PROGRESS TOWARD A HIGH-TRANSFORMER-RATIODIELECTRIC-WAKEFIELD EXPERIMENT AT FLASH∗

F. Lemery1, P. Piot1,2, C. Behrens3, C. Gerth3, D. Mihalcea1, C. Palmer4,J. Osterhoff3,4, B. Schmidt3, P. Stoltz5

1 Department of Physics and Northern Illinois Center for Accelerator &Detector Development, Northern Illinois University DeKalb, IL 60115, USA

2 Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA3 Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany

4 Universitat Hamburg, 22761 Hamburg, Germany, 5 Tech-X Corporation, Boulder, CO 80303, USA

AbstractDielectric wakefield accelerators offer many advantages

over conventional RF accelerators such as higher accelera-tion gradients and cost effectiveness. In this paper we de-scribe our experimental plans to demonstrate an enhancedtransformer ratio with drive and witness bunches. The ex-periment, pending its approval, is foreseen to be performedat the Free-electron LASer in Hamburg (FLASH) and uti-lizes unique pulse shaping capabilities using the dual-frequency superconducting linac to produce high trans-former ratios (R > 2). The beam-driven accelera-tion mechanism will be based on a cylindrical-symmetricdielectric-lined waveguide (DLW). The experimental setupis described, and start-to-end numerical simulations of theexperiment will be presented.

INTRODUCTIONThe last decade has witnessed an increasing interest to-

ward the development of compact electron accelerators.These accelerators could serve as backbone for compactshort-wavelength light sources that could have a wide rangeof applications. A popular configuration consists of a“drive” electron bunch with suitable parameters propagat-ing through a high-impedance structure or plasma mediumthereby inducing an electromagnetic wake. A following“witness” electron bunch, properly delayed, can be accel-erated by these wakefields [1].

Collinear beam-driven acceleration techniques havedemonstrated accelerating fields in excess of GV/m [2, 3].The fundamental wakefield theorem [4] limits the trans-former ratio – the maximum accelerating field over the de-celerating field experienced by the driving bunch – to 2 forbunches with symmetric current profiles. Tailored buncheswith asymmetric , e.g. linearly-ramped, current profilescan lead to transformer ratio R > 2 [5]. Recently, newmethods to tailor bunches below picosecond timescaleshave been developed [6, 7], illuminating the path towardhigh-frequency (THz) beam-driven acceleration schemeswith enhanced transformer ratios. Most recently, a simple

∗This work was sponsored by the DTRA award HDTRA1-10-1-0051to Northern Illinois University, the German’s Bundesministerium fur Bil-dung und Forschung and by the DOE contract DE-AC02-07CH11359 tothe Fermi research alliance LLC.

technique that uses a radiofrequency (rf) linear accelera-tor (linac) operating at two frequencies was shown to becapable of generating electron bunches with quasi-rampedcurrent profiles [8] which could produce a transformer ratio∼ 6 when combined with dielectric wakefield accelerationusing dielectric-lined waveguides (DLW’s). The shapingtechnique was demonstrated at the Free-electron LASer inHamburg (FLASH) facility and an experiment to demon-strate an enhanced transformer ratio is being planned [9].

EXPERIMENTAL SETUPThe main goal of the experiment being planned at the

FLASH accelerator is to demonstrate acceleration in aDLW with R > 2. In addition, FLASH’s flexible repe-tition rate could also enable the investigation of dynamicaleffects in DLWs. A description of the FLASH facility canbe found in Ref. [10, 11]. In short, the electron bunches aregenerated via photoemission from a cesium telluride pho-tocathode located on the back plate of a 1+1/2 cell normal-conducting rf cavity operating at 1.3 GHz.

Figure 1: Overview of the enhanced-transformer-ratio ex-periment under preparation at FLASH. The nominal beamenergy is ∼ 700 MeV.

