnostic chamber. In the event of a vacuum failure the valve will close to protect the beam line vacuum.

Running an XAS experiment involves getting the sample into the measurement position, configuring the beam line for a scan and running the scan. The most demanding part of the experiment tends to be getting the sample into position or transferring the sample so this will be discussed at length. Alignment of the sample to the beam is accomplished with the aid of the zero order light and is fairly routine. The SGM beam line interface software, called IDAV (Integrated Data Acquisition and Visualization), is used to configure the beam line, run the scans and perform preliminary data analysis.

Sample Preparation

The factors that have to be considered when preparing samples include the following:

  • Safety.
  • Physical size and consistency.
  • Sample concentration.
  • Vacuum compatibility/volatility.
  • Number of samples.
Safety

Each year there are hundreds of different research groups coming and going from the CLS and each group brings in different samples. Although very few samples actually present a significant hazard, the sheer number of samples implies that vigilance is required in sample handling. This is why all of the samples that you wish to measure must appear on your permit and be approved by the CLS Health, Safety and Environment (HSE) group. All samples must be stored safely and clearly labeled to prevent accidental exposure or dangerous chemical reactions. It is the responsibility of the visiting scientists to be familiar with all hazards associated with their samples and to communicate these hazards to others working in the vicinity. This includes the beamline staff and users on other beamlines that are sharing the lab space. Material Safety Data Sheets (MSDS) must be available for all sample materials with any hazards and it is the users responsibility to familiarize themselves with the MSDS of all CLS materials they work with in the CLS lab space.

Physical size and consistency

On the SGM SSA chamber samples can be powders, crystals, bulk material or films on different substrates. The sample size is limited by the sample holder system. Samples should be no larger than 10 mm^2, or in the case of circular shaped samples, 16 mm in diameter. Samples should not extend more than 6 mm from the sample disc when mounted or they will interfere with the transfer mechanism. Metal samples can be welded to the sample discs and most others can be affixed with conductive, double sided carbon tape. Carbon tape will be supplied by the beamline.

Sample Concentration

It is important to consider the concentration of the element under study when preparing samples. Low concentration samples are more difficult to measure and require more careful preparation. Never place a sample with high concentration of the element of interest on the same sample disc as a sample with a low concentration of that same element. Contamination of the low concentration sample with the high concentration sample can easily occur.

Sample Transfer

The SGM sample holder system was upgraded in April 2009 to accomodate sample cooling and motorization. Upgrades were also done to the beamline interface, IDAV (Integrated Data Acquisition and Visualization), to allow for control of the sample position and for saving and restoring the sample positions. These upgrades required a completely new sample transfer system. A system based upon the omnicron style pincer and sample clip was chosen.

Sample Transfer Precautions

It is possible to damage the endstation, manipulator or transfer arm while doing the transfer. Please keep the following things in mind when transferring:

  • Always double check the pressure in the transfer arm. Opening the valve between the chamber and the load lock is the easiest way to vent the beamline. You must be sure that the pressure in the load lock is below 5e-6 Torr before opening this valve.

Never use force. It is possible to damage the manipulator and transfer arm by applying too much force to the transfer arm. If something doesn't seem to fit properly, do not keep trying to get it to fit by using additional force. The system was designed so that very little force is required so if it seems to be jammed just stop and try to improve your alignment.

  • Be sure to close the last valve in the beamline and turn off the detector high voltage before transferring. Closing the last valve in the beamline by clicking 'Close Vacuum' on the interface will protect the beamline from being vented if the endstation is vented. Turning off the detector high voltage during transfer protects the detector against accidental venting. It also reduces the chance of electrocution durning transfer, which is very unlikely.
  • Never adjust the manipulator position when the sample arm or screwdriver are attached to it. Motion of the manipulator while the transfer arm or screw driver are attached can easily damage any one of the components. Be sure to retract the transfer arm and screwdriver to do fine adjustments to the manipulator position. Before you move the manipulator to the measurement position, be sure that the both the transfer arm and the screwdriver are fully retracted.

