HIGH POWER RF GENERATION FROM A W-BAND CORRUGATEDSTRUCTURE EXCITED BY A TRAIN OF ELECTRON BUNCHES

D. Wang∗, C. TangTsinghua University, Beijing, 100084, China

M. Conde, D. Doran, W. Gai, G. Ha, W. Liu, J. Power, E. WisniewskiArgonne National Laboratory, Lemont, Illinois, 60439, USA

S. Antipov, C. Jing, J. QiuEuclid TechLabs, LLC, Solon, Ohio, 44139, USA

V. DolgashevSLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA

AbstractWe report on the generation of multi-megawatt peak RF

power at 91GHz, using an ultrarelativistic electron bunchtrain to excite electromagnetic fields in a high-impedancemetallic corrugated structure. This device can be used as apower source for high gradient acceleration of electrons. Toachieve precise control of the wakefield phase, a long rangewakefield interferometry method was developed in which theRF energy due to the interference of the wakefields from twobunches was measured as a function of the bunch separation.

INTRODUCTIONHigh power RF sources with frequencies beyond mi-

crowave regime has many applications in particle accelera-tors, communications, radar, etc. One approach to realizehigh power RF of high frequency is based on the beam-driven RF generation. Beam-driven devices use wakefieldsfrom a high charge drive bunch (or bunch train) to generateRF pulses with stable phase and amplitude necessary to ac-celerate a witness beam in both the collinear accelerationand the two beam acceleration schemes [1]. The individualRF pulse generated by a single bunch are due to the interac-tion of the structure’s EM modes with the EM fields of thebunch and the total RF pulse generated by the bunch train isthe superposition of the individual RF pulses.

NUMERICAL STUDYWake Function of the TM01 ModeThe sketch of the high-impedance W-band corrugated

structure is show in Fig. 1(a). The W-band radiator consistsof two identical copper plates, one side of the plate wasmilled out to form a series of grooves, or half capsule cells,which form periodic resonant cavities when the correspond-ing sides of the two plates face each other. Similar to theconventional disc-loaded Traveling Wave (TW) acceleratingstructure, the fundamental mode of structure is TM01 mode.It is designed to have 120 degree phase advance per cell. Theaperture between the two plates 2a = 0.94mm when the fre-quency of the TM01 mode f0 = 91GHz; the period lengthin Z direction d = 1.1mm; the half length of groove in the∗ wangdan11@mails.tsinghua.edu.cn

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Figure 1: (a) sketch of the W-band structure, (b) wake func-tion and frequency spectrum of the TM01 mode.

Y direction ymax = 1.25mm; and the depth of the groovexmax = 0.9mm. Besides, the group velocity vg = 0.105c,R/Q = 83.3 kΩ/m, structure attenuation α = 3.54/m, andloss factor per unit length κ = 13.3MV/m/nC. The totallength of the grooved structure Ls = 123mm.

The wake function is used to describe the integrated effecton the witness particle from the wakefield excited by a driveparticle along the whole structure [2]. The wake function ofthe TM01 mode in the W-band structure is:

w‖ (s) = −w0 · H (s)(1 − s/lw )(s ≤ lw ) · cos(ks) (1)

where w0 = 2κLLse−αs

lw /Ls is the amplitude of wakefieldwith the consideration of the structure attenuation, k iswave number, H (s) is the Heaviside step function. Andthe wakefield duration excited by a single particle (bunch)lw = Ls (1/βg − 1) = 3.4 ns.Figure 1(b) shows the comparison of the numerical sim-

ulation of the wakefield function of the W-band structureusing CST [3] and the analysis envelope calculation by Eq. 1.The wake function in the TW structure has a decreasingenvelope versus s, which implies the wakefield travels out ofthe structure in a finite time. The finite duration of the wake-field leads to the RF power saturation after a certain number

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05 Beam Dynamics and Electromagnetic Fields

D10 Beam-beam Effects - Theory, Simulations, Measurements, Code Developments

of bunches in a train. The black dots indicates the separa-tion between sub-bunches in the train is 23.06 cm (1.3GHz),the long range wakefield from the sub-bunches will add upinside the structure.

