Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than ten times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76 +- 0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies

Cary, J. R.

Cormier-Michel, E.

Esarey, E.

Geddes, C. G. R.

Leemans, W. P.

Nakamura, K.

Plateau, G. R.

Schroeder, C. B.

Toth, Cs.

UNT Digital Library

APS/123-QEDPlasma density gradient injection of low absolute momentumspread electron bunchesC.G.R. Geddes,1, ∗ K. Nakamura,1, † G.R. Plateau,1, ‡ Cs. Toth,1 E. Cormier-Michel,1, §E. Esarey,1, § C.B. Schroeder,1 J.R. Cary,2, ¶ and W.P. Leemans1, ∗∗1Lawrence Berkeley National Laboratory,1 Cyclotron Rd, Berkeley CA 947202Tech-X Corporation, 5621 Arapahoe Ave. Ste. A, Boulder CO 80303(Dated: December 22, 2007)AbstractPlasma density gradients in a gas jet were used to control the wake phase velocity and trappingthreshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal andtransverse momentum spreads more than ten times lower than in previous experiments (0.17 and0.02 MeV/c FWHM, respectively) and with central momenta of 0.76 ± 0.02 MeV/c. Transitionradiation measurements combined with simulations indicated that the bunches can be used as awakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread athigh energies.PACS numbers: 52.38.Kd, 41.75.Jv, 29.25.Bx∗Electronic address: cgrgeddes@lbl.gov†Also at University of Tokyo, Japan‡Also at E´cole Polytechnique, France§Also at University of Nevada, Reno¶Also at University of Colorado, Boulder∗∗Also at University of Nevada, Reno, and U.C. Berkeley1Laser wakefield accelerators (LWFAs)[1] sustain gradients of hundreds of GV/m, enablingreduction of accelerating distances by a thousandfold compared to conventional technologies.The laser ponderomotive (radiation) pressure drives a plasma density wave or wake, whoselongitudinal field accelerates electrons [1, 2]. If the wake exceeds a threshold amplitude,electrons can be self-trapped from the bulk plasma and accelerated. Recently, such experi-ments produced bunches with a few MeV/c momentum spread near 100 MeV/c momenta byextending the laser propagation distance using a guiding channel [3] or large spot size [4, 5]in combination with self-guiding [6]. Channeled LWFAs have now also produced buncheswith ∼ 20 MeV/c longitudinal and 2 MeV/c transverse momentum spread at 1 GeV [7].The GeV energies obtained in compact LWFAs are sufficient for applications includingfree electron lasers [8] and THz [9, 10] and X-ray [11] radiation sources, and multiple unitscould be staged for high energy physics applications. However, the 1-20 MeV/c (few %)longitudinal and ∼0.2-2 MeV/c transverse momentum spreads of present experiments needto be reduced, and pulse-to-pulse stability and tunability improved, for these applications. Inself-trapped experiments, beam quality was best when operating at the trapping threshold [4,5, 12], and relatively stable operation [7, 13] was only observed in a narrow parameterwindow, restricting tunability.To improve bunch momentum spread and stablity, several methods have been proposed tocontrol electron injection. The short plasma wake wavelength, λp =√pic2m/e2ne, typically∼ 10−100 µm, requires injection of a femtosecond electron bunch with femtosecond timing.Here ne is the plasma density, m and e the electron mass and charge, and c the speed of light.Experiments [14] demonstrating injection using the colliding laser pulse method [15, 16]showed the usefulness of controlled injection in tuning the electron bunch energy, but so farhave not reduced bunch momentum spread.An alternative method to control trapping has been proposed using plasma density gra-dients with density decreasing in the laser propagation direction, or downramps [17–19]. Ina downramp, λp increases with propagation. This causes the wake fronts behind the laser tofall further behind as the laser propagates, decreasing the wake phase front velocity vφ eventhough the laser group velocity vg is increased due to the decreased