Basic Transfer Instructions

  • Step 1. Prepare the beamline for transfer.
    • Close the last vacuum valve in the beamline by pressing 'Close Vacuum' on IDAV.
    • Turn off the high voltage system by pressing the 'Turn HV Off' button on IDAV.
    • Ensure that the XEOL lens is retracted and the screwdriver and transfer arm are fully retracted.
    • Press the 'Transfer' button under 'Saved Position' in the SSA Endstation tab on IDAV. This will drive the sample to the transfer position.
  • Step 2. Retrieve the sample.
    • Check the vacuum in the transfer arm. The gauge is located in the rack near the transfer arm and is labeled 'EA2 Transfer Arm'. It must be below 2.5e-6 Torr before the sample can be retreived.
    • Open the valve between the load lock and the chamber. Rotate the knob on the valve counter clockwise until the red bump protrudes from the handle. Be careful not to accidently turn the knob on the fluorescence yield detector as they look similar.
    • Push the transfer arm into the main chamber and up to the sample.
    • Align the sample clip with the transfer arm pincers. By pressing the 'Transfer' button in the saved positions, the sample should already be very close to the right position. If small modifications to the position are needed use the beamline laptop. Do not try to rotate the motor stages by hand. This will take the motors out of calibration.
    • Close the pincer grip on the sample clip. This is done by rotating the rear knob on the transfer arm. The direction is indicated on the knob. The pincers should close completely. Small rotations of the transfer arm can help get the clip to fit into the slots in the pincers properly.
    • Insert the vacuum screwdriver into the receptical on the sample manipulator and rotate counter clockwise to release the sample. After several rotations, the sample should be detatched from the manipulator.
    • Retract the sample into the load lock. Please do this with the sample holder oriented horizontally (sample facing up) to prevent the sample from scraping the inside of the chamber.
  • Step 3. Park sample.
    • With the sample fully retracted, release the nut holding the sample garage. Be sure to hold onto the knob of the linear feedthrough before releasing it as there is considerable force pulling it down. Adjust the position of the garage so there is an empty spot below the height of the transfer arm.
    • Move the transfer arm so the sample is directly above the empty sample holder. Release the knob on the feedthrough and pull it up so the sample fits into the holder. Release the pincer grip and retract the transfer arm.
    • At this point, proceed to Step 5 if you wish to vent the load lock and switch samples in the garage. Proceed with the next step to insert one of the samples already in the garage.
  • Step 4. Retrieve and insert new sample.
    • Adjust the linear feedthrough so the height of the required sample clip aligns with the slot in the pincers. It may help to move the pincers quite close to the sample clip. Lock the pincers onto the sample.
    • Lower the garage feedthrough so the sample comes out of its holder, retract the sample, then fully raise the garage.
    • Rotate the transfer arm so the sample is facing down then slide the sample into the chamber. Rotate it so the sample is facing the beam and slip it into the manipulator.
    • Use the screwdriver to lock the sample into place in the manipulator.
    • Release the pincers and retract the transfer arm and screwdriver.
    • Close the valve separating the load lock and the chamber.
    • Press 'Measure' on the Saved Positions page of IDAV to move the sample back to the measurement position.
  • Step 5. Venting the load lock.
    • Proceed with this step if you wish to remove samples from the load lock and/or insert new samples into the load lock.
    • Close the valve between the load lock and the chamber. Be sure the valve is closed tightly.
    • Turn off the CCG gauge (red LED numbers) labeled 'EA2 Transfer Arm' in the rack near the transfer arm
    • Close the angle valve connected to the turbo pump on the load lock.
    • Turn off the power bar on one of the chamber stands legs.
    • Watch the transfer arm TCG (green numbers – labeled 'Transfer Arm') pressure increase. When it reads ~750 Torr, open the viewport door on the load lock.
    • Using the large forceps, grab the sample clips and remove the samples from the garage. Adjust the linear feedthrough to help access all three samples.
    • Insert the new samples by firmly gripping the sample clip with the forceps and placing them into the holders in the garage.
    • Close the viewport door.
    • Open the angle valve connected to the turbo pump on the load lock.
    • Turn on the power switch on the power bar on the chamber stand leg.
    • Wait until the pressure in the load lock is 5e-6 Torr before following the procedure in Step 4 to insert the next sample.

