Page 5 - Transport studies in high-performance field reversed configuration plasmas

FIGURE 3. Non-moving to moderate commoving to commoving of target motions from the total detachment from the electron sheet (a) to totally commoving case (e). (a) TNSA. Laser generates energetic electrons on the front surface of the thick target. Electron travel through the target to emerge from the rear side with broad energy spread. These electrons exit into vacuum to pull ions. However, most electrons are pulled back to the immobile target before ions gain much energy. Electrons at the margin of the electron cloud are ejected out from the electron space charge. (b) The Mako-Tajima scenario. Electrons with a delta function energy spectrum enter from the metallic immobile (real) surface. Electrons rush out in vacuum to pull ions. However, most electrons are pulled back to the immobile boundary before ions gain large energy. Some electrons are ejected forward. The electron dynamics have much in common with case (a), although the electron spectrum is broad and has a tail in (a). (c) A case study with an ultra thin target that is immobile. One significant difference of (c) from (a) is that the electron energy directly determined by the laser and its ponderomotive potential beyond the rear surface of that target. Thus the energy of ions is expected to be narrow in its width and to have higher maximum than (a). (d) When the target is sufficiently thin, the rear surface of the target (and some times entire target) begins to move, while the laser interacts with the target. (e) When the target is pushed with the laser ponderomotive force (such as the circularly polarized laser pulse) without too much heating of electrons, ions in the target as a whole are trapped in an accelerating bucket with tight phase space circles. If and when the laser leaks through and electrons are ejected forward, the bucket may begin to collapse. Cases (c)–(e) belong to the regime of CAIL, while (e) is in particular in the RPA conditions. (from Ref. [33])
LASER WAKEFIELD ACCELERATORS AND ION ACCELERATION
The main lessons learned from the 1970’s collective accelerator research effort (particularly those in which TT was involved in or observant of) are two fold. In order to scoop up ions at rest and bring them to higher energies, we first have to make the driver (the electron bunch in the case of Rostoker’s lab) screech to stop at the entrance metal boundary and from there on, electrons start to pick up velocity. However, when this occurs, the driver strongly interacts and the plasma becomes unstable and generates unnecessary disturbances. This is similar to the tsunami
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