The bunch is then accelerated in a 1.3-GHz and 3.9-GHzsuperconducting accelerating modules (respectively ACC1and ACC39) before passing through a bunch compressor(BC1). The ACC39 3rd-harmonic module was installed to

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03 Particle Sources and Alternative Acceleration Techniques

A15 New Acceleration Techniques

nominally correct for nonlinear distortions in the longitu-dinal phase space (LPS) and enhance the the uniformityof the electron bunch. Downstream of BC1, the bunchis accelerated and can be further compressed in BC2. Alast acceleration stage (ACC4/5/6/7) brings the beam to itsfinal energy (maximum of ∼ 1.2 GeV). The beam’s di-rection is then horizontally translated using a dispersion-less section referred to as dogleg beamline (DLB). Nomi-nally, the beam is sent to a string of undulators to producelight (ranging from extreme-ultraviolet to soft X-ray) viathe self-amplified stimulated emission free-electron laser(FEL) process. However, for our experiment, the buncheswill be vertically sheared by a 2.856-GHz transverse de-flecting structure (TDS) operating on the TM110-like modeand horizontally bent by a downstream spectrometer [12];see Fig. 1. Consequently the transverse density measuredon the downstream Cerium-doped Yttrium Aluminum Gar-net (Ce:YAG) scintillating screen is representative of theLPS density distribution. The horizontal and vertical coor-dinates at the Ce:YAG screen are respectively xs � ηδF ,where η � 0.75 m is the horizontal dispersion function,and ys � κzF where κ � 20 is the vertical shearing factorand (zF , δF ) refers to the LPS coordinate upstream of theTDS.

PHOTOINJECTOR GENERATION OFDRIVE AND WITNESS BUNCHES

The produced ramped bunches have been shown to becapable of producing high-peak electric fields with trans-former ratios significantly higher than 2 using a dielectric-loaded waveguide (DLW) structure [9]. To demonstratethe capability of the generated ramped bunch to supportan enhanced transformer ratio, the wakefield would haveto be driven by a high-charge (drive) bunch followed by alow-charge (witness) bunch. Ideally the drive bunch wouldonly lose energy while the witness bunch would be delayedto only sample the accelerating portion of the wakefield.The latter would also imply that the witness bunch lengthshould be shorter than the wavelengths of the excited wake-field modes. Instead, for the first round of experiments weplan on having a longer witness bunch thereby samplingboth the accelerating and decelerating field. This choicestems from the lack of flexibility in the witness-bunch gen-eration: producing two bunches with controllable spacingand individual duration at the DLW experiment location ischallenging in the current FLASH photoinjector configura-tion. The witness bunch will be generated by temporallysplitting the photocathode uv laser pulse with a birefrin-gent α-BBO crystal [13, 14]; see Fig. 2 (a). Such a methodwould provide two identical pulses with temporal separa-tions given by δτ = GL where L is the crystal thicknessand G � −0.96 ps/mm is the group velocity mismatch. A21-mm thick crystal would therefore provide a δτ � 20 pswhich is longer than the laser rms duration σt = 6 psand allows for a clear separation between the two elec-tron bunches according to simulation performed with AS-

Figure 2: Photocathode drive laser manipulation schemeto produce the drive-witness bunch set (a) and correspond-ing simulated electron beam LPS 0.5-m downstream of thephotocathode using the FLASH photoinjector setup (b).

TRA [15]; see Fig. 2 (b). In addition, the ratio of the drive-and witness-pulse intensities could be controlled by vary-ing the angle between the optical axis of the crystal and theincoming laser polarization. Therefore such a simple mod-ification of the laser system would therefore provide a drivebunch followed by a low-charge population that would beenergy-modulated by the drive-bunch’s wakefields.

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Figure 3: Longitudinal phase spaces (false color images)and associated current and energy profiles (white traces)downstream of the first stage compression (a) and at thelocation of the planned DWFA experiment (b).

Simulations with ASTRA and a 1D-1V model were per-formed to explore whether a shaped drive bunch followedby a witness bunch could be achieved at the anticipatedlocation of the DLWA experiment. The resulting LPS pro-vides the needed shape as shown in Fig. 3. In the sim-ulations, the drive and witness bunches have a respectivecharge of 500 and 50 pC. The 500-pC drive-bunch charge isconsistent with our previous shaping experiment [8] whilethe 50-pC witness bunch charge is large enough to be ob-served on a Ce:YAG screen such as the one used in thesingle-shot LPS diagnostics.