density. This reduces thethreshold for trapping by reducing the velocity electrons must achieve, and allows controlover the trapping process by tailoring the gradient.In this Letter, control of particle trapping in an LWFA using plasma density gradients2is demonstrated for the first time, resulting in the production of stable electron bunchessuitable for use as an injector with more than ten-fold lower momentum spread and jitterthan previous experiments. Momentum of the bunches was stable at 0.76 ±0.02 MeV/c,longitudinal (transverse) momentum spread was 0.17 (0.02) MeV/c FWHM, and pointingstability was 2 mrad or 0.002 MeV/c RMS, over hundreds of shots. Bunch charge was on theorder of 0.5 nC. Simulations show that the bunches are indeed produced through modificationof the trapping threshold by the density gradient in the gas jet target. Measurementsfurther show that the laser mode is well transmitted, and with simulations also indicatethat the bunch is ultrafast. This allows coupling of the gradient to a low density plasma topost-accelerate the bunches, and simulations show that such coupling produces high energybunches with greatly reduced momentum spread, as required for applications.In the experiments, a pulse from the LOASIS Ti:Sapphire laser [20] was focused on thedownstream edge of a thin slit gas jet oriented transversely to the beamline (Fig. 1). Thepeak power was 10 TW (0.5 J in 47 fs FWHM), focused to a 7.5 µm FWHM spot. The plasmadensity profile along the laser propagation direction, measured by an interferometer [21], wasapproximately Gaussian with a peak density of 2.2 ±0.3× 1019 cm−3 and a FWHM of 750µm ±100 µm. The laser ionized the hydrogen gas and drove the plasma wake. The wakeis excited strongly within a Rayleigh range ZR ∼ 200 µm of the laser focus [2], allowingeffective selection of the density gradient using the laser focus location. Focusing upstreamproduced an increasing or flat density, published previously [12], while focusing downstreamproduced a decreasing density to control trapping.Electron bunches were characterized using a magnetic spectrometer with a bend angleof 55o. The bunch was dispersed onto a phosphor screen (LANEX) imaged by a CCD,covering a range of ±14% about a central momentum determined by magnet current. Themomentum resolution was±5%. Beam divergence was observed in the out-of-plane direction.An insertable bunch phosphor screen (BPS), imaged by a CCD, measured the spatial profile.Charge was determined by cross-correlating the phosphor signals with an integrating currenttransformer (ICT). Gamma ray detectors monitored Bremsstrahlung radiation from theelectron beam dump, and this was used as a simple on-line identification of high energyelectron production due to the restricted spectrometer acceptance [22]. Electron bunchlength was inferred from THz radiation emitted from the plasma edge [9, 10]. A 10 µmsilver coated nitrocellulose pellicle transmitted the electron bunch and diverted the laser to3a mode imager CCD [21] to measure the laser profile at the plasma exit.Scanning the jet position relative to the laser focus (Fig. 2) demonstrated control oftrapping and acceleration by selecting the density gradient at focus. Focusing upstream op-timized production of high momentum electrons via conventional self-trapped acceleration,evidenced by the peak in the gamma signal [22]. The magnetic spectrum here was expo-nential up to tens of MeV/c (see [12]). The mode imager showed strong self-modulationand filamentation of the laser into multiple spots, also consistent with past results in thisjet [12].Focusing at the downstream edge of the jet produced stable electron bunches with anorder of magnitude lower absolute momentum spread and jitter than previously observed inLWFAs [23]. The charge measured on the ICT increased slightly, but the gamma signal de-creased, indicating lower momentum electrons. Magnetic spectrometer data (Fig. 3) showedbunches with central momentum stable at 0.76 MeV/c ±0.02 MeV/c