Kuan Hiang Kwek

Diagnosing collisions of magnetized, high energy density plasma flows using a combination of collective Thomson scattering, Faraday rotation, and interferometry (invited).

A suite of laser based diagnostics is used to study interactions of magnetised, supersonic, radiatively cooled plasma flows produced using the Magpie pulse power generator (1.4 MA, 240 ns rise time). Collective optical Thomson scattering measures the time-resolved local flow velocity and temperature across 7-14 spatial positions. The scattering spectrum is recorded from multiple directions, allowing more accurate reconstruction of the flow velocity vectors. The areal electron density is measured using 2D interferometry; optimisation and analysis are discussed. The Faraday rotation diagnostic, operating at 1053 nm, measures the magnetic field distribution in the plasma. Measurements obtained simultaneously by these diagnostics are used to constrain analysis, increasing the accuracy of interpretation.

Wavefront reversal technique for self-referencing collimation testing

We present a wavefront reversal technique to produce a dual-field fringe pattern for self-referencing collimation testing in wedge-plate lateral-shear interferometry. The method requires only a suitably placed cubic beam splitter to produce two replicas of the fringe field formed by the wedge-plate lateral-shear interferometer. One of the replicas has a fringe pattern that is the reverse of the other. With these two fringe fields, the collimation testing has a built-in reference, and the detection sensitivity is twice that of a single-wedge-plate technique.

Dual-prism interferometer for collimation testing

An air-wedge lateral-shear interferometer using two prisms is presented. With a variable shear, the interferometer is suitable for testing collimation of a wide range of beam sizes down to a few millimeters in diameter. No antireflection coatings are necessary. Collimation for a light source with short coherent length is also demonstrated.

A double-prism lateral shear interferometer for wavefront analysis and collimation testing

A novel method of using two prisms to perform lateral-shear interferometry for wavefront analysis and collimation testing over a wide dynamic beam size range is presented. Two right-angled prisms are utilized to establish an adjustable air gap with a small wedge angle. A test beam directed into the air wedge will result in reflected beams that are laterally sheared and produce an interference pattern which can be analyzed in the conventional manner. The thickness of air gap can be adjusted to allow a wide range of beam sizes to be tested down to a few millimeters in diameter. By using rightangled prisms instead of shear plates, several advantages such as the elimination of unwanted beam reflections, increase in the amount of shear per unit separation, and the enhancement of intensity and contrast of the output beams. The gap separation and wedge angle can be varied to maintain high sensitivity over the wide beam size range. Theoretical and experimental investigation into the sensitivity achievable in collimation testing with the method will be reported.

A Gigawatt High-Voltage Generator for Coherent Soft X-Ray Generation

A fast gigawatt high-voltage generator was developed to drive an argon-filled capillary for the generation of pulses of coherent short wavelength radiation in the extreme ultraviolet and soft X-ray region. A four-stage double Marx bank that is capable of delivering voltage pulses of 150 to 350 kV at 290-ns transition duration was used to pulse charge a 5-Omega-impedance water-filled pulse-forming line. A fast current pulse with amplitude ranging from 9 to 21 kA and 35-ns transition duration was generated through a 20-cm-long and 3-mm-diameter alumina channel filled with argon. Laser emissions in the Ne-like Ar at 46.9 nm have been observed in the pressure range of 0.14-0.38 mbar. It was also observed that over the spectral range from 20 to 70 nm, no other transitions, except the 46.9-nm line, experienced high gain and strong amplification.

Coiled arrays as a tool for modifying implosion dynamics in a wire-array z-pinch

Summary form only given. Coiled arrays, a cylindrical array in which each wire is formed into a single helix, suppress the modulation of ablation at the fundamental wavelength. Instead, ablation flow is modulated at the wavelength of the coil, and arrays with large coil wavelength produce an organized mode of implosion in which the global instability can be controlled. The ablation and implosion dynamics of coiled arrays in this regime were studied using a combination of resistive voltage probes and Faraday rotation. These experiments were carried out on the MAGPIE generator at Imperial College. This research was sponsored by the DOE under Cooperative Agreements DEF03-02NA00057 and the Imperial College Junior Research Fellowship scheme.

Recent wire-array Z-pinch experiments at imperial college

Recent wire-array Z-pinch experiments performed on the MAGPIE generator at Imperial College are presented at currents up to 1.5 MA. Experiments have been conducted using a variety of array configurations, including radial wire arrays, cylindrical arrays, and coiled arrays. This research was sponsored by Sandia National Laboratories, Albuquerque; and the NNSA under DOE Cooperative Agreement DE-F03-02NA00057

Wide dynamic beam size range lateral-shear interferometer

A novel method for testing the collimation of coherent optical beam is presented. Two prisms are used to define a thin air wedge with a wedge angle. A wide range of beam sizes can be tested down to a few millimeters in diameter. By redirecting the transmitted beam back to the air wedge so that it experiences a shear in the reversed direction, a double-shearing interferometry can be established. The gap separation and wedge angle can be varied to maintain high sensitivity (twice that of the wedge plate design) over the wide beam size range.