J.M. Gahl

Initial plasma-filled backward-wave oscillator experiments using a cathode-mounted plasma prefill source

The introduction of plasma into the slow-wave structure (SWS) of a backward-wave oscillator (BWO) has been shown to increase microwave power output and generation efficiency, as well as provide several other benefits. Researchers at Niigata University in Japan have performed a linear analysis that has shown that the optimal plasma-filled BWO configuration is one in which the plasma is confined close to the axis of the SWS, while the electron beam driving the device is kept close to the walls. Previous schemes for preinjecting plasma have utilized sources mounted downstream of the SWS and have relied upon the converging field lines of the electron beams guiding magnetic field to compress the plasma close to the axis as it entered the SWS. This paper presents data from experiments using a plasma source mounted internal to the cathode structure of a high-power BWO, thus providing injection of the plasma directly on axis in the strong, uniform field region of the BWO. Data describing plasma prefill density at different axial positions within the SWS prior to operation is presented. The plasma prefill has been found to both enhance and reduce the microwave generation efficiency, depending on the plasma density. Details of the novel plasma injection system are also provided.

Heat load material studies: Simulated tokamak disruptions

It is clear that an improved understanding of the effects of tokamak disruptions on plasma facing component materials is needed for the ITER program. Very large energy fluxes are predicted to be deposited in ITER and could be very damaging to the machine. During 1991, Sandia National Laboratories and the University of New Mexico conducted cooperative tokamak disruption simulation experiments at several Soviet facilities. These facilities were located at the Efremov Institute in Leningrad, the Kurchatov Atomic Energy Institute (Troisk and Moscow) and the Institute for Physicial Chemistry of the Soviet Academy of Sciences in Moscow. Erosion of graphite from plasma stream impact is seen to be much less than that observed with laser or electron beams with similar energy fluxes. This, along with other data obtained, seem to suggest that the “vapor shielding” effect is a very important phenomenon in the study of graphite erosion during tokamak disruption.

Electron emission from slow-wave structure walls in a long-pulse, high-power backward wave oscillator

Pulse shortening is a phenomenon common among all long-pulse, electron-beam-driven high-power microwave sources. Although the electron beam driving the source may continue to propagate through the interaction region of the device for several microseconds or more, the duration of the emitted microwave pulse is typically no more than /spl sim/100 ns. Most explanations of this phenomenon put forth involve the introduction of plasma into the interaction region and/or the degradation of beam quality. This paper describes experiments conducted on the University of New Mexicos Long-Pulse Backward Wave Oscillator (BWO) Experiment which investigate the behavior of beam electrons in the slow-wave structure (SWS) during microwave generation. A current probe having a small aperture at a variable radius is placed within the SWS to monitor the beam current profile at different radii as a function of time. Results from these experiments reveal the appearance of electrons between SWS ripples at times corresponding to the peaks of the microwave pulses in separate experiments. A drop in the main beam current is observed shortly thereafter. The source of the electrons within the ripples is thought to be field emission or secondary electron emission from the SWS walls or emission from plasmas generated there.

Performance of boron containing materials under disruption simulations and tokamak divertor plasma testing

Abstract The behavior of thick B 4 C coatings on different graphites under high power electron beam irradiation, pulsed plasma irradiation and under DIII-D divertor plasma action was investigated. SiCB 4 C coating on graphites produces in General Atomics and pyrolytic boron nitride were also tested in the plasma gun device. In the following tests at these facilities, the samples were examined using SIMS, EDAX, X-ray crystallography, profilometry and Auger spectroscopy. B 4 C coatings showed excellent durability under high heat flux irradiation and in conditions of real tokamak divertor plasma. The obtained results indicated that the use of RGT graphite (the graphite with high thermal conductivity) improves the durability of the B 4 C coating significantly. Pyrolytic boron nitride showed very small removal of the matter and no mechanical damage.

Beryllium and graphite performance in ITER during a disruption

Plasma disruptions are considered one of the most limiting factors for successful operation of magnetic fusion reactors. During a disruption, a sharp, rapid release of energy strikes components such as the divertor or limiter plates. Severe surface erosion and melting of these components may then occur. The amount of material eroded from both ablation and melting is important to the reactor design and component lifetime. The anticipated performance of both beryllium and graphite as plasma-facing materials during such abnormal events is analyzed and compared. Recent experimental data obtained with both plasma guns and electron beams are carefully evaluated and compared to results of analytical modeling, including vapor shielding effect. Initial results from plasma gun experiments indicate that the Be erosion rate is about five times larger than that for a graphite material under the same disruption conditions. Key differences between simulation experiments and reactor disruption on the net erosion rate, and consequently on the lifetime of the divertor plate, are discussed in detail. The advantages and disadvantages of Be over graphite as a divertor plasma-facing material are discussed.

