Particle Energization in an Expanding Magnetized Relativistic Plasma

Abstract

Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the relativistic expansion of a magnetized collisionless plasma into a vacuum, we report a new mechanism in which the magnetic energy is efficiently converted into the directed kinetic energy of a small fraction of surface particles. We study this mechanism for both electron-positron and electronion (m i /m e =100, m e is the electron rest mass) plasmas. For the electron-positron case the pairs can be accelerated to ultra-relativistic energies. For electron-ion plasmas most of the energy gain goes to the ions. An outstanding problem in astrophysics is the acceleration of high-energy particles. The challenge is to find natural mechanisms which can efficiently convert bulk energy, whether it is magnetic, bulk motion, thermal or gravitational energy, into the relativistic energy of a small number of nonthermal particles. Here we report the results of PIC simulations [1], which suggest a new mechanism for the energization of relativistic particles via magnetic expansion.

Sustained Acceleration of Over-dense Plasmas by Colliding Laser Pulses

“…This leads to continuous acceleration of both the UL pulses and the dwindling population of trapped fast particles . The phase space evolution (Fig.2d) of this colliding pulses accelerator (CPA) resembles that of the DRPA [7].…”

“…Such transmission of EM waves a b c d through an overdense plasma could not be achieved using a single UL pulse, because there the plasma thickness remains >> 2 relativistic skin depths (Fig.1). During transmission, the B fields of the opposing pulses add while E fields cancel (Fig.2b), setting up a state similar to the DRPA initial state, and the subsequent evolution resembles the DRPA [7]. As the transmitted UL pulses reemerge from the plasma, they induce new drift currents J at the trailing edge of the pulses (Fig.2c), with opposite signs to the initial currents (Fig.2b), so that the new J x B forces pull the surface plasmas outward.…”

“…Preliminary results suggest that, for very thin e-ion plasma slabs which can be compressed to < two relativistic skin depths, the CPA concept remains viable. Most energy is eventually transferred to ions via charge separation, similar to the e-ion DRPA [7,15]. Fig.6 compares CPA runs for e-ion plasmas at different ion densities.…”

“…A dwindling number of fast particles also get accelerated indefinitely by the comoving EM force, reaching maximum Lorentz factors greater than the usual ponderomotive limit [6] γ max > a o 2 /2 >>(Ω e /ω pe ) 2 . This novel phenomenon is called the diamagnetic relativistic pulse accelerator (DRPA) [7]. DRPA is strictly a nonlinear, collective, relativistic phenomenon, with no analog in the weak field (Ω e /ω pe <1), low density (ω o >ω pe ) or test particle limit.…”

Optimizing Laboratory Experiments for Dynamic Astrophysical Phenomena

“…[20] and [21]). Generally speaking, effects involved require a kinetic description of the plasma particles, and one cannot use the hydrodynamic scalings discussed in the previous sections.…”