Plasma-based accelerators hold great promise to accelerate electrons, positrons, protons and ions to high energies over short distances and with high quality. They are one of the possible technologies that could revolutionize directly or indirectly many fields of science (Free Electron Lasers and Linear Colliders in particular), medicine, and industry. Plasma-based accelerators can be driven by laser pulses [1] (LWFA) or by particle bunc>es [2] (PWFA). A driving pulse can excite plasma wave in which electrons are trapped and gain energy as long as they are in phase with the accelerating region of the field. In just fifteen years, laser-driven plasma accelerators have advanced from making 10 MeV beams with ~100% energy spread to GeV bunches with a few percent energy spread. The steady increase in maximum energy was enabled by the rise of multi-hundred terawatt laser facilities around the world. A great effort in this direction is ongoing also at INFN within the FLAME, a 300TW power laser.
The progress for beam-driven experiments has been even more remarkable, thanks to the development of high quality ultra-short electron bunches, with the maximum energy gained in the plasma increasing from a couple hundred MeV to over 40 GeV in just two years, as shown in Fig. 1.

Fig. 1 - summary of LWFA and PWFA results

In the PWFA the plasma wave is excited by the space charge forces of the driving electron bunch that displace the plasma electrons, Fig. 1. In that way the driving electron pulse can transfer a large fraction of its kinetic energy to a subsequent bunch placed at a proper distance. The accelerating field scaling law

Shows that high charge (Nb) and short bunches (σz) play an essential role in achieving ultra-high gradients. For example with a beam of Nb = 2x1010 particles (corresponding to Q~3.2 nC) and σz = 20µm, accelerating gradients of the order of 100 GV/m can be driven in a plasma with a density of 1017 cm-3 Moreover since the dependence on the bunch length is inverse quadratic, more challenging accelerating gradients of the order of 1 TV/m could be obtained with shorter bunches: σz = 0.8 µm and Nb = 108 (corresponding to Q~20 pC), in a plasma with a density of 7x1019 cm-3.

In a PWFA driven by a single electron bunch, the peak accelerating field is, in principle, limited to twice the value o the peak decelerating field within the bunch (transformer ratio R=2). Therefore the maximum possible energy gain for a trailing bunch is less than twice the incoming energy. Several methods has been proposed to increase the accelerating field. A very promising method is the so called ramped bunch train [3] and consists of using a train of NT equidistant bunches wherein the charge increases along the train producing an accelerating field resulting in a transformer ratio proportional to the number of driving bunches, see fig. 2. For this application, it is essential to create trains of high-brightness femtosecond long microbunches with stable and adjustable length, charge, and spacing.

Moreover a new regime called weak blow out [4] has been recently investigated. It requires operation in the quasi-nonlinear regime, where one uses beam with relatively low charge and longitudinal and transverse beam size smaller than a plasma wavelength, σr, σz « λp ~ 200µm . In this case, the beam density may exceed that of the plasma, producing blowout (strongly non linear regime) [5], but due to the small total charge, producing a disturbance that behaves in many ways as linear, having frequency essentially that of linear plasma oscillations.
A lot of efforts are now ongoing worldwide to produce the required bunch train configurations [6]. The method we propose to achieve the required bunch train quality is based on the so called Laser Comb Technique [7] that we have proposed some year ago and which has been recently tested with the SPARC photoinjector in a not optimized configuration and without the plasma [8]. In this injector operating mode, the photocathode is illuminated by a comb-like laser pulse in order to produce a train of sub-picosecond high-charge density pulses within the same RF gun accelerating bucket. Downstream of the gun exit, the work done by the space charge force produces a linear energy chirp along each pulse, which can be exploited to compress the initial charge profile with an RF accelerating structure, operating in the velocity bunching mode [9], as shown if Fig. 3.
With such a train of NT bunches a resonant excitation of plasma waves can be performed with a convenient scaling with NT:

For example train of 4 bunches with 16 pC/bunch separated by one plasma wavelength (160 µm), propagating in a plasma of density 3x1022particles/m3 can generate an accelerating field in excess of 3 GV/m, as shown in a preliminary simulation reported in Fig. 4. Useless to mention that electron pulse trains with some hundreds pC charge, a sub-picosecond length and a repetition rate of some terahertz can be useful also to drive pump and probe or multi-color free-electron laser (FEL) experiments, generation of narrow-band terahertz radiation [10] and to drive Dielectric Wake Field Acceleration (DWFA) experiments [11].

The comb scheme (comb laser pulse and RF compression) represents an active method to generate THz repetition rate bunch trains without the introduction of beam losses. We have demonstrated experimentally the control of pulse spacing, length, current and energy separation by properly setting the accelerator.

SL_COMB home -Laser-Comb Technique @ SPARC - General issues on comb beam manipulation - Experimental results

[1] E. Esarey, C. B. Schroeder, and W. P. Leemans, Physics of laser-driven plasma-based electron accelerators, Reviews of Modern Physics, Volume 81, July-September 2009
[2] Patric Muggli, Mark J. Hogan , Review of high-energy plasma wakefield experiments, C. R. Physique 10 (2009) 116-129
[3] P. Schutt, T. Weiland, and V. M. Tsakanov, in Proceedings of the Second All-Union Conference on New Methods of Charged Particle Acceleration (Springer, New York, 1989).
[4] J. B. Rosenzweig, private communication
[5] J. B. Rosenzweig, N. Barov, M. C. Thompson, and R. B. Yoder, Energy loss of a high charge bunched electron beam in plasma, Phys. Rev. Special Topics - Accelerators And Beams,Volume 7, 061302 (2004)
[6] P. Muggli et al., Simple method for generating adjustable trains of picosecond electron bunches, Phys. Rev. Special Topics - Accelerators And Beams,Volume 13, 052803 (2010)
[7] M. Ferrario. M. Boscolo et al., Int. J. of Mod. Phys. B, 2006
[8] M. Ferrario et al, Advanced Beam Dynamics Experiments with the Sparc High Brightness Photoinjector, Proc of IPAC 2010, Kyoto, Japan
[9] M. Ferrario et al., Experimental Demonstration of Emittance Compensation with Velocity Bunching, Phys. Rev. Letters 104, 054801 (2010)
[10] E. Chiadroni et al., Characterization of the Thz Source at SPARC, Proc of IPAC 2010, Kyoto, Japan
[11] M. C. Thompson et al, Gigavolt per Meter Breakdown Limits on Wakefields Driven by Electron Beams in Dielectric Structures, Phys. Rev. Letters 100, 214801 (2008)
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