Plasma-based Particle Accelerator for High Energy Physics
In a wakefield accelerator, a plasma electron density wave is driven using either a high energy charged particle beam or an intense focused laser pulse. The resulting plasma wave supports high electric fields, and electrons can be accelerated to high energies in these fields. The advantage of a plasma-based accelerator is that plasmas support much higher electric fields compared to conventional accelerators.
When a plasma wave is driven by a relativistically intense (I > 10^18 W/cm^2), ultra short (T < 100 fs) focused laser pulse, the radiation pressure of the laser expels all of the electrons transversely, and forms a region completely devoid of electrons behind the driving laser pulse. Plasma electrons can then be self-injected into this plasma "bubble", and become accelerated.
Our research focuses on the bubble regime of wakefield accelerators, and specifically on the dynamics of self-injection. We study self-injection both analytically and numerically. We have published papers on this process, focusing on non-stationary Hamiltonian models and particle-in-cell simulations.
High Energy Density Plasma Physics: From Fast Ignition to Astrophysical Jets
Relativistic electron beam propagation through a background plasma occurs in a variety of physical systems, including inertial confinement fusion and astrophysical scenarios. In the originally envisioned Fast Ignition scheme of inertial confinement fusion (ICF), underdense relativistic electron beams (~MeV) excited by a laser are the final transport mechanism for depositing energy into the core of a pre-compressed target pellet, sparking the ignition. It is thus of great interest to understand the physics of this transport process both far away from and close to the core.
The current resulting from an underdense, `fast' moving electron beam excited by the laser is quickly neutralized by an induced plasma return current which is dense and `slow' moving. This is an unstable configuration that results in collective instabilities. For relativistic beams, the dominant instability is the transverse electromagnetic Weibel Instability (or WI). The WI leads to beam pinching and filamentation, beam and plasma heating, and the generation of strong magnetic fields. It is an important mechanism for beam slowing because it induces transverse thermalization and transfers energy from the beam to fields and background plasma. The WI consists of a linear stage, in which beam density and magnetic field strength grow exponentially in time, and a nonlinear stage, in which beam filaments formed during the linear stage merge together.
Electron beam propagation has been studied using fully 3D particle-in-cell (PIC) codes, but it is generally difficult to model the dynamics over large time and spatial scales with PIC codes. To mitigate this problem, the Shvets research group uses a hybrid approach that treats the beam kinetically and background plasma as a fluid to model the beam-plasma system. Recent work has focused on electron beam deceleration during the nonlinear stage of the WI in the presence of plasma electron-ion collisions. An analytic description of this process has been developed, which is supported by computational results, and is pending publication.
Laser-driven ion acceleration
Laser-driven ion acceleration has recently proven capable of generating mono-energetic ions with characteristics suitable for a variety of applications. Important examples are proton therapy for treatment of deep tumors in cancer patients, and the fast ignition scheme of inertial confinement fusion. In both cases the advantage of using ions, often protons, is their ability to deposit a their kinetic energy in a small target volume, known as the Bragg peak. Laser-plasma interactions can also support accelerating fields orders of magnitude higher than conventional accelerators, and produce beams of much higher intensity.
Acceleration of ions is accomplished with an ultraintense laser incident on an ultrathin foil, which instantly is instantly ionized into an overdense plasma. In the radiation pressure acceleration (RPA) regime, electrons in the foil are pushed via the laser ponderomotive force, and set up the electrostatic acceleration field, which accelerates the ions. Circularly polarized lasers prevent electron heating, as the ponderomotive force is constant in time.
The key challenge is reducing the spread in ion energy. A primary factor in energy spectrum degradation is the stability of the ions. In the ions’ rest frame they are resting on the laser front, and therefore are subject to the classic Rayleigh-Taylor Instability seen when a heavy fluid is positioned above a lighter fluid.
We are working to develop a model of this instability for accelerated ions, and are investigating strategies to reduce the degradation of the energy spectrum. For example in a hydrocarbon foil, the hydrogen ions (protons) quickly separate from the carbon ions and are accelerated on a stable interface, while shielded from the instability at the carbon-laser boundary.
Structure-based Laser-driven particle accelerators
Recently we have proposed an alternative accelerator structure driven by a CO2 laser. The Surface Wave Accelerator Based on SiC (SWABSiC) is based on a polar semiconductor, SiC, that supports infrared surface waves at the SiC/vacuum interface because its dielectric permittivity is negative within the tuning range of the CO2 laser. The SWABSiC structure can be employed as (i) an accelerator, in which case an external laser beam is needed to supply energy to the accelerated beam; or (ii) a radiation source that uses a pre-bunched electron beam to generate mid-IR Cherenkov radiation. Application (ii) can be potentially useful as a diagnostic tool for high-energy beams (e.g., at FACET or ATF facilities). So far we have succeeded in fabricating and optically testing the SWABSiC (at low power) . Using a line-tunable CO2 laser, we have succeeded in experimentally measuring phase velocities and quality factors of the accelerating and deflecting modes of the SWABSiC. The results (to be published) are summarized in the figure. To our knowledge, this is the first metal-free fabricated and optically characterized macroscopically-sized laser-driven accelerating structure.