Chengmu Luo

A three-frame Mach–Zehnder interferometer for measuring dense magnetized plasmas

A three-frame Mach–Zehnder interferometer (TFMZI) was developed for taking three pictures of the imploding plasma produced by one shot of fast pulsed discharge. TFMZI consists of a YAG laser, a beam splitter, and a Mach–Zehnder interferometer. By passing through the beam splitter the laser beam was split into three beams with an interbeam delay of 13 ns and each laser beam will give a snapshot of the implosion state. The measurements with the TFMZI were conducted on a small gas-puff z-pinch device and gave good results. In principle an upgraded four or (more) frame Mach–Zehnder interferometer could be constructed if much improvement was made on the TFMZI.

Measuring the gas flow from a supersonic nozzle used in a 1.5-MA gas puff Z pinch

A supersonic nozzle for a 1.5-MA gas puff Z pinch was designed and the gas flow injected out from the nozzle was measured. A miniature ionization gauge was developed to measure the density profile of the gas shell and two pressure transducers were used for determining the Mach number of the gas flow. It was found that the gas flow is not distributed symmetrically about r/sub 0/, the mean radius of the exit aperture, and the radial location for the peak of the density profile moves inwards as time goes on. When the plenum pressure is 5 atm, the maximum line mass density measured was 43 /spl mu/g/cm and the Mach number of the gas flow deduced from experiments is 4.3, which is close to the expectation: line mass density of about 50 /spl mu/g/cm and a Mach number higher than 4. The measurements indicate that if a plenum opens into a large dead volume via a finite opening valve in a gas-puff system it will not produce a steady-state solution. Then, any agreement between the measurements and the calculations based on the steady state should not be expected.

A simple knife-edge design for initial phase optimization in plasma focus

A simple knife-edge design was described that enabled easy optimization and investigation of the initial phase for the plasma focus devices. It enhanced the initial breakdown process along the insulator surface by allowing free adjustment or fine-tuning of the insulator length and by forming a sharp knife-edge with effective field emission. The plasma pinching was much improved with neutron yield equal or above that predicted by the scaling law. This knife-edge design is especially suitable for the optimization of medium and large-scale plasma focus devices where it would be otherwise rather difficult to modify the insulator configuration directly.

A compact repetitive Marx generator

A compact repetitive Marx generator has been designed, built and tested. The generator of 8 stages is an L-C ring that consists of 8 capacitors (0.039 uF per capacitor) and 16 isolating inductors (128 uH per inductor). The generator was quickly charged to 25 kV within a charging time less than 52 microseconds by a DC charging source via a charging spark gap and a matching resistor of 81 Ohm that is equal to the characteristic impedance of the LC ring. A noise-immune two-channel trigger system was constructed for repetitively triggering one charging spark gap and the first two discharging spark gaps (there are 9 discharging spark gaps in the generator) in order of experiment. Due to the limited capacity of the DC charging source the generator was tested at a low repetition rate of 5 Hz with an output voltage about 170 kV (efficiency 85%).

Diagnostics for a small gas-puff Z-pinch plasma device

Experiments were carried out on a small gas-puff Z-pinch plasma device with a capacitor bank of 16 /spl mu/F and a charging voltage of 22 kV. A compact Thomson ion energy analyzer was installed on the device for determining the energy spectra of ion beam emitted from the Z-pinch plasma. The energy spectra of Argon ion beams with single, double, and triple charges were determined with the analyzer. A Mach-Zehnder laser interferometer was installed to measure the electron density and the movement of the Z-pinch plasma. The electron density of the plasma just before the pinch is larger than 9.00/spl times/10/sup 18/ cm/sup 3/, the corresponding radius and the pinch velocity of the plasma are 1.42 mm and 1.86 cm//spl mu/s, respectively.

