Machine FAQ (NC Linac)

(April 21, 2021) T. Maxwell

At present, the available photon energy range is ~ 200 eV up to 25 keV covered by the soft x-ray and hard x-ray FEL undulators. The soft x-ray system will provide access to the 200 eV to 2 keV range. The hard x-ray system will cover the range from 1 keV to 25 keV with the higher photon energy range being commissioned through Run 18.

It is important to understand that this is the range of FEL operation. The x-ray optics transmission and instrumentation must also be taken into consideration for the determination of  the experimental energy ranges.

The new Variable Gap Undulators (VGU’s) provide a means to change photon energies by changing the undulator gap (variable strength). This is a continuing area of development expected to result in faster energy changes and photon energy scanning capabilities. To date, soft x-ray photon energy can be varied up to a factor of 2 without significant degradation of performance for typical parameters. Hard x-ray photon energy can be similarly varied 25% or more.

Energy changes based on accelerator beam energy change are more performant and require anywhere from 5 minutes (small energy adjustments of 5-50%) to 30 minutes (energy adjustments of a factor of 2-3) with some accelerator tuning required in to re-establish the full X-ray pulse energy. This retuning is faster when the energy is increased rather than decreased.

Please ask the ACR 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) via accelerator beam energy without requiring any other adjustments (although some small X-ray power variations may accompany this). This fast energy control can be implemented directly by users in a programmed fashion.  Please ask your LCLS point of contact in advance about preparing this in coordination with the ACR.

(April 21, 2021) P. Krejcik, T. Maxwell

For normal conducting linac operation a fast kicker located at the end of the linac can now deflect the 120 Hz linac beam into either the HXR or SXR undulators, allowing interleaved operation of the two beamlines. See FAQ 13 for details on the rates available in each beamline.

The variable gap undulators in each beamline allow the x-ray wavelength to be set independently for each beamline. Since the electron bunches are coming from the same linac the program coordinators will discuss with the users in each beamline what the best choice of beam energy, bunch length and bunch charge will be for each combination of experiments.

The brightness of the x-rays from each undulator depends both on the gap setting and the electron energy, and the reason the exact choice of electron bunch parameters must be negotiated and worked out with the program coordinators.

For the special case of fully interleaved delivery to both beamlines, as described in FAQ 12, there exists the capability to independently vary the electron bunch energy over a small range in order to allow Vernier scans of the X-ray wavelength for each user.

We also continue to expand the capability to provide different average electron beam energies to both undulators to enable simultaneous operation at very different photon energies for secondary undulator operation. This includes operation of the soft x-ray undulator as low as 700 eV during most hard x-ray delivery, and as high as 7-8 keV for the hard x-ray program during primary delivery to most soft x-ray programs. Again, please discuss with your beamline physicist and program coordinators regarding this new capability.

(July 23, 2019) T. Maxwell

As noted in the LCLS Parameters table, the expected pulse energy for the new undulator systems is about 0.6-1.5 mJ for the hard x-ray line for a nominal FWHM pulse duration of 30 fs, and ~2 mJ in the soft x-ray undulator for a nominal 50 fs pulse duration. The number of photons in the pulse depends on the photon wavelength (energy) and duration.

These are equivalent to 0.2e12 to 9e12 photons/pulse for the hard line and 0.8e13 to 60e13 photons/pulse for soft.

Very roughly speaking, reducing(/increasing) the pulse duration comes with a proportional reduction(/increase) in the expected number of photons per pulse for a given photon energy.

If the electron bunch is shortened by increasing compression (generating a shorter X-ray pulse), generally 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. Therefore the number of photons in the pulse may not drop as fast as a simple pulse-length scaling suggests and in some cases even increases. Note that this pulse length reduction by increasing compression also generally includes a corresponding increase of the FEL SASE bandwidth.

(July 23, 2019)

Please always ask your LCLS point of contact first and they will help you reach the appropriate expert(s) where needed.

(July 23, 2019) H.-D. Nuhn

The FEL x-ray pulse has good transverse coherence (M = 1 - 1.5), since for normal operation the transverse coherence area is larger than the spot size. However, systematic errors in the FEL may slightly degrade the transverse coherence and require additional tuning and correction where critical.

