Physical mechanisms leading to high currents of highly charged ions in laser-driven ion sources

Abstract

Heavy ion sources for the big accelerators, for example, the LHC, require considerably more ions per pulse during a short time than the best developed classical ion source, the electron cyclotron resonance (ECR) provides; thus an alternative ion source is needed. This can be expected from laser-produced plasmas, where dramatically new types of ion generation have been observed. Experiments with rather modest lasers have confirmed operation with one million pulses of 1 Hz, and 10" C 4 + ions per pulse reached 2 GeV/u in the Dubna synchrotron. We review here the complexities of laser-plasma interactions to underline the unique and extraordinary possibilities that the laser ion source offers. The complexities are elaborated with respect to keV and MeV ion generation, nonlinear (ponderomotive) forces, self-focusing, resonances and "hot" electrons, parametric instabilities, double-layer effects, and the few ps stochastic pulsation (stuttering). Recent experiments with the laser ion source have been analyzed to distinguish between the ps and ns interaction, and it was discovered that one mechanism of highly charged ion generation is the electron impact ionization (EII) mechanism, similar to the ECR, but with so much higher plasma densities that the required very large number of ions per pulse are produced.

Laser-driven generation of fast particles

“…The first one is the S-LPA mechanism, described in the previous section. The second one uses the, so-called, double-layer effect in expanding plasma corona [7,8]. The double-layer is formed on the front of expending plasma by a cloud of hot (fast) electrons escaping from the plasma.…”

“…These ions form a fast ion group, as opposed to the thermal, slower ion group accelerated by thermokinetic forces. The mechanism of acceleration using the double-layer in plasma corona dominates in the case of long driving pulses [7][8][9][10], but it can be also efficient in the case of short pulses of relatively low contrast (producing sufficiently thick underdense preplasma layer) [44,45]. Both the double-layer and S-LPA mechanisms were demonstrated to be efficient in generation backward-emitted heavy ion beams.…”

“…Studies of laser-driven generation of highly charged ions with the use of long-pulse (ns or subns) lasers have a much longer history and have been performed by several research groups [7][8][9][10][52][53][54][55][56][57][58]. Some results obtained by the Czech-Polish group with the use of subns high-energy iodine lasers are presented in Table 3 [10].…”

“…Though the laser-driven ion source (LIS) can be considered itself as a separate, compact ion accelerator, it can also be used as an ion injector in large conventional accelerators [8,60,61]. The potential advantage of the laser ion injector is the high ion current, the small transverse and logitudinal emittances as well as the possibility of production of ions of arbitrary elements.…”

“…However, powers and intensities, I L , of the used lasers were low (I L < 10 12 W/cm 2 ), so energies of produced particles were low (~keV) as well. The high-intensity experiments began in the seventies and up to the middle of the nineties they were performed almost exclusively with long nanosecond and subnanosecond laser pulses of intensities up to 10 16 W/cm 2 [7][8][9][10]. A real breakthrough in the field, especially in laser-driven generation of fast ion beams, took place, however, on the turn of the century, when short picosecond and subpicosecond laser pulses of powers from TW to PW level were employed in the experiments.…”