IDAV

This section will describe the beamline interface and provide instructions on how to use it effectively.

Getting Started

Logging in

When you sit down at the beamline interface computer, be sure to log in using your user account information. If there is no one else logged in, a login prompt should be visible on the screen. If there are multiple windows, perhaps the beamline interface, on the screen, than it is likely that someone else logged into the computer. Follow the instructions for logging on to User Accounts.

Starting IDAV

Once you are logged in to your own account, you can start IDAV by clicking the icon on the desktop:

The IDAV interface should appear:

The interface window is divided into a primary control area and a notebook page style area. The primary control area contains controls for turning the beam on and off, setting the beamline for visible light, controlling the slits and setting are reading back the photon energy. The notebook pages are used for monitoring and setting other beamline parameters including the undulator harmonic, the scan configuration and the high voltage system.

Scan configuration

To set up a scan configuration, click on the "Scan Config" tab.

Click on the "Load Config" tab and navigate to the 'Configuration Files' directory in your home directory. For standard experiments, select the "Default_Configuration_for_Dataacquisition.config" file. This will setup the beamline for TEY and FLY x-ray absorption measurements. A default energy range and step size will be filled into the region boxes. Click on the two boxes labeled "use auto-suffix" and "use auto-numbering". Your interface screen should now look like this (note the two check marks for auto-suffix and -numbering, and the filepath against "Load Config"):

Setup the directory where your data will be stored by clicking on "Set Output" box. You should store your data in a subdirectory of your home directory. You can create a new subdirectory using the following button:

When you are in the folder you want to use, click on "Select Directory" to apply that directory as your storage directory.

Enter the desired filename in to the text box on the "Scan Config" page. A file number and .dat suffix will automatically be added when the file is written.

Note: if you perform the above steps out of sequence, you may lose the filepath and filename, and possibly also the selection of auto suffix and auto numbering. If you are having problems check that:

a) the correct filepath is displayed b) the correct filename is displayed c) both auto-suffix and auto-numbering have check marks.

Now set the desired energy range and step size you wish to scan. You can easily add and remove regions using the "Add Region" and "Delete Region" buttons. Copy the "BL1611-ID-1:Energy" string from Region 1 into new regions. Be sure not to use same end and start value for subsequent regions, as the beamline will stop scanning at that point.

Aligning Your Sample

Let's assume you have succesfully transferred a sample into the endstation, and have it facing the beam. You need to launch the endstation camera program, if it's not already running. On the interface window, click on the "Cameras" tab, and select camera 3.

To make sure the beam is hitting the sample, you can use visible light from the beamline. This is acheived by setting the monochromator to transmit the Zeroth order of diffraction - i.e. to allow specular reflection - and is thus often referred to as "zero order" light. Click on the "Visible Light On/OFF" button. You will see three things happen (and hear at least one other):

1) The Energy Feedback will start to increase dramatically. We are setting the grating to allow specular reflection, which the software interprets as a very high photon energy. This will take a good few 10s of seconds.

2) The entrance slit will move to its fully open position (250 ?m)

3) The exit slit will move to its fully open position (400 ?m)

When the mono reaches the visible light setting, the Beamline Status tab will show "stopped".

The things you might hear are the EA1 and EA2 window valves closing - these have a glass insert that allows visible light through but blocks the X-rays. This is important as full zero order undulator light may damage your sample.

There should now be a white light spot visible on your sample. If there isn't check: a) there is still beam b) that there is ~10-7 A on the EA1 Io mesh (EA2 will be blocked by the window valve). You can find the EA1 Io signal by clicking on on the "SSA endstation" of the interface window. c) the chamber lighting is off. Sometimes the bright light in the chamber makes it hard to observe the beam spot.

