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Machine FAQ

An Office of Science User Facility
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​[ Machine Parameters Table ]

 Machine FAQ

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1) What is the photon energy range which LCLS can provide and how long does it take to switch?
Answer Collapse/Expand Text:
(8/25/16) LCLS Machine Phys.
At present, the available photon energy range is 270 eV up to 10 keV. Photon energies as high as 12.8 keV may be reached with advanced notice and reduced reliability.
Energy changes can require anywhere from 5 minutes (small energy adjustments of 5-50%) to 45 minutes (energy adjustments of a factor of 2-3). In addition, some accelerator tuning may be required in order to re-establish the full x-ray pulse energy (e.g., to achieve more than 2 mJ may require another hour or more). This retuning is generally faster when the energy is increased rather than decreased.  Please ask the operator for a time estimate when requesting photon energy changes.

It is also possible to make very fast, but small photon energy changes of +-2% (relative photon wavelength) without requiring any other adjustments (although some small x-ray power variations may accompany this).  Such fast energy control can be implemented directly by users in a programmed fashion.  Please ask the operator for help setting this up.

2) What is the highest pulse energy available (number of photons in the pulse) and how does it vary with photon energy and pulse length?
Answer Collapse/Expand Text:
(8/26/16) LCLS Machine Phys.
Typically the pulse energy is about 2 - 3 mJ but operators, if given time (1-2 hrs), can frequently exceed this.  Of course the number of photons in the pulse depends on the photon wavelength (energy).  At 8.3 keV the highest number of photons in the pulse has been about 2E12, whereas at 830 eV the highest number of photons in the pulse is about 2E13. A linked plot ( figure ) shows FEL energy in mJ vs photon energy.

If the electron bunch is further compressed in length (generating a shorter x-ray pulse), the number of photons in the x-ray pulse is reduced since the pulse is shorter.  However, the increased peak current of the electron beam in the undulator makes a more efficient FEL (up to a limit) and therefore the number of photons in the pulse does not drop as fast as a simple pulse-length scaling might suggest.  In fact, the peak x-ray power tends to rise as the electron bunch is further compressed, although not without limit.  A linked plot (figure) shows the photon pulse energy and the photon peak power as the electron peak current is increased.  Another plot  shows the pulse energy vs electron peak current from May to August, 2010 (FEL vs photonEngy and bunch length).

3) Whom do we contact, both before and during the experiment, to request changes or special parameters, and who do we contact for questions on x-ray beam characteristics?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
Always ask your instrument scientist first and he/she will help you find the right persons. 

4) What is the LCLS x-ray pulse length, how long does it take to change it, and how does the pulse energy and peak power vary with pulse length?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
The x-ray pulse length is approximately a copy of the electron bunch length, which is set by the linac bunch compression parameters.  A shorter electron bunch length produces a higher peak current in the undulator.  At the shortest  photon wavelengths (~8 keV) the peak current required to achieve FEL gain saturation is 3000 A or higher.  However, at longer wavelengths (800 eV and below) a lower peak current is possible (down to 500 A).  Therefore, the pulse length at longer wavelengths can be set at anywhere from 300 to 40 fs FWHM (i.e., 500 A to 4000 A at the nominal 150-pC bunch charge), but at the shortest wavelengths the pulse length must be no longer than 30-50 fs FWHM (i.e., 3000-5000 A) or the FEL power will be very low and quite unstable from shot to shot.  At mid-range wavelengths the choice of pulse length range is proportionally wider, but more limited than at 800 eV. Currently (2015) we have primarily been running with a bunch charge of 150 pC which shortens the nominal bunch duration to 30-50 fs for hard X-rays and about 100 fs for soft X-rays.

Changing the pulse length (within these limits) is quite fast and can be accomplished in 1-2 minutes.  In fact, the electron bunch length (actually the final peak current) is under closed loop control and the desired setting can easily be entered, with a minute or two required for full stabilization.