RMS and momentumspread stable at 0.17 MeV/c FWHM (20 %) ± 0.02 MeV/c RMS over 28 sequential and 45total diagnostic shots. In the undispersed plane, the vertical bunch divergence was measuredto be 20 mrad FWHM ± 1.8 mrad RMS, with 1.5 mrad RMS pointing deviation.Bunch divergence (all momenta) was measured on the BPS (Fig. 4) [24], showing asmall divergence bunch surrounded by a broad background. The narrow feature has adivergence of 26 (14) mrad in the horizontal (vertical) plane with RMS deviations of ±1.8(2.5) mrad over 30 shots. Bunch pointing was stable, with 1.8 (1.2) mrad RMS and 8 (5)mrad peak-to-peak deviations in the horizontal (vertical) plane. Vertical data agree withthe magnetic spectrometer, indicating that this narrow feature is the MeV bunch, while thebroad background is likely lower momentum electrons. The observed divergence indicatestransverse bunch momenta of ∼ 0.02 MeV/c.Correlation of the signal on the magnetic spectrometer and BPS to the ICT was used todetermine the the charge of the bunch at 0.76 MeV/c, giving Qbunch of 0.3 - 1 nC. Chargestability was 40% RMS in this sequence.Accelerator performance was stable over three runs and 7 days; such stability will becrucial for LWFA applications and has not been previously observed. Central momentumwas stable between 0.76 and 0.78 MeV/c, FWHM energy spread between 0.16 and 0.19MeV/c, and divergence between 17 and 23 mrad. Similar bunches were observed on eightrun days, and were stable for at least hundreds of shots at a time (as long as observed).4The experiments were modeled [25] with 2D particle-in-cell simulations (VORPAL [26]),which show that the ramp decreased vφ as described in [17], producing trapping withoutsignificant plasma modulation of the laser (which is unstable), and yielding trapping andacceleration to ∼ 1.5 MeV/c (Fig. 5). Trapping of electrons occurred where the density was∼ 5×1017 cm−3, such that λp was resonant to the laser pulse length. At this density vφ is toohigh in a homogeneous plasma to allow trapping by this laser, emphasizing the ramp’s effect.Absolute momentum spread was < 0.2 MeV/c, as observed in experiments. Consistent withexperimental stability, the simulation parameters were scanned and MeV-class beams wereobserved for plasma lengths 0.5-1 mm, densities 1.8-2.2×1019 cm−3, and laser powers 8-10TW.In the simulations, the low trapping threshold decreased transverse wake fields at trap-ping, yielding low transverse emittance. Transverse momentum was ∼0.05 MeV/c (diver-gence 30 mrad) compared to 0.2-2 MeV/c observed in homogeneous plasmas. The simulatedbunch size of ∼10 µm long by 5 µm diameter, together with the experimental and simulateddivergences, indicated a normalized emittance ²n ∼ 0.2− 0.4 pi mm-mrad. As λp lengthensin the downramp, the bunch expands to ≥ 50 µm in dimension, explaining the absence ofspace-charge blow-up [27] of the experimental bunches. Remaining differences with experi-ments may result from the 2D nature of the simulations or parameter uncertainties.The absolute momentum spread and the transverse momentum inferred from divergenceare 10 to 100-fold below values obtained in homogeneous plasmas, and the measurementsand simulations below indicate that the momentum spread is not significantly degraded inacceleration, so that use of these bunches as an injector may greatly improve LWFA beamquality.Suitability of the bunches as an LWFA injector was evaluated using THz emission from thevacuum-plasma boundary [9, 10] to infer bunch length τb. THz energy was measured using abolometer, and emission in four wavelength bands was measured using Teflon and Fluorogoldfilters to cut the spectrum. Ratios of emission in each band were used to infer τb, assumingGaussian bunch profilies. Fluorogold measurements were consistent with τb ∼ 210 ± 100fs, and Teflon with τb ∼ 300 + 600,−200 fs, agreeing within error bars (Fluorogold has asharper cutoff, giving better precision) [28]. Simulations indicated that the bunch expandsto