Long Range Wakefield Interferometry MethodIn order to figure out the phase information of the W-band

RF pulse, we put up the scheme of the long range wakefieldinterferometry method with two drive bunches. Considertwo drive bunch case : each bunch generates a wakefieldpulse as shown in the Fig 2 (a), the wakefield electric fieldgradient ~E add up with phase information. It is just a sim-ple superposition of two vectors of ~E1 = E1 cos(θ1) and~E2 = E2 cos(θ1), where E1, θ1 and E2, θ2 are the wake-field gradient and phase excited by bunch 1 and bunch 2respectively. The RF power P ∝ | ~E |, the expected to-tal power Pt ∝ |E1 |

2 + |E2 |2 + 2E1E2 cos(∆θ), where

∆θ = θ2 − θ1 = k∆Z21. With the wakefield wave num-ber k and the separation between two bunches ∆Z21, theexpression of ∆θ = k∆Z21, which means when scanningthe separation between two bunches, the added RF powervaries as a cosine function which repeating the wave lengthof λ = 2π/k as shown in Fig 2 (b). Thus the wakefieldwaveforms are mapped and the wakefield phase informationare known in this way .

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Figure 2: (a) sketch of the superposition of the long rangewakefield from two drive bunches, (b) wakefield mappingas the total RF energy varies with the relative delay betweentwo drive bunch.

EXPERIMENT MEASUREMENTSet Up at the AWA FacilityThe experiment was performed at the Argonne Wake-

field Acceleration Facility (AWA) at Argonne National Lab-oratory [4]. Figure 3 shows the layout of the experiment.A 65MeV electron bunch train was used to excite the W-band structure. The train from the linac was focused withquadrupoles and its trajectory was adjusted with trim mag-nets to pass it through the center of the W-band structure.The charge of a single bunch in the train was varied from0.1 nC to 3 nC during the experiment. Each bunch had alongitudinal Gaussian distribution and corresponding rmsbunch length of 0.2mm to 0.6mm. Note, the longer bunchlengths at higher charge are due to stronger space chargeeffects. The integrating current transformers (ICT) wereused to record the charge of the drive beam before and afterthe W-band structure respectively to optimize the percentageof charge transmitted.

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Figure 3: Schematic of the experimental setup.

The RF extracted from the accelerating structure was char-acterized by two direct methods and one indirect method.The RF pulse exited the structure through the WR10 hornantenna, passed through a quartz window mounted on thevacuum chamber, and propagated into the detection system.A calibrated photo-acoustic energy meter was used to mea-sure the RF pulse energy and a Michelson interferometerwith a helium-cooled bolometer was used to measure thetime domain characteristics of the RF pulse. In addition tothe direct measurements of the RF pulse, the kinetic energyof the electron bunches was measured with a spectrometer toindirectly characterize the wake since the electron buncheslose energy to generate the wakefield. Taken together, thesemethods were used to characterize the interaction of variousbunch trains with the W-band wakefield structure.

Single Bunch Results

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Figure 4: single bunch experiment results. (a)measured auto-correlation curve, (b) measured frequency spectrum (FFT ofthe data in figure (a)), (c) comparison of the measurementswith simulations on the RF power verses the drive bunchcharge.

For the single bunch experiment, the measured autocorre-lation curve with a interferometer is shown in Fig 4(a), thefundamental frequency ( f0) is 91.3GHz, and the relativelybroad bandwidth BW = 13.1GHz is due to the limit rangeof the interferometer. And the RF pulse energy as a functionof beam charge was also measured in the single bunch exper-iment. The compared to simulation are shown in Fig 4(b).The electromagnetic code CST was used to simulate the RFenergy generated by the beam with RF losses (structure at-tenuation) included. The RF power depends on the electronbunch parameters according to P ∝ Q2

b· F (σz ) [5], where

Qb is the single bunch charge and F (σz ) is the bunch formfactor. To estimate the form factor F (σz ) as a function ofQb , the beam dynamics code ASTRA [6] was used for start

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to end beamline simulations to determine the electron beamparameters in theW-band structure. This showed that F (σz )decreases with Qb due to space charge effects which resultsin the slower power growth with charge than would occur ifF (σz ) were constant.