The total energy in the x-ray pulse depends on the pulse length.  Less pulse energy is observed with a shorter pulse (keeping the electron bunch charge constant).  However, the increased peak current of the electron beam in the undulator makes a more efficient FEL (up to a limit) and therefore the number of photons in the pulse does not drop as fast as a simple pulse-length scaling might suggest.  In fact, the peak x-ray power tends to rise as the electron bunch is further compressed, although not without limit.  A linked plot (figure) shows the photon pulse energy and the photon peak power as the electron peak current is increased (with a 250-pC constant bunch charge).
Note: For even shorter xray pulses, please also refer to the Slotted Foil and Low Charge FAQs below. For online measurement of xray pulse duration, please refer to the XTCAV FAQ below. 

5) What is the degree of coherence of the x-ray pulse in both transverse and longitudinal directions and how is this affected by the choice of photon energy?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
The x-ray pulse has good transverse coherence, since for normal operation the transverse coherence area is larger than the spot size. However, if the undulator has a significant misalignment or there is betatron (beam size) mismatch, the transverse coherence area can be reduced enough to degrade the transverse coherence.

The FEL does not generally have full temporal coherence. In SASE mode, for the 150-pC bunch charge case (nominal) and assuming an electron peak current of 5 kA (typical with hard x-rays) with a uniform current profile, then the electron duration is about 30 fs FWHM. Given the fact that the power e-folding length (gain length) is about 3.3 m and the saturation length is about 65 m, there are about 200 temporal spikes in the x-ray profile, which means that the frequency bandwidth is about 200 times larger than the transform limited bandwidth. For the low charge case (20 pC), the electron bunch length is very short (<10 fs), and hence there are about 30 spikes, i.e., the frequency bandwidth is about 30 times the transform limited bandwidth.


Note: Longitudinal coherence is improved in Self Seeded Modes. See related FAQ.

6) How does the x-ray transverse beam size vary with photon energy and electron peak current settings?
Answer Collapse/Expand Text:
(03/05/14) JT
The x-ray transverse divergence and therefore beam size downstream of the undulator changes as a function of energy. The size at a given experiment will depend on the distance, source point, and divergence. See the table in FAQ answer 7 for divergence. See an example of size versus energy in the middle of the FEE (at the direct imager) in this figure.

7) Where is the approximate location of the x-ray source point along the undulator, how does this vary with photon energy and peak current, and how can we move this source point?  In addition, what is the source size and divergence and how does this scale with photon energy and peak current?
Answer Collapse/Expand Text:
The x-ray source point is located about 1-2 Rayleigh lengths upstream of the end of the last inserted undulator.  The source point location can be moved further upstream by removing down-stream undulators (about 4 m per undulator).  For a given undulator configuration, the relative source point location, i.e., the distance of the source point to the end of the last inserted undulator, as well as the rms source size and the far-field rms divergence depend on photon energy, electron beam charge, and peak current.  For photon energies between 0.5 keV and 2.0 keV and for electron beam charges between 20 pC and 250 pC we estimate:

- the relative source point location to be between -2 m and -15 m (where negative numbers indicate here that the source is upstream of the end of the last inserted undulator),
- the far-field rms divergence to be between 6 µrad and 30 µrad, and
- the rms waist size to be between 11 µm and 25 µm.

The figures, linked below, show the rms waist size, the rms divergence, and the relative source point location (wrt the end of the last inserted undulator) all vs. photon energy:

0.5 - 2.0 keV 2.5 - 4.0 keV 4.5 - 6.0 keV 6.5 - 8.0 keV
waist size 20 pC 20 pC 20 pC 20 pC
  250 pC 250 pC 250 pC 250 pC
divergence 20 pC 20 pC 20 pC 20 pC
  250 pC 250 pC 250 pC 250 pC
source position 20 pC 20 pC 20 pC 20 pC
  250 pC 250 pC 250 pC 250 pC

Tables (2 keV , 4 keV , 6 keV , 8 keV ) of x-ray beam parameters as function of photon energy, bunch charge, and peak current is also available here.

8) Where are the direct imager and the “YAGXRAY” screens located with respect to the undulator exit? 
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
Referring to the LCLS undulator coordinate system (LCLS-TN-03-8), which has its z-axis parallel to the electron beam in the undulator, some distances of interest are listed below (note that the "last undulator segment" is assumed to be number 33, while at times, some of the undulators are offline and the last segment is further upstream

YAGXRAY is 45 meters downstream of the end of the lastundulator (33).


