B. Luther-Davies

Penumbral imaging of high energy X-rays from laser-produced plasmas

Abstract We present a new coded aperture imaging technique which uses a simple large circular aperture and we apply it to imaging 20 keV X-ray emission from a laser-produced plasma. In the method known as penumbral imaging, the source casts a shadow of the aperture at the film plane. We ensure that the source is smaller than the aperture and are able to obtain high quality reconstructions of the source using deconvolution. We have achieved a resolution of about 25 μm in initial tests limited in this case by asymmetries in the coding aperture.

An experimental study of magnetic fields in plasmas created by high intensity one micron laser radiation

Magnetic fields generated in plasmas formed by focusing intense picosecond (∼20 psec) duration Nd: laser pulses onto disk targets have been studied using simultaneous polarimetry and interferometry for a wide range of laser and plasma parameters. Megagauss fields, which extend throughout the plasma corona, have been observed during the pulse, but subsequently these decay rapidly and are unobservable only 100 psec after the pulse peak. The general morphology of the field together with this rapid decay suggests a role for the superthermal particles in magnetic field generation and saturation (via convection). Hard x‐ray (∼20 keV) imaging has been used to relate the regions of energy deposition by superthermal electrons to the magnetic field structure. The results are compared with the predictions of recent two‐dimensional (2‐D) computer simulations.

The determination of electron density profiles from refraction measurements obtained using holographic interferometry

Abstract The angles a fourth-harmonic holographic interferometer probing beam is refracted upon passing through a neodymium laser-produced plasma have been measured by varying the focus during reconstruction of the hologram. A method (not requiring the numerical inversion of an integral equation) for using such refraction measurements to give the plasma electron density profile is shown to produce a profile in agreement with that obtained from the phase information on the reconstructed in-focus interferogram.

An electro-optically Q-switched actively mode-locked Nd:YLF laser with feedback stabilised pre-lase

Abstract This paper describes the performance of a pulsed, actively mode-locked, actively Q-switched solid state laser oscillator which uses negative feedback to provide a period of pre-lase up to 1.2 ms long during which stable transform limited optical pulses are formed within the cavity. When Q-switched, single pulses with energies of up to 1.0 mJ are generated.

The use of a regular array of apertures in penumbral imaging

Abstract Coded aperture imaging using a regular array of apertures is investigated and a simple technique is presented whereby the recorded image may be made to appear as if it were generated by a single aperture but with a much greater collection efficiency. The technique is applied to penumbral imaging, which was the subject of a previous paper, and is shown to give significant improvements. It is also shown that the method will allow tomographic resolution of the source and that this resolution may be enhanced using super-resolution techniques.

A simple laser pulse tailoring/synchronization system for laser plasma interaction experiments

Abstract A system is described for accurately synchronizing pulses from single longitudinal mode Q-switched Nd: YAG oscillator (QSO) with those from a passively mode-locked Nd: YAG oscillator (MLO). Single mode operation is achieved by applying external negative feedback stabilization to the QSO to bring it into quasi-CW operation before Q-switching is triggered by the pulse from the MLO. The relative jitter between the peaks of the two pulses did not exceed 1 ns.

Electron density profiles of short pulse 1 μm laser-produced plasmas

Abstract This paper presents electron density profile measurements of the critical density region of picosecond neodymium laser produced plasmas which are consistent with a model of profile modification due to resonance fields interacting with a rippled critical density surface.

Evidence of intensity dependent profile modification in laser-produced plasmas inferred from X-ray continuum measurements

Abstract Measurements of the optimum angle for resonance absorption θ m in laser-produced plasmas as a function of laser intensity have been made from X-ray continuum measurements. It has been found that θ m increases with increasing laser intensity. The result has been interpreted as indicating that the plasma density scale length decreases with increasing laser intensity. The functional dependence so determined is found to be in close agreement with the results of simulations by other workers.

Evidence of filamentation (self-focusing) of a laser beam propagating in a laser-produced aluminium plasma

The thresholds for self-focusing (filamentation) of a laser beam in a laser-produced plasma have been computed for propagation of nanosecond duration pulses in short (<or=180 mu m thick), high-density (0.05-0.9 Ncr where Ncr is the critical density) multiply ionised absorbing plasmas. The authors also report results of an experimental study where they have observed filamentary structures in the X-ray emission from aluminium plasmas accompanied by an increase in the divergence of the light transmitted through them. Both observations indicate self-focusing has occurred at an intensity near to that predicted by the computer code, this being approximately=1013-1014 W cm-2 for nanosecond duration pulses in plasmas formed by irradiating light solids (Z<or=13).

Complex second harmonic emission spectra from plasmas produced using short pulse, high intensity, 1μ m laser radiation

Abstract Complex second harmonic spectra emitted from plasmas generated using short pulse high intensity 1 μm laser radiation can be most easily explained by a generation mechanism involving a decay instability of resonance absorption operating in combination with modulation of the spectrum by return current driven ion acoustic waves.