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MICROBUNCHING OF THERMIONIC CATHODE RF GUN BEAMS IN
THE ADVANCED PHOTON SOURCE S-BAND LINAC ∗J. Dooling† , A. Brill, N. Kuklev, I. Lobach, A. Lumpkin, N. Sereno, Y. Sun
Argonne National Laboratory, Lemont, IL, USA
Abstract
We report on measurements of beams from thermioniccathode (TC) rf guns in the Advanced Photon Source S-Band
Linac. These measurements include the macropulse out ofboth new and existing TC guns as well as the observation of
microbunching within the micropulses of these beams. A
gun chopper limits the macropulse FWHM duration to the
10 ns range. Our objectives were to analyse the new TC gun
and investigate microbunching within a TC-rf-gun-generated
beam. Our diagnostics elucidated longitudinal beam structures from the ns to the fs time scales. Coherent transition
radiation (CTR) interferometers responding to far-infrared
wavelengths were employed after each compression stage toprovide the autocorrelations of the sub-ps micropulse durations. The first compression stage is an alpha magnet andthe second a chicane. A CCD camera was used to image
the beam via optical transition radiation from an Al screen
at the end of the linac and also employed to measure coherent optical transition radiation (COTR) in the visible range.
The COTR diagnostic observations, implying microbunching on a fs time scale, are presented and compared with alongitudinal space-charge impedance model.

INTRODUCTION
The injection system of the Advanced Photon Source(APS) has relied on thermionic cathode (TC) rf guns aselectron-beam sources since 2001 [1]. This will continue
to be the case for the APS Upgrade with the multi-bend
achromatic magnet lattice installation in the 6 GeV storage
ring currently in progress [2]. There are two such S-Band
TC rf guns installed in the linac so that one is a "hot" spare.
We report the basic testing of the beams from new generation TC rf guns manufactured by RadiaBeam Technologies(RBT) [3] in terms of the ≈10 ns long macropulse composed of micropulses with an S-Band repetition frequency(2856 MHz). We obtained basic macropulse data for a newly
installed RBT gun to comparison with one of a set of threeTC rf guns purchased from AET Associates (AET) in 2001.
The AET guns have been the primary electron source for the
APS since that time.
Our interest was to measure the charge of each micropulse
within the macropulse structure from the chopper-gated [4],TC-rf gun [5–7]. The extraction of charge from the TC rf
gun involves high-power rf applied to the cathode; thus, theextracted charge comes in a series of micropulses. With themacropulse temporal distribution, we can then determine∗ Work supported by the U.S. D.O.E., Office of Science, Office of Basic

Energy Sciences, under contract number DE-AC02- 06CH11357.

† dooling@aps.anl.gov

the charge per micropulse. Only the AET gun was employed
for the bunch compression experiment discussed below.
In the APS linac, the micropulse duration after the alphamagnet is sub-ps. After further compression in the chicane,longitudinal space charge (LSC) induced microbunching
within the micropulses generates coherent optical transition
radiation (COTR) when striking the downstream intercepting Al screen. The COTR (visible wavelengths, fs temporalscale) was transported out of the tunnel to a CCD camera,
and the images were recorded. We present a comparison of
the observed COTR enhancements with the predictions of a
LSC-impedance model.