DIRECT IMAGER is 87 meters downstream of the end of the last undulator.

9) What is the shortest pulse length possible, how does this vary with photon energy, and what pulse energy is available at the shortest pulse?  In addition, how long does it take to establish this pulse length and also to go back to nominal conditions?  Can it be changed any time of day or night?
Answer Collapse/Expand Text:
(8/25/16) LCLS Machine Phys.


Normally LCLS delivers X-ray pulses with roughly 2 mJ of energy and 50 to 100 fs fwhm duration.  There are two ways much shorter pulses can be made, though at the expense of about a factor of ten in energy per pulse.

1) With the low charge method an electron bunch of 20 pC  is maximally compressed and sent through the undulator.  The resulting x-ray bunch length has a fwhm of 2-4 fs. Changing to low-charge mode typically takes less than one hour.

2) Using the Slotted Foil method a small portion of a full charge bunch lases to produce FEL radiation, while the rest of the bunch is degraded and produces only spontaneous radiation. This results in an FEL pulse similar to that of the low charge method. Inserting the slotted foil typically takes ten minutes and once inserted, pulse length changes take seconds.

Operations shifts are able to perform the change in either direction at any time, but it is best to ask for the change in advance to make sure resources are available.

The XTCAV diagnostic, located just after the undulator, can be used to measure x-ray pulse profiles on a shot-to-shot basis. It has an intrinsic resolution of 1-4 fs, with better resolution for longer wavelengths (see below).



10) What are the temporal characteristics of the x-ray pulse, including number of spikes, spike duration, peak power in each spike, and how does this vary with photon energy and peak current?
Answer Collapse/Expand Text:
(02/03/14 - AB)
The temporal characteristics of the LCLS x-ray pulse are chaotic due to the SASE process. The x-ray pulse profile consists of many random spikes that vary from shot to shot.  Typically the spike duration is about 1-2 fs for soft x-rays (280 eV to 2 keV) and down to 300 attoseconds for hard x-rays (8 keV). The total number of spikes is approximately the ratio of the x-ray pulse length (see item 4) to the spike duration. The peak power in each spike can be anywhere between 1-100 GW, depending on electron peak current, and can fluctuate from spike to spike.  A linked figure shows the typical x-ray power profile at the wavelength of 0.15 nm and 1.5 nm, operated at under-compression mode with 250 pC.  The current profiles of the electron bunch with double horns are also shown in the figure.

11) What is the bandwidth of the x-ray pulse and to what extent can it be minimized or maximized and how long does it take to make such changes?  Can the x-ray pulse be energy chirped and if so, what is the final bandwidth available (FWHM)?
Answer Collapse/Expand Text:
(8/25/16) LCLS Machine Phys.
From measurements, the FWHM bandwidth is 0.1-0.2% for the hard x-rays and 0.5% for the soft x-rays.  The minimum bandwidth can be obtained with an electron bunch that is nominally compressed by the second bunch compressor (“under-compression”), and as the FEL just reaches saturation. Electron beam energy chirp can also map to the photon bandwidth and is adjusted per user request. In this case, the energy-chirped electron bunch can drive a frequency-chirped FEL that can have relatively large bandwidth.  Measurements suggest that FWHM bandwidth up to 1% is possible for hard x-rays and up to 3% for soft x-rays.  This figure shows the x-ray rms bandwidth along the undulator distance at the wavelength of 0.15 nm and 1.5 nm, operated at under-compression mode with 180 pC. 

A soft x-ray spectrometer is located in SXR and can be used to measure the single-shot x-ray spectrum from 600 eV to 2 keV. A linked figure shows two typical SXR spectra around 1.2 keV at the nominal machine condition (when electron bunch is under-compressed in BC2). Likewise, a transmissive hard x-ray spectrometer located in the FEE can be used while tuning and/or recorded by experiments for bandwidth determination.


In the hard x-ray region, a 4-bend crystal monochromator is also used to measure the FEL spectra around 8 keV by scanning the electron energy (hence the measurement is multi-shot instead of single-shot). For examples, the left plot of this linked figure shows the FEL spectra when the electron beam is under-compressed in BC2 for several peak current values. The right plot shows the FEL spectra when the electron beam is over-compressed in BC2 (indicated by negative peak current values) and clearly demonstrate the spectral broadening effect due to the electron energy chirp.



