Richard F. Hubbard

Plasma wakefield generation and electron acceleration in a self-modulated laser wakefield accelerator experiment

A self-modulated laser wakefield accelerator (SM-LWFA) experiment was performed at the Naval Research Laboratory. Large amplitude plasma wakefields produced by a sub-picosecond, high intensity laser pulse (7×1018 W/cm2) in an underdense plasma (ne≈1019 cm−3) were measured with a pump–probe coherent Thomson scattering (CTS) technique to last for less than 5 ps, consistent with the decay of large amplitude plasma waves due to the modulational instability. A plasma channel was observed to form in the wake of the pump laser pulse, and its evolution was measured with the pump–probe CTS diagnostic. The trailing probe laser pulse was observed to be guided by this channel for about 20 Rayleigh lengths. High energy electrons (up to 30 MeV) have been measured using an electro-magnetic spectrometer, with the energy spectra and divergence of lower energy (up to 4 MeV) electrons obtained using photographic films. Highly nonlinear plasma waves were also detected using forward Raman scattering diagnostics and were obser...

Plasma current and conductivity effects on hose instability

Hose instability dispersion relations, which include a self‐consistent treatment of the spatial and temporal evolution of plasma conductivity and plasma current, are derived for a relativistic beam propagating in weakly ionized gas. A simplified conductivity model is used which neglects temperature dependence of the electron mobility. In some regimes the results are dramatically different from those found previously for a beam propagating in a fixed conductivity channel. For example, the hose growth rate is found to decrease with increasing current Ib for a beam propagating in initially neutral gas, even though the plasma return current fraction increases rapidly with Ib. As another example, it is found that an externally driven discharge current can completely eliminate hose instability in a fixed conductivity channel, but causes only a weak decrease in growth rate when the plasma conductivity is modeled self‐consistently. OFF

Relativistic effects on intense laser beam propagation in plasma channels

Propagation characteristics of a radiation beam in a preformed, tapered plasma channel are analyzed by means of an envelope equation for the beam spot size. The model allows for relativistic focusing and ponderomotive channeling, radial and axial density gradients, and is valid for arbitrary intensity. The characteristics of laser beam propagation are shown to be governed by two parameters, the ratio of laser power to the critical power for relativistic focusing, and a dimensionless focusing strength parameter that includes contributions from both relativistic and channel focusing. The envelope equation provides a unified approach for exploring diverse applications such as designing a tapered laser wakefield accelerator or a plasma lens. The model is employed in interpretation of pump–probe laser propagation experiments and an x-ray source experiment. Full-scale simulations of a plasma channel lens are presented and shown to be in excellent agreement with the analytical results.

Guiding and damping of high-intensity laser pulses in long plasma channels

An experimental demonstration is presented of large-distance (up to 6.6 cm, or 75 Rayleigh lengths) guiding of high-intensity (>1016 W cm-2) laser pulses in a plasma channel produced by a capillary discharge. Theory and simulation are used to describe damped oscillations that occur in the laser spot size as well as laser-induced ionization within the channel. Good agreement is obtained between theory, simulation, and experiment.

GeV acceleration in tapered plasma channels

To achieve multi GeV electron energies in the laser wakefield accelerator (LWFA) it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects, wakefields, and relativistic nonlinearities. In the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. Long pulses (several plasma wavelengths) experience substantial modification due to these instabilities. The short pulse LWFA, although having smaller accelerating fields, can provide acceleration for longer distances in a plasma channel. By allowing the plasma density to increase along the propagation path electron dephasing can be deferred, increasing the energy gain. A simulation example of a GeV channel guided LWFA accelerator is presented. Simulations also show that multi-GeV energies can be achieved by optimally tapering the plasma channel.

Conditioning electron beams in the ion‐focused regime

Relativistic electron beams propagating through dense gas are subject to the resistive hose instability, a virulent kink instability that restricts the effective range of high‐current beams. Previous studies have shown that the instability can be suppressed by centering the beam and tailoring its emittance prior to injection into the gas. One means of centering and tailoring a beam is to use short ‘‘conditioning’’ cells that operate in the low‐pressure, ion‐focused regime. In this paper, analytic models are developed to understand and assess the performance of such cells.

Streamerless guided electric discharges triggered by femtosecond laser filaments

The time evolution of electrical discharges induced and guided between the cathode of a Van de Graf generator and a ground sphere was studied using a 100 fs Ti:Sapphire laser. Nonlinear focusing and ionization effects produce optical and plasma filaments in the discharge region. Streak camera images often exhibit streamers that propagate towards the cathode, but sudden discharge triggering is frequently observed with no streamer precursors. The typical discharge triggering delay time was measured to be 150 ns. Similar time delays were obtained from an air chemistry code used to model the long time behavior of the plasma induced by the short laser pulse. The model shows that ohmic heating of the filament plasma persists over long time scales and inhibits the decay of electron density due to recombination and attachment processes. Eventually the rise in electron temperature causes the avalanche rate to increase to the point where breakdown occurs. The hydrodynamic density reduction process reported by Tzortzakis et al. [Phys. Rev. E 64, 057401 (2001)] is also taken into consideration. Its main effect is found to be a hastening of the breakdown process.

Measurements of energetic electrons from the high-intensity laser ionization of gases

Electrons ionized from tightly bound atomic states by a high-intensity laser pulse can gain energies from one to millions of electron volts dependent on the intensity of the pulse. We have currently been investigating hundreds of kilovolt to megavolt electrons produced by ionization of krypton and argon with terawatt laser pulses. Angular and energy distributions have been measured to determine the usability of this electron source as an injector for higher energy accelerators. Studies have included pressure dependence, angular ejection angle energy dependence, and polarization dependence. In particular, the energy-dependent ejection angle of electrons has been used to produce electron beams with energies peaked at 600 keV. Numerical simulations of these electrons show that 4 MV electron beams with excellent beam quality and femtosecond pulse widths can be produced from this electron source using higher power laser pulses.

Longitudinal compression of short laser pulses in air

We have performed laboratory experiments to study long distance propagation of large bandwidth ultrashort laser pulses in air. Initial pulse length, frequency chirping, and laser pulse energy were varied where the maximum propagation distance was up to 105 m. We have demonstrated the compression of initially negatively chirped low intensity laser pulses due to the linear group velocity dispersion of air. The characteristics of the compressed pulse such as pulse duration and spectral chirping were found to be significantly affected by the laser pulse intensity, with higher intensities corresponding to longer minimum compressed pulse duration.

Velocity control and staging in laser wakefield accelerators using segmented capillary discharges

To achieve multi-GeV electron energies in the laser wakefield accelerator, it is necessary to propagate an intense laser pulse long distances in plasma channels while maintaining a proper phase with the accelerated electrons. We have demonstrated a method that allows control of the laser group velocity in long, multistage plasma channels. The control is achieved by modifying the index of refraction through a variation of the plasma density using a segmented capillary discharge.