Dielectric wakefield accelerators have shown great promise toward high-gradient acceleration. We investigate the performances of a possible experiment under consideration at the FLASH facility in DESY to explore wakefield acceleration with an enhanced transformer ratio. The experiment capitalizes on a unique pulse shaping capability recently demonstrated at this facility. In addition, the facility incorporates a superconducting linear accelerator that could generate bunch trains with closely spaced bunches thereby opening the exploration of potential dynamical effects in dielectric wakefield accelerators

/DESY

/Fermilab

/Hamburg U., Inst. Theor. Phys. II

Behrens, C.

Elsen, E.

Flottmann, K.

Gerth, C.

Kube, G.

Lemery, F.

Mihalcea, D.

Osterhoff, J.

Piot, P.

Schmidt, B.

Stoltz, P.

U., /Northern Illinois

UNT Digital Library

EXPERIMENTAL PLANS TO EXPLORE DIELECTRIC WAKEFIELDACCELERATION IN THE THZ REGIME∗F. Lemery1, C. Behrens3, E. Elsen3, K. Flöttmann3, C. Gerth3, G. Kube3, D. Mihalcea1,J. Osterhoff3,4, P. Piot1,2, B. Schmidt3, P. Stoltz51 Department of Physics and Northern Illinois Center for Accelerator &Detector Development(NICADD), Northern Illinois University DeKalb, IL 60115, USA2 Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA3 Deutsches Elektronen-Synchrotron DESY, Notkestraße 85 D-22607 Hamburg, Germany4 Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany5 Tech-X Corporation, Boulder, CO 80303, USAAbstractDielectric wakefield accelerators have shown greatpromise toward high-gradient acceleration. We investigatethe performances of a possible experiment under consider-ation at the FLASH facility in DESY to explore wakefieldacceleration with an enhanced transformer ratio. The ex-periment capitalizes on a unique pulse shaping capabilityrecently demonstrated at this facility. In addition, the facil-ity incorporates a superconducting linear accelerator thatcould generate bunch trains with closely spaced bunchesthereby opening the exploration of potential dynamical ef-fects in dielectric wakefield accelerators.INTRODUCTIONThe rapidly developing field of accelerator physics isshowing great promise for applications to many othertechnologies and sciences such as high energy physics,medicine, imaging, material sciences, energy productionand propulsion. Today’s conventional accelerators use ra-diofrequency (rf) cavities which have maximum accelera-tion gradients of∼ 100 MV/m [1]. The emergence of high-gradient accelerating structures such as plasma or dielectricwakefield accelerators [2, 3] opens the door towards morecompact and cost effective accelerators.This paper concentrates on beam-driven wakefield accel-erators that employ dielectric-loaded waveguides (DLW).In such a scheme, a high-charge drive bunch excites elec-tromagnetic wakes in the DLW allowing a properly de-layed witness bunch to experience a strong acceleratingfield. Collinear beam-driven acceleration techniques havedemonstrated accelerating fields in excess of 1 GV/m [2, 4].The fundamental wakefield theorem [5] limits the trans-former ratio (R ≡ E+/E−) – whereE+ is the maximumaccelerating field in the wake, andE−is the maximum de-celerating field experienced by the driving bunch – to 2 forbunches with symmetric current profiles. Tailored buncheswith asymmetric (e.g. a linearly-ramped) current profilescan lead to a transformer ratio> 2 [6].