Proceedings of IPAC2012, New Orleans, Louisiana, USA WEEPPB001

03 Particle Sources and Alternative Acceleration Techniques

A15 New Acceleration Techniques

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SIMULATION OF THEEXPERIMENTAL SETUP

To understand the effects of the DLW on the buncheswith drive-witness distributions shown in Fig. 3 (b), weused VORPAL [16] to simulate the propagation of the bunchthrough the DLW. The three-dimensional particle-in-cellprogram VORPAL uses the finite difference-time domain(FDTD) method to solve Maxwell’s equations and includesan advanced technique known as cut-cell boundaries to al-low accurate representation of curved geometries within arectangular grid.

An example of the simulated LPS downstream of theDLW is compared to the initial LPS in Fig. 4. In all of oursimulations, a DLW diamond structure (with relative per-mittivity εr = 5.7) is considered. The initial and final LPSare fitted using a spline, and the energy modulation due tothe DLW is inferred using a difference technique. The typ-ical induced modulation with peak-to-peak amplitude is inexcess of ∼ 1 MeV which should be easily resolved usingthe time-resolved single-shot LPS described in the previoussection; see Fig. 5.

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Figure 4: Longitudinal phase spaces upstream (blue) anddownstream (red) the DLW structure with inner and outerradius of respectively a = 180 and b = 240 μm.

HARDWARE STATUSThe experimental setup will consist of three DLW struc-

tures with different dielectric layer thicknesses that will beremotely insertable into the beam. Besides being mountedto a bi-axis translational stage for transverse alignment, tworotational stages will also enable the precise angular align-ment of the structure. Currently, the exact location of theexperiment remains to be specified. One option is to lo-cate the DWFA experimental station downstream of thelast accelerating module (ACC7). The close proximity ofour experiment to the superconducing cavities puts a strin-gent requirement on the vacuum quality. Therefore the fi-nal choice for an appropriate alignment stage will dependon the outcome of vacuum tests aimed at evaluating the re-

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Figure 5: Reconstructed time-dependent energy loss alongthe bunch’s longitudinal ordinate ζ for three DLW with in-ner and outer radii (in μm) shown in the legend. The shadedarea indicate a region where the energy loss is not accu-rately reconstructed due to the large LPS slope (see Fig. 4).

quired and sustainable capabilities of these stages in ultra-high vacuum. A Ce:YAG screen upstream of the structurealong with other available diagnostics at FLASH (beamposition monitor, beam-loss detectors) will enable precisecentering and focusing of the electron beam through theDLW.

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Field”, DESY report M-82-10, April 1982; W. Bialowons etal., Proc. EPAC 1988, 902 (1988).

[2] I. Blumenfeld et al.., Nature 445 741(2007) .

[3] M.C. Thompson et al., Phys. Rev. Lett., 100 21(2008) .

[4] R. D. Ruth et al., Part. Accel. 17 171(1985) .

[5] K. L. F. Bane et al., IEEE Transactions on Nuclear Science,NS-32 5(1985) .

[6] R. J. England et al., Phys. Rev. Lett. 100, 214802 (2008).

[7] P. Piot et al., Phys. Rev. ST AB 14, 022801 (2011).

[8] P. Piot et al., Phys. Rev. Lett. 108 (2012) 034801.

[9] F. Lemery et al., Proc. IPAC11, 2781 (2011).

[10] W. Ackermann et al., Nature Photonics 1 (2007) 336.

[11] P. Piot et al., Proc. IPAC11, 2805 (2011).

[12] C. Behrens and C. Gerth, Proc. FEL2010, 133 (2010).

[13] S. Zhou et al., Appl. Opt. 46 (2007) 8488.

[14] J. G. Power and C. Jing, AIP Conf. Proc. 1086, 689 (2009).

[15] K. Flottmann ASTRA: A space charge algo-rithm, User’s Manual, (unpublished); available athttp://www.desy.de/∼mpyflo/AstraDokumentation

[16] C. Nieter, J. R. Cary, J. Comp. Phys. 169 448(2008) .

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03 Particle Sources and Alternative Acceleration Techniques

A15 New Acceleration Techniques