EXPERIMENTAL DESCRIPTION
A schematic of the linac with components relevant for thepresent discussion is presented in Fig. 1. Compression isprovided by the alpha magnet at the end of the gun beamline and the L3 chicane after the first four accelerating structures in L2. Coherent Transition Radiation far-infrared (CTR
FIR) interferometers, positioned in the L2 section (L2:CTR)
and after the chicance (L3:CTR), provide longitudinal pulse
length data [8]. Average current is monitored immediately after the gun and macropulse charge is measured after the alphamagnet in the linac beamline. Optical transition radiation(OTR) and COTR are generated by an Al screen (S5:OTR,
as shown in Fig. 1) when struck by the electron beam. An
optical transport line guides the light to a camera systemoutside of the tunnel.
A block diagram of the macropulse duration measurement
is presented in Fig. 2. Raw beam signals from the four
button pickups are input to 20 dB directional couplers (MiniCircuits ZGDC20-33HP+). The coupled signals ("c") are
combined to form the sum output (Mini-Circuits ZN4PD642W-S+) which is fed into one channel of a fast oscilloscopewith 50 Ω input impedance. The main, uncoupled signals
continue on to the analog conditioning electronics associatedwith the rest of the S-band linac BPMs [9–11]. Because of
capacitive coupling, the DC component of the macropulse is
not present. Alternatively, the summing unit can be bypassed
to provide coupled input to the oscilloscope from all four of
the BPM channels. Four channel input allows for observationof beam position within the macropulse. The BPM rf signals
are digitally undersampled by the oscilloscope’s analog-todigital converter following the discussion in Ref. [12].
A new 1.5-cell, TC rf gun developed by RBT was installed at the RG1 location in the linac during the December
2022-January 2023 APS maintenance period. We wished tomeasure the macropulse waveform of this gun to comparewith that of the operating TC gun. Further tests with the RBT

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Figure 1: Linac schematic. The main accelerating sections are designated LX where X=1,2,4, and 5. Sections 2, 4, and 5
each have four 3-m SLEDed S-band accelerating structures. A1 and A2 are the alpha magnets associated with two TC rf gun
beamlines RG1 and RG2. L2:P1 represents the BPM location for the macropulse measurements. L*:CM are current/charge
monitors, ES is a spectrometer, triangles represent dipole magnets, and OTR images are collected at the S5:OTR station.

Figure 4: Left: Raw macropulse signal from the RG1 RadiaBeam gun. Right: Peak detect envelope of the same signal.

Figure 2: Macropulse measurement schematic.

Figure 5: Macropulse envelope waveforms from the RG2
AET gun with varying chopper voltages for 190 mA (left)and 300 mA (right).
Figure 3: Left: Raw macropulse signal from the AET RG2
gun. Right: Peak detect envelope of the same signal.
gun will be conducted once accelerator operations resume in
Fall 2023. Only the AET gun, installed in the RG2 location,
was used in the bunch compression experiment discussed
below.

MACROPULSE WAVEFORMS
An example of a raw macropulse waveform from one of
the AET guns is plotted in Fig. 3, left. Figure 3, right shows
the same signal after peak-detection processing to yield theenvelope. This AET gun is located in the RG2 station; this is
the gun beamline closest to the entrance of the linac known
as the L2 section. A sampling rate, 𝑓𝑠 of 20 GS/s provides
7 samples per fundamental rf period (350 ps) and yields a
relatively smooth envelope after peak detection.
Macropulse data acquired for one of the new RadiaBeam
guns is presented in Fig. 4. In this case, the gun is locatedin the RG1 station, approximately 3 m further from the L2

linac section than the RG2 gun. Transmission losses RG1
are typically higher than for RG2. Each station has its own
chopper system; in the case of RG1 the chopper pulse islonger than that for RG2.
The macropulse waveforms were also observed as the
chopper voltage as well as the average gun output current
were varied. Waveforms at four chopper voltages are presented in Fig. 5 left and right for gun currents of 190 and
300 mA, respectively. Cathode heater power was varied tochange the average gun current for the waveforms presented
in Fig. 5. At higher voltages, we observed a double peak inthe macropulse envelope. In this case, the beam is swept
past the slit on the rising edge of the voltage pulse and again
on the falling edge. Gun chopper voltage is the method used
to control the charge per macropulse in the linac while thegun cathode heater power is typically maintained at a constant value. Macropulse duration varies from approximately
7 to 11 ns for both current levels as the chopper is raised
from 20.5 to 23.5 kV. Current and charge data collected fromcurrent monitors in the linac during these measurements arepresented in Table 1.