Pulse shortening in high-power backward wave oscillators

Pulse shortening is a phenomenon common to all high-power microwave devices. Whereas in electron beam driven sources the electron beam propagation in the device may be for several microseconds or more, the microwave pulse duration is typically no greater than approximately 100 ns. Specific reasons for pulse shortening may vary among devices, but all explanation of the phenomenon put forth involve the introduction of plasma into the interaction region near the walls and/or the degradation of the beam quality. To gain a better understanding of pulse shortening in high power backward wave oscillators (BWOs), an investigation is being conducted at the University of New Mexico (UNM) on the UNM Long-Pulse BWO Experiment. Recent experiments have involved monitoring the beam current in the slow-wave structure (SWS) at different radii as a function of time. The current waveforms are correlated with the time histories of microwave pulses measured in separate experiments. The results reveal the appearance of electrons between SWS ripples at times corresponding to when the microwave signal peaks. A drop in the main beam current is observed shortly thereafter. Coatings of TiO2 and Cr have been placed on the inner surface of the SWS in an effort to suppress electron emission. Initial results with the TiO2 coatings have shown a measurable increase in microwave pulse width.

Plasma gun experiments and modeling of disruptions

Abstract Potentially high erosion due to ablation from plasma disruptions looms as a nemesis for advanced fusion devices such as ITER. Under some conditions, it is believed that the material ablated during a disruption forms a “vapor shield” that mitigates subsequent ablation. This paper presents new experimental data that identifies an absorption surface in the ablating plasma that is above the irradiated armor surface. We also present a summary of progress in modeling using a 1-D Lagrangian hydrodynamics code. Experimental erosion data will be reviewed from the PLADIS facility, a plasma gun at the University of New Mexico on several materials, including Be, tungsten, copper, graphites. Profile measurements of the crater topology showed the eroded Be surface to be much rougher than that of carbon and to demonstrate erosion rates that were almost factors of four greater than graphite. Plasma guns at the D.V. Efremov Scientific Research Institute and at TRINITI, both in the Russian Federation, are being utilized to confirm spectroscopic vapor shield data.

Diagnostics and analysis of incident and vapor shield plasmas in PLADIS I, a coaxial deflagration gun for tokamak disruption simulation

Tokamak disruption simulation experiments have been conducted at the University of New Mexico using the PLADIS I plasma gun system. Earlier work had characterized the plasma-surface interaction in terms of parameters such as incident energy from bucket calorimeter measurements and rough measurements of beam area from flat damage targets. A variety of new plasma diagnostics have been used to further investigate the characteristics of the incident plasma beam and vapor shield plasma in a simulated tokamak disruption. These diagnostics have included laser interferometry, two-color pyrometry, emission spectroscopy, and other methods to quantify the characteristics of the incident and vapor shield plasmas of a simulated tokamak disruption. The synthesis of different beam area measurement techniques is used to determine the radial structure of the plasma beam. Vacuum ultra violet spectroscopy is used to determine the thickness and internal structure of the vapor shield plasma. Results from two-color optical pyrometry and surface pressure measurements are used to determine the dynamics of vapor shield formation.

Results from the US/USSR exchange for heat load material studies of simulated disruptions

The motivation behind exchange I.2 of the USSR/US exchange program of cooperation for magnetic confinement fusion is to more closely simulate tokamak disruptions with a variety of plasma devices within the Soviet Union and the United States and to characterize the effect these simulated disruptions have on candidate PFC materials. Earlier work conducted in the Soviet Union by a team of Soviet and American researchers showed ablation of graphites exposed to a disruption like heat flux from a plasma flow was significantly less than that previously expected [J.M. Gahl et al., Proc. ICFRM-5, J. Nucl. Mater. 191–194 (1992) 454]. Work has continued and results from recent work at the University of New Mexico are in general agreement with earlier results from the Soviet Union. New results from work in the United States and the Soviet Union will be presented.

Test wire for high voltage power supply crowbar system

The klystron microwave amplifier tubes used in the low energy demonstration accelerator (LEDA) and to be used in the accelerator production of tritium (APT) plant have a strict upper limit on the amount of energy which can be safely dissipated within the klystrons vacuum envelope during a high voltage arc. One way to prevent damage from occurring to the klystron microwave amplifier tube is through the use of a crowbar circuit which diverts the energy stored in the power supply filter capacitors from the tube arc. The crowbar circuit must be extremely reliable. To test the crowbar circuit, a wire that is designed to fuse when it absorbs a predetermined amount of energy is switched between the high voltage output terminals. The energy required to fuse the wire was investigated for a variety of circuits that simulated the power supply circuit. Techniques for calculating wire length and energy are presented along with verifying experimental data.