Select the "SSA Endstation" tab in the beamline interface. Using the manipulator controls, move the sample into the beam. Note that the zero order light is a good guide of where the x-ray beam will be, but not a representation of the beam size. The exit slits are opened to 400 ?m to allow the maximum intensity of visible light through to the sample in the zero order configuration. The exit slit will likely be closed to less than 100 ?m for most measurements, so the actual spot size will be smaller. Also note that since the visible light is going through a crown glass viewport (to block x-rays) the position of the x-ray beam can be slightly different than the position of the visible light. However, this difference is very small and so the x-ray beam should hit the sample somewhere inside the spot where the visible light beam is hitting the sample.

Now hit the "Visible Light On/OFF" tab again. The energy feedback will return to the set point, and both slits will return to their set points.

Grating And Harmonic Selection

Let's take another look at the interface window, with the "Adv. Controls" tab selected:

Make sure that: a) The three tabs in the "Energy Tracking" section are all white, and not grey. b) End station "ea2" is selected c) The undulator scan mode is set to "scan"

The posters at the beamline show the ranges and relative fluxes of the three diffraction gratings in the beamline. Make sure the scan range you want is covered by the grating selected. As a rough guide:

Note that where the energy ranges of two gratings overlap, the higher of the two will give better (sometimes much better) resolution, but less flux. The data on the beamline posters give a good idea.

To change the grating, click on the tab of the grating you want. You will be asked if you're sure, and then the grating selection mechanism will start. This is entirely computer controlled, and will take a few 10s of seconds, during which the Beamline Status tab will show "moving". When in position, it will change to "stopped". The selected grating tab will be white.

Note that if you do not match the undulator harmonic with the grating as per the table above, you may get very little flux. To change the harmonic, click on the button you want. Again you will be asked to confirm, and then the undulator will move. This will take a few seconds, and again the Beamline Status will show "moving".

Run a Simple Scan

Let's take another look at the interface window with the "Scan Config" tab selected:

To set the scan range you want, edit the "Start", "Delta" (step size) and "End" in the "Region 1" line above. You will now need to turn on the Fluorescence detector. Click on the "HV group is Off" button. You will see "All HV is off" change to "Some HV is ramping up" and the tab changes from grey to yellow. When the detector is fully on, this changes to "Some HV is on" and stays yellow.

Now click on the "Beam On" button. You will hear one or more valves open, and after about 3 seconds you will see "BEAM OFF" change to "BEAM ON". There are now X-ray photons hitting your sample.

Now click on the "Acquire" button. You will see a new window pop open, which will show the data as it is acquired:

Note that it may take several seconds for the scan to start - three devices have to move to set a photon energy: the undulator (which tracks with the photon energy), the diffraction grating, and the exit slit. During this time the Beamline Status will show "moving".

X-ray Excited Optical Luminescence

Equipment

The CLS employs the Ocean Optics QE65000 for measuring optical emission from samples. The QE65000 is a Czery-Turner type spectrograph with a thermo-electrically cooled CCD and allows for collection of relatively weak optical emission between 200 and 800 nm (check these numbers-TZR). The input to the spectrometer is a SMA905 fiber optic coupler. A large (600 to 1000 micron)scientific grade fiber optic couples the spectrometer to a SMA905 fiber optic feedthrough. Inside the chamber a 5mm collimating lens collects the photons from the sample and passes them through a short, unjacketed, 1000 um fiber to the feedthrough. The chamber video camera is also a key part of the XEOL system as it gives quick feedback on the XEOL intensity and the stray light level in the chamber.