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12th Int. Beam Instrum. Conf.
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Table 1: RG2 Current and Charge Measurements
Chopper
voltage
(kV)
20.5
21.5
22.5
23.5

190 mA
I𝐶 𝑀
Q𝐶 𝑀2
(mA)
(nC)

300 mA
I𝐶 𝑀
Q𝐶 𝑀2
(mA)
(nC)

191
189
191
191

303
302
303
300

0.85
1.07
1.26
1.37

1.04
1.37
1.56
1.64

MICROPULSES AND MICROBUNCHING
The gun output current after the alpha magnet is modeled
as a train of charge delta functions where each charge pulse
is separated from its neighbor by one rf period. The current
waveform may be expressed as,
2𝜋𝑛𝐼𝑔 (𝑡) = 𝐴(𝑡)𝛿 𝑡 − 𝑡 𝑠 −
,
(1)
𝜔𝑟 𝑓
where 𝐴(𝑡) is the time-varying amplitude of the macropulse,𝑡 𝑠 is time slip or jitter, 𝛿 is the delta function, and 𝜔𝑟 𝑓 is theradian frequency of RG2 rf drive power. The total charge isthen determined integrating Eq. (1) over time,∫∑︁𝑄=
𝐼𝑔 (𝑡)𝑑𝑡 =𝐴 (𝑡 𝑛 )(2)𝑛where 𝑡 𝑛 = 𝑡 𝑠 + 2𝜋𝑛/𝜔𝑟 𝑓 . Respective calibration factors
for the AET RG2 gun operating at currents of 190 mA and
300 mA are 56.33 ±0.97 pC/V and 55.36 ±1.49 pC/V; themaximum charge per micropulse at these currents are 39.99±0.88 pC and 49.88 ±1.66 pC.
For bunch compression studies, we are interested in the micropulse peak current. Compression studies were conductedwith the RG2 AET TC gun beam. Two coherent transition
radiation (CTR) interferometers are installed in the APS
linac [8, 13]; one in the L2 section (L2CTR) between thesecond and third accelerating structures (AS), and anotherfollowing the chicane in L3 (L3CTR), see Fig. 1. The four
AS of L2 may be phased to chirp the pulses prior to the chicane to facilitate bunch compression. The beam energy after
the L2 section was 150 MeV. The L2CTR sees the effect of
the alpha magnet compression, whereas L3CTR observes
compression provided by the chicane. Autocorrelations of
the CTR signals at these two locations are presented in Fig. 6.
Assuming a Gaussian profile for the compressedbunch, the√rms bunch duration can be approximated as 2𝜎𝑧 /𝑐. For the
autocorrelation scans shown in Fig. 6 this yields 300 fs and
130 fs from L2CTR and L3CTR, respectively. We assumed
that the micropulse length is approximately constant overthe macropulse duration. Using the FWHM duration, 50 pC
results in a current of 170 A after the L3CTR. This current
level is consistent with predictions made in Ref. [5].
During bunch-compression studies, gun output current
was increased to 650 mA. Phasing the L2 AS to maximize
compression, COTR was observed on the S5:OTR monitor, at an energy of 375 MeV [14]. Examples of OTR and

Figure 6: Autocorrelations of the coherent transition radiation signals from the L2CTR (left) and L3CTR (right) alongwith Gaussian fits to the central peak. RG2 (AET) source.
COTR emission are presented in Fig. 7. Emission intensity
contour and profile data with L2 rf phased away from maximum compression are shown in the top row; the bottom row
presents enhanced emission (COTR) at the maximum compression phase. The macropulse beam charge was monitoredat L5:CM during data acquisition and was stable to less than
3 percent for both compressed and uncompressed measurements. The COTR/OTR intensity ratio is about 6.5 at themost enhanced spatial location. The two COTR-enhanced
locations in the lower image (maximum compression) are approximately 𝜎𝑥 =380 µm by 𝜎𝑦 =50 µm in size. Compressed
micropulse peak current was approximately 200 A.
Examining the effect of longitudinal space-charge (LSC)
impedance, we plot in Fig. 8 Eq. (26) from Ref. [15] for
𝑅56 = 65 mm. With an rms energy spread of 2 keV, a gainof 5.7 is obtained at 800 nm. Signal intensity is proportional
to the product of the COTR gain and the sensitivity of the
CCD sensor integrated over the wavelength range of interest(400-1000 nm). Picking a COTR gain value in the red endof the range is meant to represent a rough estimate of theenhancement ratio. The gain is in reasonable agreementwith the enhancement factor determined for the data plottedin Fig. 7. This is the first time a comparison has been doneof empirical COTR gain from a TC rf gun source with an
LSC impedance model.
Previous simulations of the LSC instability showed thatmicrobunching in the visible wavelength regime can be supported if the slice energy spread of the microbunched portionof the beam is approximately 2-4 keV [16]. Using a PC rf
gun source, an LCLS-I experiment generated COTR gain
consistent with a slice energy spread of 3 keV. In the contextof Ratner’s LSC model, a 2 keV slice energy spread would
be indicated to explain the observed COTR gain for the microbunched portion of the TC rf gun beam. This value ismuch smaller than the measured ensemble energy spread.
Broadband visible regime COTR enhancements have also
recently been reported in laser-plasma-wakefield-accelerated
beams [17–19], but this is a different microbunching mechanism. In this case, intense fields from a 100-TW-classdrive laser interacting with the plasma electrons lead to microbunching.

