Nonresonant beat-wave excitation of relativistic plasma waves with constant phase velocity for charged-particle acceleration

Abstract

The nonresonant beat-wave excitation of relativistic plasma waves is studied in two-dimensional simulations and experiments. It is shown through simulations that, as opposed to the resonant case, the accelerating electric fields associated with the nonresonant plasmons are always in phase with the beat-pattern of the laser pulse. The excitation of such nonresonant relativistic plasma waves is shown to be possible for plasma densities as high as 14 times the resonant density. The density fluctuations and the fields associated with these waves have significant magnitudes, facts confirmed experimentally using collinear Thomson scattering and electron injection, respectively. The applicability of these results towards eventual phase-locked acceleration of prebunched and externally injected electrons is discussed.

Robust Autoresonant Excitation in the Plasma Beat-Wave Accelerator

“…In comparison to other PBWA schemes, including the fixed beat-frequency approach, either at (RL/TD) or below (TSS) linear resonance, the chirped (DMG) scheme, involving downward chirping from resonance, or the non-resonant PBWA, scheme, recently proposed by Filip et al [45,46], involving strongly forced waves at frequency shifts well below resonance in a marginally underdense plasma, the autoresonant/APTR PBWA enjoys a number of advantages, in terms of plasma wave amplitude, robustness, and quality. In previous sections we have seen how, for given drive laser intensity, autoresonant excitation yields longitudinal fields that can be considerably higher than the RL limit set by relativistic detuning of the plasma wave.…”

“…Diffractive effects can substantially lower the longitudinal group velocity of the laser [19] With variations in the group velocity v g expected to be small, phase coherence of the plasma wave will depend on how closely v p follows the essentially constant v g =v g of the laser. Particle-in Cell (PIC) simulations of Filip et al [46] suggest that for the RL scheme, the effective phase velocity of the nonlinear plasma wave (measured in terms of the progression of the field maximum) can vary appreciably, i.e., 10% to 20%, reflecting phase slippage of the Langmuir wave primarily as a result of ponderomotive blowout and relativistic detuning, while the plasma wave produced in their non-resonant PBWA scheme exhibits substantially less phase slippage. By working within the QSA, in 1D geometry without transverse density variation, we cannot independently assess any such slippage effects for the present scheme, but we anticipate that it will be similarly small by virtue of the autoresoant phase-locking.…”

“…and in (46) we have used symmetry to reduce the integration path to the segment where p ≥ 0. By making the change of variables φ = φ + − (φ + − φ − ) sin 2 (u), the action (46) …”

Robust Autoresonant Excitation in the Plasma Beat-Wave Accelerator: A Theoretical Study

“…Particle-in Cell (PIC) simulations of Filip et al [46] suggest that for the RL scheme, the effective phase velocity of the nonlinear plasma wave (measured in terms of the progression of the field maximum) can vary appreciably, i.e., 10% to 20%, reflecting phase slippage of the Langmuir wave primarily as a result of ponderomotive blowout and relativistic detuning, while the plasma wave produced in their non-resonant PBWA scheme exhibits substantially less phase slippage. By working within the QSA, in 1D geometry without transverse density variation, we cannot independently assess any such slippage effects for the present scheme, but we anticipate that it will be similarly small by virtue of the autoresoant phase-locking.…”

“…and in (46) we have used symmetry to reduce the integration path to the segment where p ≥ 0. By making the change of variables φ = φ + − (φ + − φ − ) sin 2 (u), the action (46) can be calculated with the help of a standard integral table, e.g., equation 2.584-13 on p. 163 of [38]:…”