Simple method for generating adjustable trains of picosecond electron bunches

Abstract

A simple, passive method for producing an adjustable train of picosecond electron bunches is demonstrated. The key component of this method is an electron beam mask consisting of an array of parallel wires that selectively spoils the beam emittance. This mask is positioned in a high magnetic dispersion, low beta-function region of the beam line. The incoming electron beam striking the mask has a time/energy correlation that corresponds to a time/position correlation at the mask location. The mask pattern is transformed into a time pattern or train of bunches when the dispersion is brought back to zero downstream of the mask. Results are presented of a proof-of-principle experiment demonstrating this novel technique that was performed at the Brookhaven National Laboratory Accelerator Test Facility. This technique allows for easy tailoring of the bunch train for a particular application, including varying the bunch width and spacing, and enabling the generation of a trailing witness bunch.

Tunable Subpicosecond Electron-Bunch-Train Generation Using a Transverse-To-Longitudinal Phase-Space Exchange Technique

“…An interesting class of temporal distribution consists of trains of bunches with subpicosecond duration and separation. Applications of such trains include the generation of super-radiant radiation [1][2][3] and the resonant excitation of wakefields in novel beam-driven acceleration methods [4,5]. To date there are very few methods capable of providing this class of beams reliably [6].…”

Terahertz radiation source based on self-wake beam bunching

“…This method is simple, but yields wide band THz radiation. This method in fact is used in CTR (coherent transition radiation) interferometry to determine the bunch spacing in the train [12,14,17]. Alternatively one can use an undulator, creating a THz free electron laser (FEL).…”

Design Considerations for Plasma Accelerators Driven by Lasers or Particle Beams

“…The energy gain in a beam-driven plasma wave is given by the transformer ratio R = Ay/yd nvc , where Ay is the energy gained by an electron at the peak of the accelerating field and yd nve is the energy in the drive bunch Under general considerations [21], R < 2 for plasma waves driven by symmetric beams Higher transformer ratios may be achieved by using asymmetric beams to drive the wake In particular, transformer ratios with R > 2 can be achieved using a long (k p L ^$> 1), ramped beam (l e , triangular bunch with low density at the head), or, equivalently, a train of bunches with increasing charge A higher transformer ratio enables a more compact accelerator via the use of lower energy drive beams (potentially produced from smaller conventional accelerators) Appropriately shaped ramped bunches have been produced experimentally [22], as well as ramped bunch trains [23] Experiments using a ramped bunch train in a dielectric-loaded wakefield accelerator have demonstrated high transformer ratios [24] One limitation with using long beams for high transformer ratios, is that long beams are subject to instabilities, and, in particular, the electron-hose instability [25,26] The growth rate of the electron-hose instability scales as Those ~ Yb {®ptyi i {k p L) 2 l 7 ', indicating that the most effective way to suppress hosing is to reduce the bunch length In a laser plasma accelerator, the energy gam is limited by the laser energy depletion length The laser depletion length [27], for fixed laser intensity, scales as Lj <* «~3/ 2 Since, for fixed intensity, the accelerating field of the plasma wave scales as E z ~ EQ <* n l l 2 , the energy gam in a single laser-plasma accelerator scales with plasma density as Ay ~ E Z LJ <X n~x Achieving higher energy gains in a single laser-plasma accelerator requires going to lower density, lower gradient, and longer interaction lengths Present laser-plasma accelerator experiments typically rely on selftrapping of plasma electrons The self-trapping threshold is determined by the phase velocity of the plasma wave [28] In contrast to beam-driven plasma waves, the phase velocity of the laser-driven plasma wave is a function of plasma density, and for fixed intensity, the Lorentz factor of the phase velocity scales as y p w (Oo/(O p <* n l ' 2 , where G)o = 2KC/XQ IS the laser frequency Hence, to achieve high energy gains requires operating at low plasma density, and, as a consequence of the increased phase velocity, using some form of triggered injection Several methods of trigged injection are actively being explored, such as colliding pulse injection [29,30], using plasma density gradients [31,32], and ionization injection [33][34][35][36] DRIVER PROPAGATION AND COUPLING Plasma-based acceleration can be limited by the laser-plasma or beam-plasma interactio...…”