Experimental study of gas-puff z-pinch plasma

A small neon gas-puff z-pinch device was constructed, and the plasma implosion and its radiation characteristics were studied experimentally. The plasma implosion was investigated using a three-frame Mach–Zehnder interferometer (TFMZI) that is capable of taking three pictures (5 ns exposure and 13 ns interpicture delay) within a single z-pinch shot. Thermopiles were also used in the experiments with the TFMZI to study the correlation between the total energy of x-ray emission and the plasma implosion. The ion beams from the z-pinch plasma were analysed using a compact Thomson spectrometer. Based on the experiments, the imploding velocity and electron density of the plasma shell were obtained, a conclusion that a nearly uniform and symmetric imploding plasma shell would emit a higher x-ray yield than an asymmetric one was drawn and the charge state resolved energy spectra of ion beams emitted from the z-pinch plasma were plotted.

Effect of the cavity structure on the discharge features of pseudosparks

The cavity structure of electrodes is important factor to determine the performances of pseudospark discharge devices. With the hollow cathode cavity, a very small and stable erosion pattern by electron-beam generated from the spark was formed on the opposite surface inside the anode cavity and stable concentric ring-shaped erosion patterns were observed at the central area around the apertures on the electrodes. Without the hollow cathode cavity, the patterns became irregular and unstable. The cavity structure of intermediate floating electrode can also increase the self-breakdown voltage of pseudospark switches up to 80 kV.

Preliminary results on X-ray lithography using a compact plasma focus

Summary form only given. A 1.8 kJ compact plasma focus (CPF) source operated in neon is demonstrated as a soft X-ray (SXR) source for microlithography in the wavelength range of 0.8-1.4 nm. The CPF is a four-module system. Each module uses a rail gap switching 12 capacitors each with a capacity of 0.6 /spl mu/F. It operates with peak current of 400 kA at 11.5 kV into water cooled electrodes at repetition rates up to 16 Hz to produce 300 W SXR in burst duration of several minutes. Lithographs are obtained by exposing the resist to the X-ray point source with a mask in contact with the resist. The total energy emitted is measured using PIN diodes and photoconducting diamond detectors (PCD) to be 20 J per shot with a spectrum peaked at 1 keV corresponding to the K shell lines from the neon working gas. The maximum repetition rate of 16 Hz allows lithographs to be obtained in less than 20 seconds when a reasonably sensitive resist is used.

Effect of the cavity structure on the discharge features of pseudospark switches

The cavity structure of the anode and the cathode of a pseudospark switch is an important factor for its performances. When the cathode has a cavity structure, the electric erosion on the cathode and anode is small and a concentric ring-shaped erosion pattern is observed at the central area around the apertures of both electrodes. Without the cathode cavity, the erosion becomes stronger, and the patterns become irregular and unstable. Several kinds of electrode structure have been designed and investigated. Among them, a unique cavity structure of intermediate floating electrode in pseudospark switch has a good performance. This configuration could reduce the erosion on the electrodes and increase the selfbreakdown voltage of the pseudospark switches up to 80 kV. In this paper, we have studied the influence of the electrode structure on the erosion of the electrode and the selfbreakdown characteristic of the switches.

Study on the gas shell from gas puff nozzle for z-pinch

Summary form only given, as follows. The gas puff nozzles for two neon gas-puff z-pinches were designed and tested. The first z-pinch is driven by a high current fast pulsed-power generator (1.5 MA, 80 ns) and the second one by a low current slow generator (240 kA, 2 /spl mu/s). In order to maximize the X-radiation from the z-pinch the initial radius and line mass density of the gas shell was optimized for the particular pulsed-power generator. A miniature ionization gauge was designed and constructed. With this specially designed gauge the density profiles of the puffed gas shells were measured. The density profiles were also obtained by numerical simulation. The experimental and calculated density profiles were compared and the line mass density was deduced. It was found that the calculated implosion time from 0D-model (slug-model) with the deduced line mass density agrees well with that observed in the experiment. Two fast responding pressure probes were used in the experiments on the supersonic gas shell. One probe is installed at a position just before the throat of the nozzle and another at the exit of the nozzle. The two probes method yields local Mach number of 4 for the puffed gas shell.