The FEL does not generally have full temporal coherence. In SASE mode, the frequency bandwidth can be 200 times larger than the transform limited bandwidth for very long pulse duriations (>30 fs) to 30 times the transform limited bandwidth for short pulse duriations (<10 fs).

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

July 23, 2019) H.-D. Nuhn

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 an example of size versus energy in the middle of the FEE (at the direct imager) in the below figure.

For details on aperture, final spot size, focusing, etc, please refer to the appropriate beamline and your LCLS point of contact.

Question details:

Answer:

(July 23, 2019) B. Jacobson

Each of the new undulator beamlines is instrumented with several X-ray profile screens. Historically the “YAGXRAY” camera and other imagers were used with one in the Electron Beam Dump (EBD) and one in the Front-End Enclosure (FEE), respectively. Each system uses a scintillator and an optical CCD camera to capture images of the X-ray profile.

For Run 18 onward, other imagers are designated “IM#H/S”, where # refers to the occurrence number and H/S refers to hard or soft beamline. Each unit has three scintillator options: YAG:Ce, and two diamonds. The YAGXRAY screen remains on the hard X-ray beamline and the new YAGXRAYB is the counterpart for the soft X-ray beamline.

Locations of the X-ray profile screens along the beam path are listed below for the soft and hard X-ray beamlines. In each case, the distance referenced is from the end of the last undulator – segment 21 (SXR cell 47) for the soft line; segment 32 (HXR cell 46) for the hard line.

(July 22, 2019) P. Krejcik

The normal conducting linac can operate at rates up to a maximum of 120 Hz. All macro-pulses (typically containing one pulse) are selected from this continuous and equispaced train.

The 120 Hz rate can be further subdivided into lower rates between the HXR and SXR beamlines. The resulting maximum allowed rates in the two beamlines must add to a maximum of 120 Hz, so that if the HXR receives N Hz then the SXR may receive (120-N) Hz, where N can take on values of:

0, 1, 10, 30, 60, 90, 110, 119 or 120 Hz.

The maximum rate in each beamline is assigned by the program coordinators. The assigned maximum rates can be independently lowered on demand in each undulator to any lower integer values <N in the HXR and <(120-N) in the SXR, including single shot and burst mode. Burst mode delivers a prescribed number of bunches to the user with a spacing determined by the rate N or (120-N) for the beamline.

Single shots and burst mode can be programmed by the user, selected from the maximum rate assigned to the beamline.

The rates described above always refer to the macro pulse rate from the linac, derived from the macro-pulsed repetition rate of the RF klystrons in the NC linac. Normally only one electron bunch (and therefore X-ray pulse) is delivered per macro RF pulse. However, under special circumstances the linac can also be operated in a “multi-bunch mode" where two or more bunches in close succession are delivered per linac pulse. The separation of the "multi-bunches" has to be an integer multiple of 0.35 ns (a number determined by the linac RF frequency of 2856 MHz). Yet another mode allows for "twin bunches" very close together (femtoseconds) within the same RF pulse.

These modes affect beam delivered to all destinations and are not generally compatible with parallel running. Please refer to the LCLS Parameters page for further details on these advanced modes and speak with your LCLS point of contact regarding availability.

(July 23, 2019) J. May

You're able to move your experiment's timing around, but not the beam itself. The arrival time of the x-rays to your experiment is determined by the bulk of the configuration of the accelerator. Things like the klystrons, the injector lasers and time domain diagnostics all depend on time (and phase) stable operation, as do systems involved in machine protection and beam containment. Therefore, the accelerator does not, by design, allow for this.

Related to this point, FAQ #13 speaks to the selection of beam rates. While beam is available at the assigned rate, an experiment can have delivery suppressed at any destination by enabling BYKIK.

