IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT036

TWO AND MULTIPLE BUNCHES WITH THE LCLS COPPER LINAC*F.-J. Decker†, W. Colocho, A. Halavanau, A.A. Lutman, J.P. MacArthur, G. Marcus, R.A. Margraf,
J.C. Sheppard, J. Turner, S. Vetter, SLAC, Menlo Park, CA, USA
AbstractTwo, four, and even eight bunches were accelerated
through the copper linac. Two and four bunches were delivered successfully to photon experiments in both the hard(HXR) and soft (SXR) LCLS x-ray lines. In this paper wewill concentrate on the more challenging issues, such as:
the BPM deconvolution for both bunches, RF kicks atlonger separations, tuning challenges, bridging the communications gap between the photon and electron side, thelower bunch charges for the eight bunch case, and rapid
timing scans over several ns. We will describe some of thedeveloped solutions and plans for the rest.

INTRODUCTUION
Two bunch running is now quite well established, only
longer time separation of 220 ns and beyond are more
problematic. For a general reference, see [1].

FOUR AND EIGHT BUNCHES
Eight bunches, 0.7 ns apart were accelerated in the LCLS
copper linac and brought onto the dump screen (Fig. 1).
The differences are quite obvious and come from the different intensities from the laser pulses onto the gun cathode.

Multi-Bunch Generation
The Multi-Pulse Pulse-Stacker (MPPS, see Fig. 2 in [2])splits, delays and combines light pulses of two laser (Coh1,Coh2) into four pulses each. In Figure 1 the order is: Coh1
B1+B2+B3+B4 then Coh2 B1+B2+B3+B4. Bunches 3and 4 from each laser have lower intensity and Coh1 waslower than Coh2. Lower charge bunches get off the cathodefaster and in the end get compressed more. The charges
were quite far away from equal (12.5% each). The deviations from that number in percent were: -6, +7, -29, -26,+22, +40, -6, -2, making the 6th bunch about twice as intense as the 3rd one. The average charge of 60 pC was abouta third of the typical intensity.

Charge Sensitivity
Even though the phase change due to charge is quite
small the effect on peak current and therefore CSR kick islarge, see Fig. 2 and 3. Besides the initial difference incharges which were caused by a not ideal 50/50 splits, it
was recently found that some of the mirrors for the delayedbunches were clamped so hard that the mirrors deformed
and caused the delayed laser pulses to a more elliptical thanround shape and therefore causing some intensity loss due
to apertures. For two bunches a charge difference can beused for tuning.
____________________________________________*Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515.
† Decker@SLAC.Stanford.edu

MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

Figure 1: Eight bunches 0.7 ns apart on the OTR dump
screen. The time resolving transverse cavity XTCAV wasdetuned in temperature so the eight bunches don’t overlap
in time (horizontal). The vertical dimension is proportionalto energy. All bunches are over-compressed (top), while
for the middle the compression was reduced (-32° to -30°),
so bunches B2, B5 and B6 were under-compressed. At thebottom (at -29° in L2 phase) all are under-compressed. Theenergy slope along the bunches of about +2% is real. It was
not corrected at the time.

TUPOPT036
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Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

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ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT036(blue). It gets trickier for four or more bunches and RF manipulation techniques like in [4] have to be deployed. C)
RF phase changes of a few degrees when the SLED pulse
is early are quite a concern and are the cause that the second pulse is typically further compressed and therefore has
more peak current, see Fig.4 (green).

Charge [nC]

Figure 2: BC2 peak current (blue) [and BC1 *10] versus
charge. The time separation (dotted versus dashed) comesfrom the fact that the scan was done in “zig-zag” mode (going up and down) so a small phase drift during a scan canbe easily detected since feedbacks had to be off.

Figure 4: Energy (blue, inverted) and peak current (green,in kA) at BC2 versus SLED RF timing. While there are twopoints with the same energy (red circles, 218.5 ns apart) thephase of L2 is different by a few degrees and therefore thepeak compression is quite different by a factor of two.

Phase Change during SLED Pulse

Charge [nC]

Figure 3: CSR induced transverse kick causing a trajectorychange up to 0.3 mm after BC2, or about six times the RMS
beam size (Data form 28-Jul-2021 11:03:18).

Reduced Tuning Parameters
For four and even more for eight laser pulses the tuning
parameters are restricted compared with two pulses. Theeasiest way to see this are the delays. Timing of the twolasers plus one delay stage gives three parameter for fourdelays. This is similar for intensities, therefore one bunch
cannot be adjusted in time or charge or direction. For theeight pulse case we get an additional delay stage, so fourparameter for eight bunches. They should be equal “automatically”, but typically they are not.