Precise Mapping of the Wakefield

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Figure 5: (a) Measured RF energy vs. the relative delay ofthe tailing bunch. Labels 1O ∼ 4O correspond to differentrelative wakefield phases when the leading bunch and thetrailing bunch add. (b) Measured energy distribution of thetwo bunches, in case 1O, the trailing bunch was deceleratedby the wakefield from the leading bunch; in case 3O, thetrailing bunch was accelerated.

The energy will vary between minimum and maximumas the delay of trailing bunch is scanned (Fig. 5(a)) withthe cycle repeating at the RF wavelength, 3.3mm. In case1O, the individual wakes add constructively, generating themaximum RF energy. In case 3O however, the wake of thetrailing bunch interferes destructively, so the output RF en-ergy is minimal, which does not drop completely to zerosince the RF pulses don’t fully overlap in time due to bunchseparation.

The energy change of the trailing bunch also depends onthe bunch separation (Fig. 5(b)). (Note, the leading bunchloses the same energy regardless of the position of the trail-ing bunch since its wake doesn’t affect the leading bunch.)In case 1O, the trailing bunch gets decelerated by the wakefrom the leading bunch. It losses more energy than the lead-ing bunch and contributes to the maximum RF amplitude.In case 3O however, the trailing bunch rides on the accel-eration phase so that it gains energy from the wake of theleading bunch. Energy measurements of the two bunchesare consistent with the RF energy descriptions in cases 1Oand 3O.

High Power with Bunch TrainFor higher RF output power, three drive bunches are ap-

plied in the experiment, proper bunch spacing was set withthe newly developed two-beam (wakefield) interferometrymethod described above. Three bunches with different en-ergy losses are clearly visible as shown in Fig. 6(a). The

blue dashed line gives the simulated energy spectrum of 3bunches, each of 2 nC effective charge, passing through theW-band structure. These agree well with the experimentalobservation on the spectrometer. The average energy lossof bunch 1 through 3 was 1.3MeV, 3.2MeV, 4.4MeV re-spectively, indicating coherent superposition of wakefieldsfor maximum radiated power. Given the energy meter mea-surement of 18mJ per pulse, we calculate the maximumpeak power of 4.8MW (in agreement with the simulationof a 3 × 2 nC bunch train) and peak accelerating gradient of85MV/m.

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Figure 6: (a) beam energy distributions with 3 bunches of 2nC drive beam after the W-band structure, (b) derived outputRF pulse power and the wakefield gradient.

SUMMARYIn summary, a W-band wakefield corrugated structure

driven by a bunch train was experimentally characterized. Atwo-beam wakefield interferometry measurement was devel-oped as a high sensitivity technique for wakefield character-ization and precise RF phase control in wakefield accelera-tion. A 5MW RF pulse was generated corresponding to anaccelerating gradient of 85MV/m.

ACKNOWLEDGEMENTSThe authors would like to thank C. Whiteford, T. Bense-

man and Y. Hao from ANL, Piot from FNAL and Zhe Zhangfrom Tsinghua University. Euclid Techlabs acknowledgessupport from DOE SBIR Grant DE-SC0009571. The Ar-gonne Wakefield Accelerator Facility is supported by DOEcontract number W-31-109-ENG-38.

REFERENCES[1] Hill, Marc E et al., “High-gradient millimeter-wave accelerator

on a planar dielectric substrate”, Phys. Rev. Lett, vol. 87, no. 9,p. 094801.PRL, 2001, 87(9), 094801.

[2] D. Wang et al., “Effect on beam dynamics from wakefields intravelling wave structure excited by bunch train”. in Proc. ofIPAC’15, Richmond, VA, 2015, pp. 289–292.

[3] CST, https://www.cst.com/Products/CSTS2

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[4] M.E. Conde et al., “Commissioning and recent experimentalresults at the argonne wakefield accelerator facility (AWA)”,in Proc. of IPAC’15, Richmond, VA, 2015, pp. 2472–4.

[5] Gao, F et al., “Design and testing of a 7.8 GHz power extractorusing a cylindrical dielectric-loaded waveguide”, Phys. Rev.

ST Accel. Beams, vol. 11, no. 4, p. 041301.[6] ASTRA Manual, http://www.desy.de/~mpyflo/ASTRA_

dokumentation/

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