Separate to the accelerator, and key to the question, users have access to the triggers for their experiments. Delaying your triggered acquisition systems for your diagnostics in most cases accomplishes the same thing that you would by moving the entire accelerator around. Further, with experiments running supplemental laser systems, laser timing, controlled by the same precision timing hardware as used on the injector lasers, can be controlled as desired, allowing experiment control of laser/xray separation. These systems typically have step sizes on the order of 20fs (open-loop) and with diagnostics like xray/optical cross-correlators (the "time tool"), can be operated closed loop. Always discuss necessary temporal structure with your SLAC contact prior to experiments, and the Program Deputy and EOIC during your run.

(July 22, 2019) FJD

• The shot to shot jitter for the X-ray pulse arrival time (with respect to 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 0.10-0.05% (for 0.8-8 keV photons, respectively, and based on measured electron beam energy jitter of 0.05-0.025%)
• 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. The transverse jitter is often dominated by electron beam dispersion (x, y versus E) in the undulator which can be reduced to zero.

These stability levels are reasonably accurate for the nominal 125 pC bunch charge.

When additionally in 20 pC operation, these numbers can be reduced to the few-fs level, at the expense of pulse energy at the cost of additional configuration time.

Measurements with the X-band transverse cavity have been performed recently, and are in good agreement with expectation (Appl. Phys. Lett., 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.

This mode affects beam delivered from the normal conducting linac to both undulator lines and therefore generally not compatible with parallel experiments.

(Apr. 21, 2021) T. Maxwell

The LCLS has developed several “two pulse” operating modes driven by the normal conducting linac, several of which were recommissioned over the course of Run 18. Appropriate selection of the FEL mode depends on the experiment’s energy/time separation and pulse intensity requirements.

The largest possible energy separation will be up to a factor of 2 between photon energies via the split undulator technique with up to a few tens of microjoules per pulse and fs-scale pulse delay. Other more advanced methods allow higher pulse intensities (fraction of a millijoule per pulse) only for energy separations of up to 2.5% over a wide range of pulse delays. High-level details are outlined in the multi-color section of the LCLS Parameters document (below).

Users are strongly encouraged to discuss requirements for two-color experiments with their point of contact and accelerator staff prior to submission of a proposal.

See the Dual Mode Parameters Table

(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 2^nd harmonic component as high as 0.05% of the fundamental. Typical measurements of the 3^rd harmonic component are 0.5-1.5%. A 2^nd harmonic component has been measured at 0.03-0.05% of the fundamental power.

(July 22, 2019) P. Krejcik

An X-band transverse deflecting cavity (XTCAV) is installed downstream of the HXR undulator beamline and a second XTCAV is being installed in the SXR beamline to serve a similar purpose. Each device measures the electron bunch time-energy phase space distribution. Since they are located after the undulators, 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 beamline, in each machine configuration, ACR 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 implemented. 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 record images up to the full 120 Hz beam rate.

With dual operation of the HXR and SXR beamlines with interleaved bunches (see question 12) it is also possible to simultaneously record XTCAV profiles in both beamlines on alternate bunches. The temporal resolving power of each XTCAV is a function of how strong the RF kick is in each XTCAV deflecting structure, but since the RF power comes from a single klystron the resolving power of each XTCAV will depend on how the mechanical power splitter is set (in the range 0 to 100%). The program coordinators will weigh up the XTCAV needs for each experiment and find the best compromise to allow XTCAV measurements on each beamline when needed.

Please speak with your LCLS point of contact for the most recent information on recording and analyzing XTCAV data.

(May 14, 2021) A.Brachmann, T. Maxwell

Hard and Soft X-ray Self-seeding ("HXRSS" and "SXRSS") are planned for the corresponding undulator beamlines. The original LCLS seeding systems have been re-designed and re-built for the new Variable Gap Undulator beamlines and to be commissioned. HXRSS was commissioned in May 2021, SXRSS commissioning planned for the winter of 2021. Full functionality at the beginning of the run cannot be guaranteed at this time (May 14, 2021). Please discuss availability with your LCLS Point of Contact.

Setup time is approximately 1 hour for hard and 2 hours for soft. An initial machine configuration is generally built in advance of the first day of the experiment. Additional setup time is still required as part of the experiment from day to day through the remainder.

The actual performance parameter for a particular experiment depend on a variety of factors, for example the exact energy, required bandwidth and acceptable spectral pedestal.