Search

Plasma wakefield accelerators (PWF A) have demonstrated GeV/m scale accelerating gradients; however, current PWFAs tend to increase the particle beam's emittance (volume of the beam in transverse phase-space) as the beam propagates through the plasma. The beam's emittance can be preserved by tailoring the longitudinal density profile of the plasma to properly focus the beam into the plasma. We present optical techniques to create controlled plasma densities via laser ionization. The longitudinal plasma density profile is determined by the evolution of the intensity of a high power laser pulse as it propagates through a gas. Control of the pulse intensity is achieved using a superposition of Bessel beams created by a tandem lens setup. The width of the plasma is limited by the refraction of the laser pulse off of the ionizing gas, but we have developed several optical techniques to create wider plasmas. Our simulations show that suitable plasma filaments with lengths up to a meter and widths up to a millimeter can be generated in an argon gas using a~ 1 0 TW peak-power Ti:sa laser pulse.

Beam-driven, underdense thin plasma lenses are a compact and tunable means of providing strong focusing to relativistic electron beams. It is necessary to focus electron beams to small sizes in a beam-driven plasma wakefield accelerator to preserve the beam emittance, or volume in phase space. The thin plasma lens can be used to achieve these small beam sizes that are otherwise out of reach of conventional electromagnetic beam delivery systems. Ultrashort (~ 30 fs), low energy (~ 10 mJ) laser pulses can be used to ionize a small region of gas to generate the thin plasma lens. The interplay between the ionized gas and the focusing of the laser pulse are modeled in detail to calculate experimentally achievable 3-D plasma lens profiles, which are then used in simulations of beam focusing. Plasma lens focal lengths and minimum beam sizes achievable are estimated, considering the effects of radiation reaction in both the classical and quantum regimes.

Electron beam emittance preservation in a beam-driven, nonlinear plasma wakefield accelerator relies upon matching the beam to the plasma. This occurs when the linear focusing forces of the ion column acting on the electron beam are perfectly balanced against the emittance-driven divergence of the beam. The matching condition is satisfied when the square of the beam size equals the product of the geometric emittance and the inverse betatron wave number of the beam inside the plasma. In practice, however, the matched beam size is too small to achieve with conventional beam delivery systems operating in vacuum. Here, a passive, tapered plasma density ramp at the entrance (exit) of the plasma source is described that can focus the electron beam to (from) the necessary matched size at a rate exceeding the adiabatic plasma focusing limit without diluting the beam's emittance. A numerically derived empirical model predicts both the plasma ramp shape and parameter tolerances required for beam matching as a function of the incoming beam parameters. These results can be easily scaled for the purpose of designing sequential plasma stages in a high-energy linear collider.

An integral part of plasma wakefield acceleration (PWFA) is producing, monitoring, and controlling a plasma source. Understanding the temporal evolution of plasma parameters allows for constraints on experimental lifetime and initial conditions. Recombination factors for low temperature (kBT less than the ionization energy), singly-ionized Argon and Hydrogen plasmas (nominal PWFA plasma sources) are investigated to develop a model of plasma decay. Due to the high density of the plasma sources collisional recombination and ionization dominates radiative processes, and the problem of modelling plasma decay simplifies to finding the recombination coefficients for these plasma sources. This recombination coefficient is presented as a function of plasma temperature to determine plasma decay for a variety of initial plasma conditions. Additionally, plasma temperature is modelled from the ponderomotive force of the ionizing laser pulse. Results are utilized for designing optimal PWFA experiments.

Long (L>>λ pe ) charged particle bunches traveling in dense plasmas are subject to a transverse two-stream instability, the self-modulation instability (SMI)1. This instability modulates the bunch in the radial direction with a longitudinal period approximately equal to the plasma wavelength (λ pe ∼n e 1/2). The SMI can be used to transform a long bunch into a train of short bunches that can resonantly drive wakefields to large amplitudes. An experiment known as AWAKE2 was recently approved at CERN to drive GV/m wakefields with 400GeV, ∼12cm-long proton bunches in a 10m plasma with λ pe ∼1mm. In the linear wakefield regime, the plasma response is symmetric for positively and negatively charged bunches. However, once the nonlinear regime is approached the plasma response becomes asymmetric, leading to differences that could be evidenced using the ultra-relativistic electron and positron bunches3. This asymmetry makes it more difficult to accelerate positively than negatively charged particle bunches in plasmas.