Bunch length is of prime importance to beam driven plasma wakefield acceleration experiments due to its inverse relationship to the amplitude of the accelerating wake. We present here a summary of work done by the E167 collaboration measuring the SLAC ultra-short bunches via autocorrelation of coherent transition radiation. We have studied material transmission properties and improved our autocorrelation traces using materials with better spectral characteristics

Auerbach, D.

Berry, M.

Blumenfeld, I.

Clayton, C.E.

Decker, F.J.

Hogan, M.J.

Huang, Cheng-Kun

Ischebeck, R.

Iverson, R.

Johnson, D.

Joshi, C.

Katsouleas, T.

Kirby, N.

Lu, Wei

Marsh, K.A.

Mori, W.B.

Muggli, P.

Oz, E.

Siemann, R.H.

U., /SLAC /UCLA /Southern California

Walz, D.

Zacherl, W.

Clayton, C. E.

Decker, F. J.

Hogan, M. J.

Marsh, K. A.

Mori, W. B.

Siemann, R. H.

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515Electron Bunch Length Measurements in the E-167 Plasma Wakefield ExperimentIan Blumenfeld1, David Auerbach2, Melissa Berry1, Christopher E.Clayton2, Franz-Josef Decker1, Mark J. Hogan1, Chengkun Huang2,Rasmus Ischebeck1, Richard Iverson1, Devon Johnson2, ChandrashekharJoshi2, Thomas Katsouleas3, Neil Kirby1, Wei Lu2, Kenneth A. Marsh2,Warren B. Mori2, Patric Muggli3, Erdem Oz3, Robert H. Siemann1, DieterWalz1, Walter Zacherl1, and Miaomiao Zhou21Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, CA 94025, USA2University of California at Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA3University of Southern California,University Park Campus, Los Angeles, CA 90089, USAAbstract. Bunch length is of prime importance to beam driven plasma wakefield accelerationexperiments due to its inverse relationship to the amplitude of the accelerating wake. We presenthere a summary of work done by the E167 collaboration measuring the SLAC ultra-shortbunches via autocorrelation of coherent transition radiation. We have studied materialtransmission properties and improved our autocorrelation traces using materials with betterspectral characteristics.Keywords: plasma wakefield, CTR, coherent transition radiation, bunch lengthPACS: 52.40.Mj,  41.85.Ew,  42.25.Hz, 42.25.Kb,  07.60.LyINTRODUCTIONBeam driven plasma wakefield experiments have a unique need for ultrashortbunches due to the inverse square relationship between bunch length (σz) and thewake amplitude (E), where N is the number of electrons in the bunch€ E ∝ Nσ z2 (1).Although this relationship can be analytically derived only in the case of linearplasma behavior, simulations show that the scaling holds even in the non-linear regime[1,2]. Thus it is important in plasma wakefield experiments to accurately measure thebunch length. In the E-167 plasma wakefield acceleration experiment, the electronbeam from the Stanford Linear Accelerator Center (SLAC) linac is predicted to havean RMS bunch length of 10-25 microns, in the 30-80 fs range. A diagnosticappropriate to measure bunch lengths in this range is therefore necessary [3].SLAC-PUB-12419Contributed to 12th Advanced Accelerator Concepts Workshop (AAC 2006), 07/10/2006--7/15/2006, Lake Geneva, WisconsinThere are many different methods now available. Streak cameras currently do nothave the necessary resolution. Electro-optic measurements, while having theadvantage of being single shot, are difficult to perform in the SLAC environment,where jitter in the linac can foil synchronization with the required external laser.Transverse deflection cavities are parasitic, and thus cannot be run simultaneouslywith a measurement of the plasma response. Currently the E167 collaboration usespyro-electric detectors to measure the total coherent transition radiation (CTR) energy,which is correlated to the inverse of the bunch length on a shot to shot basis. At thesame time the incoming energy spectrum of the bunch is measured and can bematched to the output of the longitudinal phase space simulation code LiTrack to yieldthe expected current profile of the bunch [4]. However, a direct measurement of thebunch length is desired.Towards this goal we have been concentrating on the use of interferometry with theCTR mentioned above. When the bunch passes from one dielectric medium to anotherit emits transition radiation which is coherent for wavelengths longer than the bunchlength. The spectrum of radiation has the following form:  € Itot (λ) = Iinc(λ)•[1+ (N −1)• f (λ)]f (λ) = e2πizλ∫ S(r r )dz2where dr r S(r r ) =1∫(2).Here Iinc is the incoherent spectrum of N electrons, f is the form factor as definedabove and S is the normalized number density of electrons in the bunch [5].In the long wavelength range (λ>σζ) this spectrum is dominated by the second termin the rhs of Eq. 2. This shows that the CTR spectrum is related to the power spectrumof the bunch profile, and can therefore yield information about the bunch [5]. Then theshape of this Fourier spectrum can be indirectly measured using autocorrelation,which can be achieved through interferometry. We can then estimate the bunch length(σz) from the width of the resulting autocorrelation trace (σA) for a Gaussian bunchprofile as:€ σ Z =12 σA (3).EXPERIMENTAL SETUP AND RESULTSThe CTR energy from a conducting foil is peaked at θ=1/γ from the longitudinalaxis, giving two distinct lobes [6]. However, this translates into a sharp peak ateffectively θ=0 for 28.5 and 42 GeV, meaning the lobes cannot be split. Thus theMichelson Interferometer was chosen for the autocorrelation as shown in Figure 1.First the electron beam was passed through a 1 micron thick titanium foil, causing itto radiate transition radiation. The CTR was then collected by an off axis parabaloidmirror (OAP) and reflected into the scanning interferometer. The interferometerconsisted of two arms, one stationary to serve as the reference arm and the other on atranslation stage to serve as the delay arm. The radiation was split at the beam splitter,traveled through both arms, then was recombined and focused by another OAP onto apyro-electric detector on the other side.FIGURE 1 .   