order 200 fs as it propagates to the THz emission surface, consistent with this data, andshow that the bunch length at production is ≤ 30 fs, allowing use as an LWFA injector.5Acceleration of the bunches to high energy will be facilitated by high laser transmission,which allows the transmitted laser to drive a wake in a subsequent plasma. Mode imager data(Fig. 4B) showed that the transmitted laser mode was similar to the vacuum mode, with nofilamentation and with laser intensity and energy transmission of more than 70% when usinga prepulse [21] to preionize the gas jet. Simulations showed similar high transmission. Stableelectron bunches were measured with laser prepulse levels below the hydrogen ionizationthreshold and at levels suffcient to create a fully ionized preformed plasma, but not withintermediate levels, likely because a small preplasma can diffract the laser pulse [29, 30],preventing it from reaching focus.The simulations showed that the bunch can be accelerated to high energy while preservingits low momentum spread by ending the downramp in a plasma channel with uniform axialdensity immediately after the particles are trapped. The transmitted laser pulse then excitesa wake in the channel that accelerates the beam to high energy. The wake is effectivelyexcited because of the high laser transmission through the downramp, and the bunch isefficiently trapped because it is shorter than a plasma period. These simulations have so farshown acceleration to beyond 20 MeV/c (limited by computational time) with 0.18 MeV/clongitudinal and 0.15 MeV/c transverse momentum spread, corresponding to < 1% energyspread and < 1 mrad divergence. Other simulations predicted that such post-accelerationnearly preserves absolute momentum spread and normalized emittance [31, 32] to highenergies, which could enable much lower momentum spread and emittance than previousLWFA experiments, potentially with 0.2 MeV/c class momentum spread and low transverseemittance at GeV and greater energies.In conclusion, experiments demonstrated a plasma gradient based technique to controlinjection in LWFAs, producing bunches with 10 to 100 fold lower momentum spread andvariation than previous laser accelerators. The bunches displayed central momentum stabil-ity of 0.76±0.02 MeV/c, momentum spread in the longitudinal (transverse) direction of 0.17(0.02) MeV/c FWHM, and pointing stability of 2 mrad or 0.002 MeV/c RMS. Normalizedbunch emittance was inferred to be on the order of 0.2−0.4pi mm-mrad, approximately ten-fold better than previous LWFAs. THz measurements and simulations indicated the bunchesare short enough (≤ 30 fs at production) for use as an LWFA injector. Such electron bunchesmay directly benefit applications such as ultrafast electron diffraction. Simulations of cou-pling these bunches to subsequent LWFAs showed that their low absolute momentum spread6was preserved as the bunch accelerated, resulting in high energy beams with 0.2 MeV/c mo-mentum spread and low emittance. This may allow bunches at GeV energies and beyondwith < 0.1% energy spread. Together with the observed stability over many run days, theseproperties will be important to collider or 4th generation light source applications of LWFAs.Work supported by the U.S. Dept. of Energy contracts DE-AC02-05CH11231, DE-FG03-95ER40926, DE-FG02-01ER41178, DE-FG02-03ER83857, DOE SciDAC, INCITE,and NERSC programs, NSF 0113907 and 0614001, and DARPA. We appreciate contri-butions from D. Bruhwiler, J. van Tilborg, D. Panasenko, V. Leurent, S.A. Gaillard, D.Syversrud, J. Wallig, and N. Ybarrolaza.[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979).[2] E. Esarey et al., IEEE Trans. Plasma Sci. 24, 252 (1996).[3] C. G. R. Geddes et al., Nature 431, 538 (2004).[4] S. P. D. Mangles et al., Nature 431, 535 (2004).[5] J. Faure et al., Nature 431, 541 (2004).[6] A. G. R. Thomas et al., Phys. Rev. Lett. 98, 095004 (2007).[7] W. P. Leemans et al., Nature Physics 2, 696 (2006).