TWO BUNCHES WITH BIG SEPARATION
There are three main issues concerning two buncheswith long separation of >200 ns. A) They experience different RF kicks and therefore have different trajectory in
the undulator. How we solve that with kickers is describedin another paper [3]. B) Equal energies for two bunches canbe achieved by timing the SLED RF pulse, see Fig. 4TUPOPT036
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The phase change can have different causes. An obvious
cause can be an RF with different phase entering or exitingon purpose, by a mistuned SLED cavity, or a different modulator high voltage and therefore a different phase. But
even with perfect phase control the sudden change in amplitude due to the 180° phase jump which creates the SLEDpulses causes different frequencies, which travel with different speeds along the three meter accelerating structure
or disc loaded waveguide (DLWG). This causes a changing
phase when the peak amplitude starts exiting the DLWG
[5]. This effect could be counteracted by an I and Q waveform control of the RF allowing an independent amplitude
and phase control.

Signals Used for Tuning
The Beam Positions Monitors (BPM) show the trajectory of the beam through the accelerator, the undulator andfinally to the dump. For two bunches the normal BPM system shows a vector sum, so a special deconvolution software was developed. Since there were changes for the undulator BPMs and their digitizers another quick way was
to just time every other BPM electronics so that they
showed mainly only one bunch (Fig. 5).
After the bunches are overlapped enough (<0.1 mm) the
fine tuning is done using the gas detector or even more preferred a signal closer to the experiment. Going through a
monochromator and detecting the two bunches with a fast
diode their intensities are plotted versus electron energy
and can therefore be aligned in energy and equalized in intensity (Fig. 6).

MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

IPAC2022, Bangkok, Thailand
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ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT036

Rapid Timing Scan
A rapid timing scan of the bunch separation over a few
minutes could be done in the 38 to 54 ns range where wakefields are weak and RF kicks still quite similar. A wholeoscillation period of a diamond crystal could be measured.

CONCLUSIONS AND FUTURE STEPS

Figure 5: BPM orbit display. In the undulator the timing offor the BPM digitizer gate was setup to only measure oneor the other bunch when they were 218.5 ns apart. This
gave enough signal to tune them to have finally the sameorbit.

There are a few parameters which have to be right for
multiple bunches and two bunches to create equally good
x-ray laser pulses in the undulator. They are mainly the trajectory in the undulator and the peak current. The trajectory
gets influenced by wakefields, RF and CSR kicks, andlaunch conditions, but can be controlled by four fast kickers (two already installed). The final peak bunch current isdetermined by the timing of the bunch with respect to theRF phase in L2. A difference can come from the initial
beam arrival time into L2 caused by different charges, different Schottky phases (laser phases), and/or phase differences of the injector stations, often the L1X phase difference is used for tuning. But also the phase of L2 is different
on both sides of the peak amplitude of the SLED pulse.
Equal charges are critical for multiple bunches (>2). Getting longer and maybe variable delays for multiple bunches
is an even more significant challenge. The split and delayon the laser table could have a longer fixed path and therefore delay, but at some point the energies will be too different and an RF waveform control for amplitude and phasehas to be implemented. Since the split and delay uses somepaths twice for the four bunch mode, and some even fourtimes for eight bunches, the tuning parameters are reduced.
This should have advantages like one setup for four, but aredifferent due to launch conditions (reflected versus transmitted or laser profiles).
The two bunch setup has all the tuning parameters available due to the use of two different lasers. Different separation delays have unique challenges. From 0.35 ns toabout 10 ns wakefield kicks dominate. The BPM vector
sum is pretty low at (n+1/2) * 7 ns, so these delays should
be avoided when running with beam based feedbacks.
Phase variations in L2 at long delays need to be controlled.

REFERENCES[1] F.-J. Decker, K.L. Bane, W. Colocho et al., “Tunable x-ray
free electron laser multi-pulses with nanosecond separation”,
Sc
Rep.
12,3253(2022).
https://doi.org/10.1038/s41598-022-06754-y[2] F.-J. Decker et al., “Two and Multiple Bunches at LCLS”, in
Proc. IPAC'18, Vancouver, Canada, Apr.-May 2018, pp.
4378-4380. doi:10.18429/JACoW-IPAC2018-THPMK042

Figure 6: Fast Intensity Monitor (FIM) signal de-convoluted for two bunches 7 ns apart (top). The individual intensities are then plotted versus the electron beam energy(bottom). The successful data transfer from the photon side
to the accelerator side made this plot possible in real timefor tuning.

MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers[3] A. Halavanau et al., “LCLS Multi-Bunch Improvement Plan:First Results”, presented at the IPAC'22, Bangkok, Thailand,Jun. 2022, paper TUPOPT037, this conference.
[4] Decker, Franz-Josef; Farkas, Zoltan David; Minty, MichikoG., “Low Current, Long Beam Pulse with SLED”, in Proc.
PAC'99, New York, NY, USA, Mar. 1999, paper MOP49.
[5] R.B. Neal, General Editor, “The Stanford Two-Mile Accelerator, The Blue Book”, 1968, pp.123, published by W.A. Benjamin, Inc. https://www.slac.stanford.edu/library/2MileAccelerator/2mile.htm

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Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1