SUMMARY
We described a method to measure the macropulse waveform of the APS TC rf guns. This method requires an oscillo-

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Figure 7: Top: Intensity contour plot (left) and line profiles (right) away from peak compression. Bottom: Intensity contour
plot and line profiles near peak compression.

Figure 8: Microbunching gain as a function of wavelength
for 𝑅56 = 65 mm with rms energy spread as a parameter.
scope with bandwidth and sampling rate sufficient to monitor
the fundamental frequency of the rf applied to extract beamfrom the gun; in the present case, this requires a bandwidth >2.5 GHz and sampling rate > 20 GS/s for 2865 MHz S-band
rf. Measurement of the macropulse envelope allows for calibration of the microbunch charge and thereby calculationof peak currents after bunch compression. AET and RadiaBeam TC guns were tested and, accounting for differencesin the chopper systems, behaved in a similar manner. Other
methods employing diodes could be explored to eliminate
the cost of an oscilloscope [20].
Additionally, we have inferred the presence of LSCinduced microbunching at visible wavelengths via the COTR
mechanism from TC rf gun beams. After two stages ofcompression and with further acceleration to 375 MeV, anenhancement factor of 6.5 was observed in OTR near maximum compression. The enhanced emission is attributed
to the generation of COTR from microbunching in the TC
rf gun-generated micropulses. Analytical models indicatethat a slice energy spread of only a few keV is needed for
microbunching (COTR gain) at visible wavelengths where
the CCD camera is sensitive. The importance of slice energy
spread as it applies to a TC-rf gun-generated beam is analogous to what was reported for PC gun [16] and LPWA [17]beams. Further investigations including spectral measurements of the microbunching phenomena are warranted.

ACKNOWLEDGEMENTS
Thanks to R. Soliday and H. Shang for assistance withanalysis scripts, of APS ANL.

REFERENCES[1] K. J. Beczek, J. W. Lewellen, A. Nassiri, and E. Tanabe, “A
Rationalized Approach to Thermionic RF Gun Design,”