∗This work was sponsored by the DTRA award HDTRA1-10-1-0051to Northern Illinois University, the German’s Bundesministerium für Bil-dung und Forschung and by the DOE contract DE-AC02-07CH11359 tothe Fermi research alliance LLC.In the following, we consider two types of dielectricstructures shown in Fig. 1. The structures consist of a cylin-der (resp. slab) with aperture radius (resp. half gap)a anddielectric layer thicknessδ = b − a. The dielectric layerhas a relative electric permittivityǫ. In this paper, we ex-plore the performances of these two types of structures inthe (a, b, ǫ) parameter space using experimentally gener-ated bunches with ramped current profiles [7].Figure 1: Geometries of interest: (a) An electron bunchpasses through a cylinder with inner radiusa, and outerradiusb. (b) Similarly an electron bunch passes througha slab-symmetric geometry of inner lengtha, and outerlengthb. In both instances, the dielectric material is locatedin the regionδ = b − a, the geometries are surrounded bya conducting sheet, and the region< a is vacuum.SIMULATIONS AND RESULTSWe have developed several numerical models capableof simulating wakefields generated as a bunch of elec-trons passes through a cylindrical or slab structure. Ourmodel is based on a semi-analytical approach developedin Ref. [8] for the cylindrical DLWs and on an extensionof the work reported in Ref. [9] for the slab DLWs [10].The developed models were benchmarked against thethree-dimensional electromagnetic particle-in-cell programVORPAL [11]. VORPAL uses the finite difference-time do-main (FDTD) method to solve Maxwell’s equations andincludes an advanced technique known as cut-cell bound-aries to allow accurate representation of curved geometrieswithin a rectangular grid.FERMILAB-CONF-11-423-APCFigure 2: Wakefields (blue trace) produced by a 500-pC bunch with ramped current distribution (green trace) [left column],scan of the peak accelerating field (E+) [middle column], and scan of the transformer ratio (R) [right column], in the(a, δ) parameter spaces. The top and bottom rows respectively correspond to cylindrical and slab DLWs. The wakefieldsplotted in the left column correspond to(a, δ) = (20, 60) µm.The (a, δ ≡ b − a) parameter space was scanned andthe valuesR andE+ were recorded. The dielectric layermaterial was chosen to be made of diamond (ǫ = 5.7) dueto its exceptional heat coefficient [> 2000 W/(mK)].We present simulation results pertaining to one of theramped current profiles measured during a current shap-ing experiment conducted at the Free-electron LASer inHamburg (FLASH) facility [12]; see details in [7]. Theresulting performances in the(a, δ) parameter space aregathered in Fig. 2 for both a cylindrically-symmetric andslab-symmetric structures. As expected from its shape, thecurrent profile yields aR in excess of∼ 6, with peakcurrents on the order of∼ 1 GV/m. For both types ofstructures theR evolution in the(a, δ) parameter spacedisplays similar features and especially several maxima.Some of these maxima correspond to high electric–fieldamplitudes (for small values ofa), while others are attainedfor more conservative values ofa ∼ 200 − 300 µm withsignificantly weaker (but still attractive) field amplitudes|E+| ∼ 100 MV/m.We also considered the wakefield properties resultingfrom bunches obtained using the typical nonlinear bunchcompression settings at FLASH. These bunches can gener-ate electric fields in excess of 2 GV/m (at the expense ofpoorR values); see an example of produced wakefield inFig. 3.Finally, in an effort to understand the more-intricate de-pendence of the wakefield properties on the dielectric con-stantǫ, we scanned its value for fixeda, b parameters (seeFig. 4). We found thatE+ andR were extremely sensi-tive to changes inǫ and could potentially achieve valuesFigure 3: Example of wakefield (blue trace) produced by a500-pC bunch (green trace) obtained via typical non-linearbunch compression at FLASH. The structure is a cylindri-cal DLW with (a, δ) = (20, 60) µmR > 10. These high values ofR are associated to val-ues ofǫ which are not a priori available in conventionalmaterials. Synthetizing materials with the requiredǫ whilehaving a low tan-loss factor might be very challenging. Itis however worth stressing that the current profile has alsoa significant impact. In particular, we showed that maximaof theR(ǫ) function could be shifted to differentǫ valuesby altering the current profiles. We plan on investigatingsuch a feature closer with the end goal of establishing a setof possible materials. Such