A Schematic of the experimental setup. Note that the reference detector, used formonitoring pulse length jitter (via total CTR energy measurement) in the beam was set upapproximately 1m upstream of the rest of the apparatus. This auxiliary measurement allowed us toselect similar events for analysis.It was realized that the materials chosen for the interferometer parts played animportant role in the resulting shape of the autocorrelation trace. Originally the beamsplitter was a 12.5 micron thick mylar sheet and the vacuum window either another12.5 micron thick mylar sheet or 1.5mm thick TPX disc. The mirrors had a firstsurface of uncoated gold and the detectors, both interference and reference, were pyro-electric. The measured traces showed dips indicative of long wavelength attenuation[3]. This problem was traced to the choice of mylar as a beam splitter and vacuumwindow. Material studies aimed at finding a more suitable candidate with betterspectral transmission characteristics resulted in pure silicon being chosen as areplacement (see Fig. 2) [7].FIGURE 2.  A comparison between the spectral response curves for mylar and silicon shows that,while both have similar issues at short wavelength with dips and humps, silicon is better suited for usein the interferometer due to its generally flatter response. It is also mechanically suitable for a vacuumwindow, and given its approximately 50% transmission, ideal for a beamsplitter at long wavelength.The current setup uses a 1.5mm thick silicon vaccum window and a 300 micronthick silicon beam splitter. The mirrors and OAP all had a first surface of uncoatedgold and the detectors were still pyro-electric.FIGURE 3.  The silicon trace has clearly improved over the mylar one. The dips are no longer present,and wings on either side of the main peak, which are indicative of a head or tail on the bunch can nowbe seen. Structures are visible at large delay on both traces, indicating either poor response of thedetector and/or absorption in air that must still be resolved.Figure 3 shows a comparison between the mylar optics and those made fromsilicon. The large dips present in the trace using mylar optics are no longer presentwhen silicon is used. Additionally, features indicative of a head or tail on the beam arealso observed (see Fig. 3).FIGURE 4. Autocorrelation traces for two settings of the magnetic bunch compression. The expectedlonger bunch, with R56=1.4mm, yielded an RMS trace width of 14.4 micron and the shorter one, withR56=3.2mm yielded an RMS trace width of 8.7 micron.Furthermore, we find that the trace widths scale with the R56 of the beamline (seeFig. 4). With R56=1.4mm, the RMS trace width was 14.4 micron, yielding a gaussianbunch length of 20.36 micron (as every micron in our scan is 2 micron of path length).With the R56=3.2mm, the RMS trace width dropped to 8.7 micron, yielding a gaussianbunch length of 12.3 micron. Note that the longer bunch trace appears to have someasymmetry. This is due to the fact that the interferometer was aligned to the shortbunch and the R56 change steered the beam slightly on the foil. This effect wasmitigated by the presence of the OAP, but was not totally eliminated.These results match our theoretical expectations. We expect bunches on the orderof 10-25 microns, which is what we measured. Also, the simulation code LiTrackpredicts that, for our operating conditions, we should see a head or tail on the bunch,evidence of which is present in the traces.CONCLUSION AND FUTURE WORKWe have shown that, using interferometry and educated choices in our window andbeamsplitter materials, we can measure bunches of 10-25 micron which we expect tobe produced by the SLAC linac. There are, however, still some experimental issuesthat need to be corrected. We must be sure that we are using the best materials and thatwe fully understand our detector response. To that end, spectral studies of thematerials and detectors are still ongoing. We will also attempt to improve themeasurement on the current setup using a gas purged environment.Further, the current scanning setup is of limited use in the experiment as a wholedue to the fact that it is not single shot. The jitter in the SLAC linac is enough to causesignificant variation in the plasma response, and so single shot capability is at apremium. When a suitable THz camera is found, we will begin construction of a singleshot autocorrelator to meet this goal.ACKNOWLEDGMENTSWork supported by: Department of Energy contracts DE-AC02-76SF00515 (SLAC),DE- FG03- 92ER40745, DE-FC02-01ER41179. DE-FG03-92ER40727, DE-FG02-03ER54721, DE-F52- 03NA00065:A004 , DE-AC-0376SF0098, and National ScienceFoundation grants No. ECS-9632735, NSF-Phy-0321345.We would also like to thank Zhao Hao and Michael Martin of LBNL for their help incompleting our spectral measurements.REFERENCES1. S. Lee, T. Katsouleas, R. Hemker, and W. B. Mori , Phys. Rev. E 61, 7014-7021 (2000).2. Joshi, C., et al, Physics of Plasmas 9, 1845 (2002).3. Muggli, P., et al., “Coherent Transition Radiation to Measure the SLAC Electron Bunch Length” inProceedings of the 2005 Particle Accelerator Conference, edited by C. Horak, IEEE, New York,2005, pp. 4102-4104.4. Hogan, M.J., et al, Phys. Rev. Letters 95, 054802 (2005).5. Blum, E.B., Happek, U., and Sievers, A.J., Nuclear Instrumentation and Methods A307, 568-576(1991).6. Ginzburg, V.L., and Tsytovich, V.N., Transition Radiation and Transition Scattering, New York:IOP Publishing Ltd., 1990, pp. 17.7. Ischebeck, R., et al, “Bunch Length Measurements Using Coherent Radiation” in Proceedings of the2005 Particle Accelerator Conference, edited by C. Horak, IEEE, New York, 2005, pp. 4027-4029.

Electron Bunch Length Measurements in the E-167 Plasma Wakefield Experiment

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Electron Bunch Length Measurements in the E-167 Plasma Wakefield Experiment

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