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This is Content
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in Proc. PAC’01, Chicago, IL, USA, Jun. 2001. https://jacow.org/p01/papers/WPAH052.pdf[2] “Advanced Photon Source Upgrade Project Final Design
Report,” Advanced Photon Source, Argonne National Laboratory, Tech. Rep., 2019. https://publications.anl.
gov/anlpubs/2019/07/153666.pdf[3] S. V. Kutsaev et al., “A New Thermionic RF Electron Gun for
Synchrotron Light Sources,” in Proc. NAPAC’16, Chicago,
IL, USA, Oct. 2016, pp. 453–456.
doi:10.18429/JACoW-NAPAC2016-TUB3CO04[4] Y. W. Kang et al., “Beam Chopper for the Low-Energy Undulator Test Line (LEUTL) in the APS,” in Proc. PAC’97,
Vancouver, Canada, May 1997, pp. 2699–2701.
[5] M. Borland, “A High-Brightness Thermionic Microwave
Electron Gun,” Available as SLAC Report 402, Ph.D. dissertation, Stanford University, 1991. https://www.slac.
stanford.edu/pubs/slacreports/reports03/slacr-402a.pdf[6] M. Borland, “An Improved Thermionic Microwave Gun
and Emittance-Preserving Transport Line,” in Proc. PAC’93,
Washington D.C., USA, Mar. 1993, pp. 3015–3018.
[7] J. W. Lewellen et al., “Operation of the APS RF Gun,” inProc. LINAC’98, Chicago, IL, USA, Aug. 1998, pp. 863–865.
https://jacow.org/l98/papers/TH4042.pdf[8] A. H. Lumpkin, W. J. Berg, M. Borland, J. W. Lewellen, andN. S. Sereno, “Initial CTR-Based Bunch Length Measurements of Linac Beams Following the APS Bunch Compressor,” in Proc. FEL’03, Tsukuba, Japan Sept. 2003, pp. 2420–
2422. https://jacow.org/p03/PAPERS/WPPB005.pdf[9] A. E. Grelick, “RF and Beam Diagnostic Instrumentation at
the Advanced Proton Source (APS) Linear Accelerator,” inProc. LINAC’96, Geneva, Switzerland, Aug. 1996, pp. 851–
853. https://jacow.org/l96/papers/THP75.pdf[12] M. Wendt, “Direct (Under)Sampling vs Analog Downconversion for BPM Electronics,” in Proc. IBIC’14, Monterey,
CA, USA, Sep. 2014, pp. 486–494. https://jacow.org/
IBIC2014/papers/WEIYB1.pdf[13] A. Lumpkin, B. Yang, W. Berg, J. Lewellen, N. Sereno, andU. Happek, “Electron beam bunch length characterizations
using incoherent and coherent transition radiation on the
APS SASE FEL project,” Nucl. Instrum. Methods Phys. Res.,
Sect. A, vol. 445, pp. 356–361, 2000.
[14] A. H. Lumpkin, “Beam Diagnostics for Coherent Optical
Radiation Induced by the Microbunching Instability,” inProc. FEL’13, New York, NY, USA, Aug. 2013, pp. 169–172.
https://jacow.org/FEL2013/papers/TUOBNO01.pdf[15] E. Saldin, E. Schneidmiller, and M. Yurkov, “Klystron instability of a relativistic electron beam in a bunch compressor,”
Nucl. Instrum. Methods Phys. Res., Sect. A, vol. 490, pp. 1–8,
2002.
[16] D. F. Ratner, A. Chao, and Z. Huang, “Three-dimensional
Analysis of Longitudinal Space Charge Microbunching Starting from Shot Noise,” in Proc. FEL’08, Gyeongju, Korea,
Aug. 2008, pp. 338–341. https://jacow.org/FEL2008/
papers/TUPPH041.pdf[17] C. Lin et al., “Long-Range Persistence of Femtosecond
Modulations on Laser-Plasma-Accelerated Electron Beams,”
Phys. Rev. Lett., vol. 108, p. 094 801, 2012.
[18] A. H. Lumpkin et al., “Coherent Optical Signatures of Electron Microbunching in Laser-Driven Plasma Accelerators,”
Phys. Rev. Lett., vol. 125, p. 014 801, 2020.
doi:10.1103/PhysRevLett.125.014801[10] R. Lill, O. Singh, and N. Arnold, “New Beam Position Monitor System Design for the APS Injector,” AIP Conference
Proceedings, vol. 648, no. 1, pp. 401–408, 2002.
doi:10.1063/1.1524426[19] M. LaBerge et al., “Coherent 3D Microstructure of LaserWakefield-Accelerated Electron Bunches,” in Proc. FEL’22,
2022. doi:10.18429/JACoW-FEL2022-THAI2[11] N. S. Sereno, M. Borland, and R. Lill, “Automated correctionof phase errors in the advanced photon source linac,” Phys.
Rev. ST Accel. Beams, vol. 11, no. 7, p. 072 801, 2008.
doi:10.1103/PhysRevSTAB.11.072801[20] M. Gasior, “First Results from the LHC Schottky Monitor
Operated with Direct Diode Detection,” in Proc. BIW’12,
Newport News, VA, USA, Apr. 2012, pp. 222–224. https://jacow.org/%20BIW2012/papers/TUPG044.pdf

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