a multivariate optimization inthe (a, b, ǫ) parameter space will be performed using a ge-netic algorithm. The overall optimization goals will be tofind compromises between high values ofR and|E+|.Figure 4: Accelerating fieldE+ (top) and transformer ratioR (bottom) as a function ofǫ. The blue and red traces re-spectively correspond to the current profiles shown in Fig. 2and 3. For these calculations,(a, δ) = (20, 60) µm.POSSIBLE EXPERIMENT AT FLASHBased on the simulation results presented in the previ-ous sections we are planing an experiment aimed at mea-suring an enhanced transformer ratio using the rampedbunches produced at the FLASH facility. In such an ex-periment, the dielectric structure would be located down-stream of the last accelerating stage once the beam hasbeen accelerated to∼ 700 MeV. After the beam passesthrough a remotely insertable DLW structure it will be ver-tically sheared by a transverse deflecting structure (TDS)and then horizontally bent with a dipole magnet (spectrom-eter); see Fig. 5. A Cerium-doped Yttrium AluminiumGarnet screen (Ce:YAG) located downstream of the dipolemagnet will provide a single-shot measurement of the lon-gitudinal phase space distribution thereby allowing for atime-resolved measurement of the drive bunch’s energyloss. Given the typical rms fractional momentum spreadmeasured during the ramped bunch experiment reported inRef. [7], we expect that a DLW structure with a conser-vative 2-cm length and its geometric parameter chosen toprovide a conservatively-low peak field ofE ≃ 200 MV/m.This would result in, at least, a 2-fold increase in fractionalmomentum spread to∼ 200 × 0.02/700 ≃ 0.6 × 10−3.Such an increase is well within the resolution of the spec-trometer. In addition, the time-resolved nature of our mea-surement should allow for a sensitivity to even smaller in-crease as details of the correlated energy developing due tothe wakefield will be observable.Finally, the availability of an 800-µm macropulse com-prising of 2400 electron bunches could also enable the in-vestation of dynamical effects in the DLW. Such a capabil-ity is unique to a super-conducting linac.Figure 5: Possible DLW test experimental configuration atthe FLASH facility. The drive and witness bunches lon-gitudinal phase space (LPS) is directly measured at theCe:YAG screen after the beam has been vertically shearedin a transverse deflecting structure (TDS) and horizontallybent in a spectrometer.REFERENCES[1] W. Wuensch, et al., Proceedings of the 2003 Particle Accel-erator Conference (PAC03), Portland, OR, USA, 495 (2003).[2] I. Blumenfeld et al.., Nature445, 741 (2007).[3] G. A. Voss and T. Weiland, “Particle Acceleration by WakeField”, DESY report M-82-10, April 1982; W. Bialowons etal., “Computer Simulations of the wakefield transformer ex-periment at DESY”, Proceedings of the 1988 European Par-ticle Accelerator Conference (EPAC 1988), Rome, Italy, 902(1988).[4] M.C. Thompson,et al., Phys. Rev. Lett.,100, 21 (2008).[5] R. D. Ruth, A. Chao, P. L. Morton, P. B. Wilson, Part. Accel.17, 171 (1985).[6] K. L. F. Bane,et al., IEEE Transactions on Nuclear Science,NS-32, 5 (1985).[7] P. Piot, et al. “Generation and Characterization of Elec-tron Bunches with Ramped Current Profiles in a Dual Fre-quency Superconducting Linear Accelerator”, preprint DESYFLASH-11-002 and Fermilab PUB-2011-366-APC (2011);see also paper WEPZ016, these Proceedings.[8] M. Rosing and W. Gai, Phys. Rev. D.42, 5 (1990).[9] A. Tremaine and J. Rosenzweig, Phys. Rev. E.56, 6 (1997).[10] D. Mihalcea, P. Piot, and P. Stoltz, to be published (2011).[11] C. Nieter, J. R. Cary, J. Comp. Phys.169, 448 (2008).[12] W. Ackermann,et al., Nature Photonics1, 336 (2007).

Experimental Plans to Explore Dielectric Wakefield Acceleration in the THZ Regime

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Experimental Plans to Explore Dielectric Wakefield Acceleration in the THZ Regime

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