MICROBUNCHING OF THERMIONIC CATHODE RF GUN BEAMSIN THE ADVANCED PHOTON SOURCE S-BAND LINAC

TUP027
J. Dooling, A. Brill, I. Lobach, N. Kuklev, N. Sereno, Y. Sun, A. Lumpkin
Argonne National Laboratory, Lemont, IL, USA

ABSTRACTMOTIVATION

We report on measurements of beams fromthermionic cathode (TC) rf guns in the Advanced
Photon Source S-Band Linac. These
measurements include the macropulse out of both
new and existing TC guns as well as theobservation of microbunching within the micropulsesof these beams. A gun chopper limits themacropulse FWHM duration to the 10-ns range.
Our objectives were to analyze the new TC gun andinvestigate microbunching within a TC-rf-gungenerated 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 eachcompression stage to provide the autocorrelations of
the sub-ps micropulse durations. The firstcompression stage is an alpha magnet and thesecond a chicane. A CCD camera was used toimage the beam via optical transition radiation froman 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.

 The injection system of the Advanced Photon Source(APS) has relied on thermionic cathode (TC) rf guns
as electron-beam sources since 2001 [1].

BACKGROUND• An AET gun was employed for the bunch compression experimentdiscussed here• Micropulse duration after the alpha magnet is sub-ps• After further compression in the chicane, longitudinal-space-chargeinduced (LSC) microbunching within the micropulses generates
coherent optical transition radiation (COTR) when striking a downstream
intercepting Al screen.
• COTR (visible wavelengths, fs temporal scale) was transported outof the tunnel to a CCD camera, and the images were recorded.
• We present a comparison of the observed COTR enhancementswith the predictions of a LSC-impedance model.

 This will continue to be the case for the APS Upgradewith the multi-bend achromatic magnet lattice
installation in the 6-GeV storage ring currently inprogress [2].
 We report the basic testing of the beams from newgeneration TC rf guns manufactured by RadiaBeam
Technologies (RBT) [3] in terms of a macropulse
composed of S-Band micropulses (2856 MHz).
 Comparison RBT gun with a TC rf gun purchasedfrom AET Associates (AET) in 2001. AET guns havebeen the primary electron source for the APS sincethat time.
 Our interest was to measure the charge of eachmicropulse within the macropulse structure from thechopper-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 the macropulse temporal distribution, we canthen determine the charge per micropulse.

EXPERIMENT S-BAND BPMS [9-11]

Linac Schematic:

MACROPULSE AND MICROPULSEMICROBUNCHING [14]Macropulse envelope waveforms with varying chopper voltages
Pick-up
Schematic:

AETgun,RG2
190 mA

AETgun,RG2300 mA

S5 OTR
Off max. compression phase L2

beam

Scope display
4-GHz, RBTgun, RG1

AETgun,RG2

RBTgun,RG1

FIR w/phasedrift

Autocorrelations of the FIR coherent transition radiation [8,13]

L2CTR
AET

L2CTR
AET

L3CTR
AET

L3CTR
AET

S5 OTR
Maximum compression phase L2

Under-sampled [12] and peak detected waveforms

ANALYSIS
Saldin [15] Eq. 26 with E=150 MeV and R56=65 mm

SUMMARY• We described a method to measure the macropulse waveform of the APS TC rf
guns, requiring an oscilloscope with bandwidth and sampling rate sufficient tomonitor the fundamental frequency of the rf.
• In the present case, this requires a bandwidth>2.5 GHz and sampling rate>20 GS/sfor 2856 MHz S-band rf.
• Measurement of the macropulse envelope allows for calibration of the microbunchcharge and thereby calculation of peak currents after bunch compression.
• AET and RadiaBeam TC guns were tested and, accounting for differences in thechopper systems, behaved in a similar manner.
• We have inferred the presence of LSC-induced microbunching at visible
wavelengths via the COTR mechanism from TC rf gun beams.
• After two stages of compression 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 microbunchingin the TC rf gun-generated micropulses.
• Analytical models indicate a slice energy spread of only a few keV is required formicrobunching (COTR gain) at visible wavelengths where the CCD camera issensitive. The importance of slice energy spread as it applies to a TC-rf gun beam isanalogous to what was reported for PC gun [16] and LPWA [17] beams.
• Further investigations including spectral measurements of the microbunching
phenomena are warranted[18,19].

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https://indico.jacow.org/event/44/contributions/347/editing/paper/685/2263/THAI2.pdf

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