A Dyson

Measurements of the hole boring velocity from Doppler shifted harmonic emission from solid targets

The fast ignitor scheme for inertial confinement fusion requires forward driving of the critical density surface by light pressure (hole boring) to allow energy deposition close to the dense fuel. The recession velocity of the critical density surface has been observed to be v/c=0.015 at an irradiance of 1.0×1019 W cm−2 at a wavelength of 1.05 micron, in quantitative agreement with modeling.

Measurements of the heat flux, inverse bremsstrahlung absorption and equilibration in an under-dense laser heated plasma

A fully ionised hydrogen plasma of initial density approximately=5*1017 cm-3 and temperature approximately=12 eV is heated using a 1.053 mu m laser beam focussed to a spot of 200 mu m diameter with an irradiance of 5*1013 W cm-2. The resulting evolution of the plasma is monitored by Thomson scattering of a second laser beam at 5265 AA. Comparison with a one-dimensional two fluid hydrodynamic simulation shows that the inverse bremsstrahlung absorption coefficient should be modified to include the strong field correction and that the maximum heat flux is about 0.1 of the free streaming limit. The ratio of the electron mean free path to the temperature scale length is about 0.1 and Te/Ti approximately=5. The level of ion acoustic turbulence is observed to be small, close to thermal. The ion-electron equilibration rate is found to agree with the usual classical value.

New techniques for laser scattering in plasmas

We report novel techniques for the scattering of laser light from plasmas to measure density and temperature. Subnanosecond laser pulses are used with fast optical streak cameras to monitor the dispersed scattered light. The spectra are recorded on photographic film. Time resolution down to 100 ps is obtained and stray light problems are eliminated so that the unshifted scattered light can be observed. The techniques have been used to measure densities in the range ne≊1016–1018 cm−3 and electron temperatures in the range Te≊5–35 eV with an accuracy of better than 10%. Laser pulses at 5265 A with an energy of about 10 J are used and give little plasma heating.

X-ray emission from plasmas formed using an excimer laser with various pulse lengths

We report experiments to study the effects of the laser-pulse duration, angle of incidence and target material on x-ray generation. Metallic targets (Cu and Al) were illuminated with picosecond and sub-picosecond KrF excimer laser pulses focused to maximum intensities of with 5 ps and with 150 fs pulses. The resulting plasma produces soft x-rays in the 1 keV range. Copper targets illuminated at about with 5 ps pulses gave the highest x-ray conversion efficiency (1.6%). In addition to the soft x-rays, a hard x-ray signal ( keV) was also detected with 450 fs pulses. This is assumed to be due to the fast electrons generated by the interaction of the higher intensity sub-picosecond laser pulse with the solid target.

Imaging of high harmonic radiation emitted during the interaction of a 20 TW laser with a solid target

We present images of the source of extreme ultraviolet (XUV) harmonic emission at a wavelength of 220 A from the interaction of a 20 TW, 1.053 μm Nd:glass laser beam focused to intensities up to 4×1018 W cm−2 onto a solid target. From these measurements we determine an upper limit to the source size and brightness of the harmonic emission to show its efficacy as a novel source of short-pulse, coherent XUV radiation. We also demonstrate the empirical scaling of the harmonic generation efficiency with irradiance up to 1019 W μm2 cm−2, and extrapolate to estimate the possible source brightness at higher irradiances. These source brightnesses are compared to those available from an x-ray laser system.

Forced Raman scattering in air by a two-frequency laser beam

Stimulated Raman scattering in atmospheric nitrogen has been observed using two copropagating high power laser beams at 1.064 mu m and 1.053 mu m from a neodymium glass laser. This is due to the near coincidence of the beat frequency at 98.2 cm-1 with a rotational line of nitrogen (transition between J=11 and J=13) at 99 cm-1. The beams were of total intensity 6*109 W cm-2 and had a common air path of approximately=30 m. Large amplitude Stokes and anti-Stokes sidebands up to one half the pump beam intensity were generated. The observations are compared with a theoretical treatment of the interaction.

Sideband generation by coherent anti-Raman scattering in quartz of a two-frequency high-power laser beam

Coherent anti-Raman scattering in quartz has been observed using two copropagating high-power laser beams at 1.064 mu m and 1.053 mu m from a neodymium glass laser. This is due to the beat frequency at 98.2 cm-1 being within the broad-band Raman gain of quartz which extends from about 20 to 550 cm-1. The beams were each of intensity 3*109 W cm-2 and had an interaction length of approximately=5 cm. The measured gain is 9.5*10-10 cm W-1 in agreement with the established value.