Fast-Ion Emission and Resonance Absorption in Laser-Generated Plasma

Abstract

tation are not yet complete, they clearly show spectrally shifted peaks away from the ruby-laser wavelength. The fact that the scattered radiation is shifted, in addition to the large enhanced signal levels detected, precludes Rayleigh scattering from excited hydrogen atoms as being responsible. Observed signals are at least two orders of magnitude greater than near-resonant Rayleigh scattering from hydrogen could ever provide for our experimental conditions. Significantly, the scattering was sufficiently enhanced that it was necessary to use 10 3 attenuation in detection to obtain counts within the dynamic range of the analyzer (800 counts). As further evidence of the highly nonthermal nature, a search for the electron feature (several hundred angstroms in width) made by removing the 10 3 attenuation yielded no scattered signal. Since for thermal scattering the peak ion to electron ratio is ~ 10, the detection sensitivity sets a lower bound of ~ 10 4 enhancement over thermal.In fact, a direct calibration using Rayleigh scattering showed typical enhancements of 10 4 -10 5 with occasional shots up to 10 6 . Now for the probed k = 2ir/\ = 9X10 4 cm" l , thermal fluctuations would imply density fluctuations (6w/n) th ~ (wX 3 )" 172 ~ 10" 3 . Thus since the Thomson-scattered signal ~ Ibn k I 2 , an enhancement of 10 4 implies 6n k /n~ 0.1 with correspondingly higher values for greater enhanced scattering. Clearly, very large levels of low-frequency ion (and electron) fluctuations have been induced by C0 2 -laser heating of the gas-target plasma.The presence of such fluctuations levels forThe investigation of "fast ions" emitted from a laser-generated plasma is an active research topic 1 " 7 of particular importance for laser fusion. The energy carried by these ions (greater than &A D~0 .5 andlc-i-k 0 of the C0 2 laser, as well as the experimentally observed hn/n ^0.1 for k\ D ~ 0.065 with 5 = 2E 0 , clearly shows significant ion turbulence over a broad geometry and spectral range. Potentially efficient absorption of laser radiation can result from these fluctuations.Since we have directly measured the fluctuation level for k\ D~ 0.5, it is interesting to calculate the anomalous heating rate ^* for our plasma assuming 6n k /n~ 0.1 to be true for a broad spectrum of k as indicated from our experimental results. Taking < cos 2 0,) *4 and < Ime/ I el 2 ) ^ 1 where brackets denote averages over k, we find i>*~ i>ci/3. Evidently fluctuation levels of 20% would make v* £ v clo Thus for short periods during the relatively long-pulse C0 2 -laser heating of the underdense plasma, anomalous absorption may very well be contributing to heating.We wish to thank M. Cervenan, B. Hadley, and P. Haswell for assistance in this experiment. -Nee, Rev. Sci. Instrum. 45, 1400.the thermal expansion energy of the plasma) is generally considered to be the result of ion acceleration by fast electrons. Examples of possible fast-electron sources are resonantly driv-Plasma was generated by focusing a 35-psec, 1.06-jum laser onto Perspex. Ion expa...

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

“…But the electrostatic acceleration by the double layer in the plasma surface was well measured later (Wagli et al 1976;Ladrach et al 1979), as was proved by the identification of one group of kiloelectronvolt ions that were independent of the optical polarization. The number of such ions (Wagli et al 1976) was indeed only of the order of the expected 10 9 . Apart from this very first involvement of the electric double layer in laser-produced plasmas, another impact came from the following basic question about the nonlinear force.…”

“…One special result is the effective dielectric constant (figure 12b). For a collisionless plasma, this function has a pole of minus infinity (White et al 1974) at the critical density, while adding a very tiny amount of collisions (absorption) causes the pole to jump from minus infinity to a very high positive value (Hora 1991). This is one example of how important it is to take into account the collision frequency in plasmas; otherwise, results can change from nearly plus infinity to minus infinity if the usual simplification of collisionless plasmas is assumed in the microscopic plasma theory (Hora 1991, Sec.…”

Plasmas at High Temperature and Density

“…The analysis of the fast ions in the keY range, however, led to another modification ifthe light was incident obliquely on the plane targets [82]. on the ion charge was a relation seen in several experiments, especially in the case of 100 keY ions and above.…”

Numerical simulation of a negative ion plasma expansion into vacuum

“…The subject of plasma expansion into vacuum has received significant attention over the last decades. This phenomenon has been studied not only for a two component plasma case, [1][2][3][4][5] but also for plasmas with three species. 6,7 In particular, the interest in the plasma expansion process can be justified by its potential application in space physics 2 and in laboratory experiments.…”

“…6,7 In particular, the interest in the plasma expansion process can be justified by its potential application in space physics 2 and in laboratory experiments. 3,4 In these applications it is common to find a large number of negatively charged dust grains that can significantly alter the properties of the plasma. Dusty plasmas are frequently approximated by ''negative ion plasmas'' ͑NIP͒ which consist of positive ions, electrons and negative ions of constant charge.…”