Comparison of the Laser Wakefield Accelerator and the Colliding Beam Accelerator R.R. Lindberg , A.E. Charman and J.S. Wurtele † U.C. Berkeley Dept. of Physics, Berkeley, CA 94720 † E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Abstract. With the advent of chirped pulse amplification and related technologies, much research been devoted to laser pulse shaping for optimal wake generation in a plasma based accelerator. Also, there has been a recent proposal for a colliding beam accelerator (CBA), which uses a detuned pump laser to enhance a standard LWFA wake. We use analytic scalings and PIC simulations to illustrate optimal wake generation in the LWFA under constraints of maximum laser energy, intensity, and bandwidth. We then compare the optimized LWFA to the CBA, finding that while the addition of a pump will increase the wake of a single pulse, the CBA is inferior to a single- or multiple-pulse LWFA of identical total laser energy. There have been several proposed concepts for using high power lasers to drive large amplitude, high phase velocity Langmuir waves suitable for particle acceleration in plasma (see, e.g. [1], and references therein). Two among these that rely on femtosecond pulses are the laser wakefield accelerator (LWFA) [2] and the recently proposed colliding beam accelerator (CBA) [3]. As chirped pulse amplification (CPA) [4, 5] and related optical techniques [6] increase laser power and our ability to precisely time, shape, chirp, and control the spectral content of pulses, it is natural to try to determine how suitably shaped laser profiles may optimally produce wakefields. We can consider this problem in terms of maximizing wake amplitude, efficiency, or some other measure of performance under appropriate physical constraints such as laser power, energy, or bandwidth. In what follows, we use the peak longitudinal electric field as our chief figure of merit, primarily because it is the potential for high acceleration gradients that most strongly motivates plasma-based accelerator research. Additionally, when the acceleration length is limited by the dephasing length (as is typical), maximizing the peak electric field is equivalent to maximizing the achievable particle energy gain. Using maximum wake amplitude as our criterion, we summarize LWFA optimization, and then compare the LWFA findings to the less familiar CBA under constraints of fixed laser amplitude and/or energy. 1. OPTIMIZATION OF THE LWFA The LWFA uses a short, intense laser pulse of width σ ω 1 p to ponderomotively excite a large amplitude longitudinal electric field with near-luminal phase velocity. Assuming a one-dimensional, tenuous plasma (ω 2 p ω 2 0 ), the wake amplitude φ eΦ mc 2 created