Setup

  • To start a XEOL experiment, place the spectrometer near the endstation (within 500 cm of the fiber optic feedthrough) and plug in the QE65000 power adapter. This supplies power for the thermoelectric cooling system and will start the cooling fans.
  • Next, plug the USB extension cable into the spectrometer and OPI1611-409(beamline computer near the Scienta endstation).
  • Log into OPI1611-409 using your user account.
  • On OPI1611-409, double click on the 'USB4000_01 Driver' icon. If the icon isn't on the desktop, open a terminal window and type '/home/sgm/bin/runSGM_USB4000_IOC'. This will start the backend process for the optical spectrometer.
  • Log into OPI1611-408 (main beamline interface computer) using your user account.
  • Start the optical spectrometer GUI by clicking on the 'USB400 GUI' icon. If this icon is not present, open a terminal and type '/home/sgm/bin/runSGM_USB4000_GUI'.
  • You should see the spectrometer interface appear on the desktop. Set the integration time to 1 second and press the 'Start' button under 'Acquire'. If there is any light in the chamber you should see a spectrum appear in the graph area of the screen. A simple test of the system is to measure the fluorescent light spectrum. Turn off the chamber light but leave one or two windows uncovered. The 1 second acquisition should show a spectrum similar to the one shown here:http://commons.wikimedia.org/wiki/File:Fluorescent_lighting_spectrum_peaks_labelled.gif

Alignment

  • Alignment of the optical axis of the XEOL lens with the beamspot on the sample is critical for acquiring good XEOL spectra. Alignment should be checked often - at least once for every sample.
  • Load a sample and move it to the approximate measurement position. For best results, rotate the sample to approximately +45 degrees so the sample is facing the XEOL lens.
  • Turn on the visible light by clicking the visible light button on the interface.
  • Disconnect the fiber optic from the QE65000 and illuminate the end of the fiber with a laser pointer or some other light source.
  • Note the position of the zero order light with respect to the light coming through the fiber. Adjust the horizontal position of the fiber light by moving the sample up or downstream. Bring the fiber light and zero order light into alignment.

Acquisition

Single Spectra

  • Turn off the beam and acquire a dark spectra by putting the acquisition mode in 'Dark Spectrum' and hitting 'Start'. The boxcar and integration time values must be set the same for the dark spectrum acquisition as they will be for the real spectrum acquisition.
  • Set the Acquisition mode back to single. Turn the beam on and press start to acquire the new spectrum.
  • Save the spectrum by using the 'Execute script' button. The fields at the bottom of the gui must be filled out properly for the XEOL acquisition to work. They should contain the following:
    • File Directory: '/home/YY-XXXX/Data-directory' where YY-XXXX is your user account number and 'Data-directory' is an existing directory where you wish the file to be stored.
    • File Root: A filename
    • File Index: A number that will be added to the filename. The number will automatically increment after each acquisition.
    • Script Directory: '/home/sgm/bin/'
    • Script Name: 'getSAspectra'
    • Check that the file is being stored properly by navigating to the save directory and ensuring the content of the file is reasonable.

XAS XEOL

  • XAS XEOL is a measurement method that records a XEOL spectrum at each point in a XAS scan. The total luminescence is stored in the XAS data file and each spectra is stored as a line in a separate data file.
  • Under the 'scan config' page in the beamline interface, load the 'XAS_XEOL.config' file.
  • Set the scan range and step size for the scan.
  • Ensure the XEOL GUI acquisition mode is 'Single'.
  • Start the scan.

Data Processing

X-ray Photoemission Spectroscopy

Equipment

The SGM is equipped with a Scienta SES100 photoemission spectrometer. It is located on the first endstation, EA1, of the beamline which is maintained at high to ultra high vacuum pressures at all times. The SES100 has four main components: the analyzer, the power supply, the CCD detector and the computer. XPS measurements can be performed in standalone mode, using just the Scienta computer, or in beamline integration mode, where the beamline data acquisition system is used to control the Scienta.

Common Setup

In standalone mode the beamline is set to a constant photon energy and the spectrometer is operated independently from the beamline. The user workstation close to the endstation is equipped with the Scienta PC as well as a standard CLS OPI. A KVM switch at the workspace can be used to switch the left monitor from displaying the left of two screens of the OPI to displaying the Scienta computer output.

