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Plasma acceleration is a technique for accelerating charged particles, such as electrons, positrons and ions, using an electric field associated with an electron plasma wave.

A plasma consists of positive and negative charged particles. When these particles of the plasma are displaced properly, a very high electric field will arise. Incoming charged particles can now use this high electric field to accelerate to relativistic speeds.

When a laser pulse or an electron bunch goes through a plasma, an electron plasma wave arises in its trace, like a wake following a ship on the ocean. Since an electron plasma wave is a longitudinal wave, high-density and low-density regions of electrons appear periodically in which the quasineutrality of the plasma is broken. An electric field originating from the breakdown of the neutrality, which is referred to as a "wakefield," accelerates charged particles inside the plasma in the direction parallel to the propagation direction of the electron plasma wave.

Plasma acceleration is categorized into several types according to how the electron plasma wave is formed: plasma wakefield acceleration (PWFA), laser wakefield acceleration (LWFA), laser beat-wave acceleration (LBWA), and self-modulated laser wakefield acceleration (SMLWFA). In PWFA, an electron plasma wave is formed by an electron bunch. In LWFA, a laser pulse is introduced to form an electron plasma wave. In LBWA, an electron plasma wave arises based on different frequency generation of two laser pulses. And in SMLWFA, the formation of an electron plasma wave is achieved by a laser pulse modulated by stimulated Raman forward scattering instability.

In a plasma wakefield accelerator, the wake is created when the space-charge force associated with the drive particles displaces the plasma electrons. The plasma ions, which are far more massive than the plasma electrons, remain stationary during the time scale of the beam passing through the plasma. Once expelled, the plasma electrons witness the space charge field of the ion column and are pulled back toward the beam axis, which results in a plasma electron density spike behind the center of the bunch (see below). The electric field associated with the density spike accelerates the particles at the back of the electron bunch.

The concept of plasma acceleration was first proposed by Toshiki Tajima and John Dawson in a theoretical article published in 1979. The first experimental demonstration of wakefield acceleration, which was performed with PWFA, was reported by a research group at Argonne National Laboratory in 1988.

The advantage of plasma acceleration is that its acceleration field is much stronger than that of conventional radio-frequency (RF) accelerators. In RF accelerators, the acceleration field has an upper limit determined by the threshold for dielectric breakdown of the acceleration tube. Therefore, to obtain high-energy electrons or ions, a long acceleration length is inevitably required, resulting in huge sizes for accelerator facilities. On the other hand, plasma acceleration, in which the acceleration field is generated in plasma, achieves an acceleration field several orders of magnitude stronger than with RF accelerators. It is hoped that a compact particle accelerator can be created based on plasma acceleration techniques or accelerators for much higher energy can be built, if long accelerators are realizable with an accelerating field of 10 GV/m.

Formula

According to the acceleration gradient for a linear plasma wave is:

${\displaystyle E=C\cdot {\sqrt {\frac {m_{e}\cdot \rho }{\epsilon _{0}}}}}$

In this equation, ${\displaystyle E}$ is the electric field, ${\displaystyle C}$ is the speed of light in vacuum, ${\displaystyle m_{e}}$ is the mass of the electron, ${\displaystyle \rho }$ is the plasma density, and ${\displaystyle \epsilon _{0}}$ is the permittivity of free space.

External links

• http://home.fnal.gov/~carrigan/Accelerators/Accelerator_master.htm
• http://pbpl.physics.ucla.edu/Research/Experiments/Advanced_Accelerators/Plasma_Acceleration/PBWA/
• http://icfa-usa.jlab.org/archive/newsletter/icfa_bd_nl_12/node26.html

• Plasma physics
• Particle accelerators