12) What is the machine repetition rate at present?  Is there a possibility to increase the rate beyond 120 Hz?  Can a single pulse be triggered in a one-shot mode and can the experiment pull the trigger?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
The machine repetition rate is 120 Hz.  Two electron bunches per 120 Hz RF pulse are also possible with up to 35 ns spacing. Single pulses can and have been triggered by experiments in a one-shot mode.
For 100 kHz or more, please stay tuned for LCLS-II...

13) Can I trigger the x-ray beam from my own experiment when I want it rather than asking the main control center?  If so, can I vary the trigger time in arbitrarily small increments without regard to the 120-Hz triggers (i.e., trigger the linac)?
Answer Collapse/Expand Text:
No.  This is not consistent with the design of the accelerator and would be very difficult, if not impossible to provide.

14) What is the shot-to-shot jitter I can expect of the pulse arrival time, pulse energy, photon wavelength, pulse length, and transverse pointing, and is there a set of parameters that can be used to minimize these?
Answer Collapse/Expand Text:
(8/25/16) LCLS Machine Phys.

The shot to shot jitter for the x-ray pulse arrival time (wrt the RF reference phase) is about 50 fs rms over 1-2 minutes, the photon pulse energy or intensity jitter is about 3-10% (for 0.8-8 keV photons, respectively), the relative rms photon wavelength jitter is about 0.12-0.05% (for 0.8-8 keV photons, respectively, and based on measured electron beam energy jitter of 0.06-0.025%), the photon pulse length jitter is about 7%.  The transverse pointing jitter is about 5-10% rms (for 0.8-8 keV photons, respectively), expressed here as a fraction of the rms electron beam size.  These stability levels are reasonably accurate for the nominal 180-pC bunch charge.

15) What is the power level and angular spread of the spontaneous radiation?  How is it discriminated from the FEL pulse, does it scale with photon energy and pulse length, and can it be minimized?
Answer Collapse/Expand Text:
In the table below are examples of calculated spontaneous radiation power levels, given as x-ray pulse energy per shot, for the first three harmonics.  These data are for an electron bunch charge of 250 pC with all undulator segments in-line.  They should scale with charge and there is no dependence on pulse length.

Photon fundamental [eV]:     800         2000       8264
1st harmonic [uJ/shot]:         6.04         15.1        62.4
2nd harmonic [uJ/shot]:        8.81        (22.0)     (91.0)
3rd harmonic [uJ/shot]:       (10.5)       (26.2)      (108)

These numbers represent the amount of x-ray energy that gets through the gas attenuator apertures with the adjustable slits open to 4 mm full width, both horizontally and vertically.  Typically the FEL energy is ~2 mJ/pulse for a 250-pC bunch charge, so the 1st harmonic spontaneous radiation is only a small fraction, ~0.3 - 3%, of the FEL energy, depending on the photon fundamental energy.

Photons with energy above 2200 eV are attenuated by the soft x-ray transport mirrors by at least a factor of 0.01 (see question 18 below).  In the table above, parenthesis are put around harmonics which would be cut off by the soft x-ray mirror system before reaching the experimental hutch.

The angular distribution of the spontaneous radiation is somewhat complicated.  All spontaneous radiation, no matter what photon energy, is confined (half-width) to roughly K/gamma, 1/gamma angles with respect to the x (horizontal) and y (vertical) planes.

Photon fundamental [eV]:   800      2000     8264
x' [urad]:                             420        266       130
y' [urad]:                             120          76         37

In practice the spontaneous x-rays completely fill the 4-mm round aperture defined by the hole in the center of each gas attenuator iris.  This is shown visually for a 13.5 GeV (8 keV fundamental) beam in the linked figure.  In this figure, only one gas attenuator valve was inserted, and spontaneous x-rays were able to penetrate it to simultaneously show both the incident distribution (mostly yellow) as well as the gas attenuator aperture (the red oval at the center).