  • 1. Login
    • Login to both the OPI and Scienta computer using your permit number (e.g. 9-1883) as the username and "Welcome1" as the password.
    • The first time you login you will be asked to change your password. Choose a suitable password and write it down. Logging in to your own account will ensure the security of your data and allow you to access your data from outside the facility.
  • 2. Configure the beamline
    • Set the KVM switch to show both OPI screens by pressing "A" on the switch.
    • Double click on the icons labeled "IDAV" and "Scienta Interface".
    • When the windows come up, move them to the right screen of the dual display.
    • Click on the "Cameras" tab on IDAV and click on "Camera 4".
    • When the video comes up, move the window to the right OPI screen.
    • Click on the 'Advanced Options' tab and then the EA1 radio button. This tells the control system that you are using the first endstation area.
    • Set the desired photon energy using IDAV. This may require a change of diffraction grating. See the section on selecting energy in the X-ray Absorption section.
  • 3. Configure the Scienta for Alignment
    • Set the KVM switch to display from the Scienta computer.
    • Start the SES100 software by clicking on the desktop icon.
    • Set up the detector for alignment (Add more specifics here)
  • 4. Alignment
    • Use the up, down, left, right, cw and ccw buttons on the "Scienta Interface" program on the OPI to move the sample into the general position in front of the analyzer and facing the beam. ***CAUTION*** it is possible to damage the sample holder by driving it into the chamber walls. Be sure to understand the position of the sample holder in relation to the chamber before you start moving. Try making small movements to start until you have the feeling for how the system is aligned.
    • Using IDAV, click on 'Visible Light'. Ensure the undulator is closed by looking on the 'Advanced Options' tab. If it is at a value greater that 20 mm, close it to 20 mm. This will ensure that the visible light is intense. When the grating has stopped rotating, and if your sample is close to the right position, you should see the beam spot on the sample holder using the video camera.
    • Move the sample into the visible light spot. Note that there is a windowed valve closed right before the endstation preventing the x-rays from damaging your sample.
    • Click on "Visible Light" again to turn off the visible light. When the monochromator stops moving, click beam on. If the alignment is close, you should see some signal on the Scienta detector output.
    • Optimize the signal using the "Scienta Interface" manipulator controls. Make small movements along the beam axis. The previous step aligned the sample with the beam so now you don't want to move the sample too much in the inboard/outboard or up/down directions. Note that the manipulator axes are not aligned to the beamline so combinations of inboard/outboard and upstream/downstream motions will be needed for the alignment.
  • 5. Configure the Scienta for Acquisition
    • Select the scan configuration window on the Scienta interface.
    • Set the pass energy and scan range for the Scienta.
    • Setup the filename and directory for your data.

Standalone Mode

  • 6. Start the Acquisition
    • Click on the 'start scan' button on the scienta interface

Beamline Integration Mode

  • 6. Using "IDAV", click on the "Scan Config" tab. Click "Load Config" and select "XAS_XPS.config" from the "Configuration Files" directory.
  • 7. Setup the filename and directory for your data files.
  • 8. Setup the scan range and step size with which you wish to acquire XPS. At each photon energy in the scan, an entire XPS spectra will be acquired and stored in a separate data file with the same name as the XAS file but ending in "_spectra.dat".

Technical Notes

In the beamline integration mode, the Scienta is being triggered to acquire by the IDAV interface. The XPS data is acquired as per the configuration that is currently active on the Scienta interface. When the XPS scan is finished, the data is assembled and sent back to the IDAV interface as an EPICS waveform record. At this point, all configuration of the Scienta is done using the Scienta interface and the only thing that EPICS does is trigger the scan to start, wait for the scan to stop and read back the data. In the future, support for more advanced options, such as setting the XPS scan range, integration time, resolution and others, will be incorporated into the XAS-XPS capabilities.

References

  1. 1  
    Chen, C.T.:"Performance of the Dragon soft x-ray beamline", ''Rev. Sci. Instrum. 60 (7)'', July 1989
    2. [1]


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