[8] D. Jaroszynski et al., Phil. Trans. Royal Soc. A 364, 689 (2006).[9] J. van Tilborg et al., Phys. Rev. Lett. 96, 014801 (2006).[10] W. P. Leemans et al., Phys. Rev. Lett. 91, 074802 (2003).[11] A. Rousse et al., Phys. Rev. Lett. 93, 135005 (2004).[12] C. G. R. Geddes et al., Phys. Plasmas 12, 056709 (2005).[13] C.-T. Hsieh et al., Phys. Rev. Lett. 96, 095001 (2006).[14] J. Faure et al., Nature 444, 737 (2006).[15] E. Esarey et al., Phys. Rev. Lett. 79, 2682 (1997).[16] G. Fubiani et al., Phys. Rev. E 70, 016402 (2004).[17] S. Bulanov et al., Phys. Rev. E 58, R5257 (1998).[18] R. G. Hemker, N. M. Hafz, and M. Uesaka, Phys. Rev. ST-AB 5, 041301 (2002).[19] H. Suk et al., Phys. Rev. Lett. 86, 1011 (2001).[20] LOASIS facility: http://loasis.lbl.gov/.7[21] C. G. R. Geddes et al., Phys. Rev. Lett. 95, 145002 (2005).[22] W. P. Leemans et al., Phys. Plasmas 8, 2510 (2001).[23] C. Geddes et al., in Proc. Particle Accel. Conf. (IEEE, Piscataway, NJ, 2007).[24] For BPS data, a second laser (2 TW) was present from other experiments but bunch propertieswere independent of its pointing. Spectrometer data (no second laser) verified that the secondlaser did not affect the bunch.[25] Laser focusing through the jet required a large simulation domain of 400 µm (240 µm) with12,000 (900) cells in the longitudinal (transverse) direction with 5 particles/cell. Peak densitywas 1.8 × 1019 cm−3 with a Gaussian profile of 900 µm FWHM. The laser started 750 µminto the ramp and focused 600 µm downstream of center, in the range that produced MeVbunches experimentally.[26] C. Nieter and J. Cary, J. Comp. Phys. 196, 448 (2004).[27] G. Fubiani et al., Phys. Rev. ST-AB 9, 064402 (2006).[28] G. Plateau et al., in Proc. Particle Accel. Conf. (IEEE, Piscataway, NJ, 2007).[29] W. P. Leemans et al., Phys. Rev. A 46, 1091 (1992).[30] T. Hosokai et al., Phys. Rev. E 67, 036407 (2003).[31] B. A. Shadwick (private communication, 2005).[32] N. E. Andreev, S. V. Kuznetsov, and I. V. Pogorelsky, Phys. Rev. ST-AB 3, 021301 (2000).8FIG. 1: Setup for plasma density gradient controlled injection using a laser focused on the down-stream edge of a gas jet.FIG. 2: Charge measured by the ICT and Gamma signal versus gas jet Z position with respectto the laser focus. The Gaussian fit to the plasma density indicates the laser focus position in theprofile. Correlation of the ICT and phosphor data showed that the MeV bunch charge was ∼ 0.3-1nC.9FIG. 3: Magnetic spectra at a jet position of -300µm, with the laser focus in the density downramp.Sequential shots (A), show the centroid (*) with respect to the average (square) over 45 shots,displaying bunch stablity. The contour scale is fixed (see Fig. 4). The integrated magnetic spectrum(B), obtained by scanning the magnet current, shows RMS error bars for charge and central energy(top), and FWHM (at half max.). Data points (blue) are connected by a black line.FIG. 4: Electron bunch and transmitted laser profiles. In the electron profile at a jet positionof -300 µm (A), the black solid contour indicates the small-divergence feature (70% of peak).Dotted (dashed) contours indicate the sizes and displacements of the smallest (largest) area bunchesobserved. The (*) indicates the centroid, and inner(outer) error bars display RMS (peak-to-peak)pointing deviation over 30 shots. The transmitted laser is unaberrated at this location(B), incontrast to the mode when focused upstream (C).10FIG. 5: Simulated wake and phase space. The laser (A, red) drives a plasma density wake (grey)in the downramp, trapping and accelerating electrons (yellow). Phase space (B, colorbar in Fig 4)shows formation of an ultrafast, narrow momentum spread bunch at 1.5 MeV/c, also visible in themomentum spectrum lineout at right.11

Plasma density gradient injection of low absolute momentum spread electron bunches

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Plasma density gradient injection of low absolute momentum spread electron bunches

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