The complicating feature of the angular distribution of the spontaneous radiation is the strong angle-energy correlation.  X-rays at the fundamental wavelength are confined to a narrow angular cone that is roughly the size of the FEL beam.  Of course there is only a small fraction of the spontaneous power in this narrow bandwidth.  At larger angles the x-ray photon energy is reduced - a fact which can allow some higher harmonics to, at some angle, have the same energy as the fundamental.

To reduce the background from spontaneous x-rays the adjustable slits can be adjusted to fit around the much smaller FEL beam.

16) To what level can we suppress the FEL pulse in order to measure the spontaneous background?  Does the suppression method affect the spontaneous emission?
Answer Collapse/Expand Text:
FEL suppression can be accomplished by lowering the peak current of the electron beam (to 500 A, or below), by setting the laser heater power as high as possible (~200 uJ of IR laser energy), and by introducing one or more orbit kicks in the undulator.  These methods don't have much effect on the spontaneous beam but, depending on the wavelength, may not be 100% affective in suppressing the FEL either.  The shortest wavelengths are the easiest to suppress.

17) How much x-ray power attenuation can be applied, how long does it take to set that up, and does this change any of the characteristics of the x-ray pulse?
Answer Collapse/Expand Text:
(1/15/14) JLT

Three decades at each wavelength can be applied within a few seconds for hard xrays, however periodically some of the solid attenuators cause wave front distortion due to damage. We replace these as we find distortion, so check with your instrument scientist as to present useable attenuators. For soft xrays, three decades are available through the nitrogren gas attenuator system. This typically takes a few minutes for adjustment.

18) What is the photon energy cutoff of the soft x-ray mirrors and how complete is the cutoff for energies above this threshold?
Answer Collapse/Expand Text:
(03/05/14) JLT

The photon energy cutoff of the soft X-ray mirrors is at approximately 2.2 keV.  At photon energies below 2.1 keV, the mirrors transmit about 30% of the FEL power to the near experimental hall, though by design should transmit 66%.  Calculation indicates the first mirror would be destroyed at energies above 2.2 keV with typical intensities. That is a pretty complete cutoff.

19) A “slotted-foil” method has been described, capable of producing 2-fs x-ray pulses, or two similar pulses with variable timing delay.  What range of the two-pulse timing delay is possible using this method and how many photons can be available in each pulse?  Does it depend on photon energy?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys. 

The slotted foil, first proposed in 2004 (Phys. Rev. Lett., 92,  074801, 2004), was installed at the LCLS in 2010, and it has been developed as a regular operation mode for short pulse generation and pulse length control. It is located at the middle of the second bunch compressor (BC2). Since no machine configuration changes are needed to set up slotted-foil mode, it provides a fast way (a few minutes to check the foil horizontal alignment) for X-ray pulse duration control.


Three slot arrays are widely used, one single-slot array and two double-slot arrays. By choosing the slot width or slot separation, the single pulse duration or double-pulse delay can be controlled. It also depends on the BC2 bunch current. A Matlab-based GUI is used to control the foil and to calculate the unspoiled electron bunch duration or delay, found to be consistent with measurement.

For 150pC operation, with 420 um bunch length from injector, energy spread after laser heater 20keV, BC1 current 220A, and BC2 Energy 5 GeV with R56 of -24.7mm, the calculated achievable pulse duration and double pulse separation can be found in the following table (BC2 current 1-2 kA is mostly for SXR, and 3-4 kA for HXR operation):


​​ single slot, pulse duration​ ​ ​​ ​double slots, pulse separation​
BC2​ ​min (fs) ​max (fs) ​ min (fs) ​ max (fs)
1 kA​ ​13 ​61 ​ 37 ​82
2 kA​ ​8 ​27​ ​16 ​36
3 kA​ ​6.3 ​18 ​10 ​23
4 kA​ ​5.8 ​14 ​7 ​17


When additionally in 20 pC operation, these numbers can be reduced to the few-fs level.

Direct measurements of the X-ray pulse duration from the foil are very challenging. Cross-correlation measurements have been made (Phys. Rev. Lett., 109,  254802, 2012).  The number of photons in the pulse scales roughly linearly with the selected slot width (photon pulse duration).  So for example, if the pulse length is established at 100 fs with 1E12 photons per pulse, and then the single-slotted foil is inserted to generate a 5-fs pulse, the number of photons will drop to about 5%, or 5E10.

Measurements with the X-band transverse cavity have been performed recently, and are in good agreement with expectation (APL, 107, 191104, 2015). This can also be recorded during tune up and/or experiment together with user data recording.


In January 2015, a second double-slot foil was installed for double-bunch mode, but can also be used in single-bunch mode to extend the double pulse delay upper limit approaching to the full duration of the bunch.  ​


20) How much seperation in time and energy is available over all methods of generating dual pulses?
Answer Collapse/Expand Text:

LCLS is currently developing several “two pulses” operating modes, where pairs of FEL pulses are produced with up to 3% photon energy separation in both the hard and soft X-ray regimes. There are two time separation regimes, 0 - 900 fs and 350 ps -38 ns.  Restrictions on energy separation, delay and achievable number of photons apply depending on the scheme used to make the two pulses.  Users are encouraged to contact LCLS for more detailed information about FEL performance in the different operation modes. The energy and time separation can be adjusted in minutes for the fs separation mode.  We will accept proposals utilizing two pulse operation. 


21) How much 2nd and 3rd FEL harmonic power usually accompany the fundamental FEL pulse and how does this vary with photon energy?
Answer Collapse/Expand Text:
(4/08/15) LCLS Machine Phys.
Measurements show the FEL can contain a 3rd harmonic component as high as 2% of the power of the fundamental wavelength, and a 2nd harmonic component as high as 0.05% of the fundamental.  Typical measurements of the 3rd harmonic component are 0.5-1.5%.  A 2nd harmonic component has been measured at 0.03-0.05% of the fundamental power. 

22) What should I know about the XTCAV and what can I do with it?
Answer Collapse/Expand Text:

(4/08/15) LCLS Machine Phys.

An X-band transverse deflecting cavity (XTCAV) was installed downstream of the LCLS undulator beamline and tested in early 2014 for user operation. This device measures the electron bunch time-energy phase space distribution. Since it is located after the undulator, time-resolved FEL lasing effects (electron energy loss and energy spread increase) can be measured. By comparing images between FEL-on and FEL-off conditions, we can reconstruct the X-ray temporal profile for each lasing shot without interrupting FEL operation.

The best temporal resolution measured is about 1 fs rms for soft X-rays (800 eV), and about 4 fs rms for hard X-rays (8 keV). Experimental results have been published in Nature Communications, 5, 3762 (2014). To record XTCAV data from the user side, in each machine configuration, MCC provides calibrations and also suppresses lasing for the user to record necessary baseline (non-lasing) images. This may take a few minutes. Once recorded, regular operation is resumed and XTCAV images are recorded at will.

A configuration for the LCLS DAQ system has been finalized. Python-based analysis scripts are available to the beamlines to perform single-shot reconstruction. These are compatible with both ordinary SASE mode, as well as a number of more advanced LCLS machine configurations.

The camera can now be recorded at the full 120 Hz beam rate.

Please speak with your beamline scientist for the most recent information on recording and analyzing XTCAV data.

23) What modes are available for seeded beams?
Answer Collapse/Expand Text:

​Seeded modes are available for Hard (HXRSS) and Soft (SXRSS) x-ray beams.

Setup time is approximately 2 hours and is part of the experiment setup before the user run starts.
The actual performance parameter for a particular experiment depend on a variety of factors, for example the exact energy, required bandwidth and acceptable pedestal.



Mode​ ​Energy Range ​Bandwidth Pulse Energy​ Pulse Length​
HXRSS​ > 4.5* keV  0.35 - 1.5 eV​ ~ 1 mJ up to 40 fs
​SXRSS ​0.4-1.2 keV

​~ 100 meV @ 400 eV

~ 150 meV @ 530 eV

~ 200 meV @ 800 eV

​< 50-100 uJ @ 20 fs

up to ~ 0.5 mJ with pedestal

​​20 - 120 fs


  * above ~ 9.5 keV the achievable pulse energy drops off.


For seeded dual energy / dual pulse beams, see the parameter table posted at question 20. 

SLAC SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy