O. Hankins

Vapor shielding and erosion of surfaces exposed to a high heat load in an electrothermal accelerator

The experimental and theoretical verification of the vapour shield concept in an electrothermal accelerator is reported. Measurements of the ablation mass loss of the insulator material (Lexan) demonstrate that the energy transmission factor through the vapor shield is of the order of 10%. If enough vapor is produced under high heat flux, the vapor shield mechanism will help limit the surface erosion in electromechanical devices. The erosion of the barrel material (aluminum) has a strong axial dependence, and the erosion depth increases with input energy. >

Visible light emission measurements from a dense electrothermal launcher plasma

Measurements of the visible light emission from dense, weakly nonideal plasmas have been performed on the experimental electrothermal launcher device SIRENS. The plasma is created by the ablation of a Lexan insulator in the source, which then flows through a cylindrical barrel which serves as the material sample. Visible light emission spectra have been observed both in-bore and from the muzzle flash of the barrel, and from the flash of the source. Recent measurements along the axis of the device indicate time-averaged plasma temperatures in the barrel of about 1 eV for lower energy shots, which agree with experimental measurements of the average heat flux and plasma conductivity along the barrel. Measurements of visible emission from the source indicate time-averaged temperatures of 1 to 2 eV which agree with the theoretical estimates derived from ablated mass measurements and calculated estimates derived from plasma conductivity measurements. >

Studies to reduce material erosion in electrothermal launchers

Plasma erosion processes on insulators and conductors, using the SIRENS electrothermal launcher, have verified the vapor shield concept. The energy transmission factor through the vapor shield was found to vary from 20% to 5% as the heat flux increases. Metals have strong axial erosion dependence, with an average erosion depth of 15-45 mu m/kJ for aluminium and 5-10 mu m/kJ for pure copper. Insulators have uniform ablation along the axial direction, with an average ablation depth of 10-14 mu m/kJ for Lexan. Aluminium has a higher erosion rate with an increase of energy input, while Lexan and pure copper have approximately equal erosion rates which are considerably less than that of aluminium. High-density graphite does not ablate at lower energies, and ablates only slightly at energies above 3 kJ (1-2 mu m/kJ), while molded dense electrographite ablates at a higher rate (1-3 mu m/kJ). Both types of graphite have considerably less ablation than other materials. Lexan and graphites showed greater evidence of the vapor shield effect than aluminium and copper, although there is tendency towards less erosion at higher values of heat fluxes. Multiple exposure of material surfaces demonstrated that insulators have better performance than metallic surfaces. The initial indications for the effect of the magnetic field applied parallel to the material surface revealed a threshold for the onset of the magnetic vapor shielding effect (above 5 T for Lexan). >

Observations of visible light emission from interactions between an electrothermal plasma and a propellant

A set of experiments has been conducted on the experimental plasma-propellant interaction facility PIPE to measure the burn rates of JA2 solid propellant as a function of plasma impingement angle with respect to the propellant surface. The propellant test stand is designed such that the angle of plasma impact on the propellant surface can be varied 0 to 90 degrees. Optical emission spectroscopy measurements were taken during these shots. Fiber optics were situated in two places, one closer to the plasma source and the second at the propellants rear edge near the point of impact of plasma and propellant. The fiber optics are connected to an optical multichannel analyzer set to observe neutral copper lines and molecular C/sub 2/ Swan Bands in the 505-585 nm spectral region. Time-averaged core are temperatures of 8,800 to 14,000 K and plasma densities of 2/spl times/10/sup 25/ to 4.5/spl times/10/sup 23/ m/sup -3/ have been deduced by measurements along the axis of the device using the relative intensities and the Stark broadening of the copper lines. As the angle of plasma impact with the propellant surface increases from D to 90/spl deg/, the overall radiant emission increases, falls and increases. However, the measured plasma temperature decreases gradually and plasma density falls by nearly a factor of two before increasing substantially at 90/spl deg/. Comparisons with the measured burn rates of the JA2 propellant versus inclination angles suggests a stronger correlation of plasma burn rate with plasma kinetic energy than with the radiative heat flux.

Theoretical and experimental studies of the vapor shielding mechanism for surfaces subjected to high heat fluxes

The self protecting nature of the vapor shield mechanism is important for prediction of surface damage caused by disruptions or run away events in tokamaks. The authors report on detailed modeling and experimental analysis of the vapor shield (VS) mechanism. The energy transmission factor through the VS boundary layer is found to be surprisingly low (about 10%) which may reduce estimates for the surface damage.

Vapor shield protection of plasma facing components under incident high heat flux

Disruption damage to plasma facing components has been found to be a limiting design constraint in ITER and other large fusion devices. A growing data base is confirming the role of the vapor shield in protecting ablated surfaces under disruption-like conditions, which would imply longer lifetimes for plasma facing components. We present new results for exposure of various material surfaces to high heat fluxes up to 70 GW/m 2 over 100 μs (7 MJ/m 2 ) in the SIRENS high heat flux test facility. Tested materials are graphite grades, pyrolytic graphite, refractory metals and alloys, refractory coatings on copper substrates, boron nitride and preliminary results of diamond coating on silicon substrates. An empirical scaling law of the energy transmission factor through the vapor shield has been obtained. The application of a strong external magnetic field, to reduce turbulent energy transport in the vapor shield boundary, is shown to decrease f by as much as 35% for fields of 8 T.

Comparative study of component erosion for electromagnetic and electrothermal launchers

The electrothermal launcher SIRENS has been used to study the erosion of critical components (rails and insulators) of plasma-driven launchers. SIRENS can produce high-density (>10/sup 25//m/sup 3/) low-temperature (1-3 eV) plasma, formed by the ablation of the insulator (Lexan), with currents up to 100 kA. The incident heat flux varies between 2 and 90 GW/m/sup 2/ over 100 mu s duration, for input energies of 1-10 kJ. Erosion studies have been performed on several insulators, pure and coated metals, alloys, and several graphite grades. The fraction of the total incident energy transmitted to the eroded surface varies from 12 to 30% for the materials tested and decreases to 5-7% as the incident energy fluence increases. Such reduction in erosion for a given incident fluence is due to the vapor shield effect. The scaling law for the energy transmission factor through the vapor shield layer was obtained for the exposed materials. >

Magnetic vapor shielding mechanism in electromagnetic and electrothermal launchers

The vapor shield formed by the ablation of wall materials in plasma-driven launchers absorbs a large fraction of the incoming energy and reduces the surface erosion. Decreasing the turbulent energy transport through the vapor shield may result in further reduction of the surface erosion. Applying a strong magnetic field parallel to the surface (magnetic vapor shield) is a possible approach. SIRENS, an electrothermal launcher, is equipped with a pulsed high intensity magnet, and has been used to investigate the magnetic vapor shield effect. The magnetic field has been applied at different field intensities (2.6 to 8.75 T), for launcher input energy varying from 1 to 5 kJ. A 25-35% reduction of the surface ablation has been achieved at 6.25 T for energy inputs of 1-2 kJ, with a trend of further reduction at higher values of the magnetic field. >

Performance of Graphites and Tungsten during Exposure to Pulsed Plasmas

AbstractHeat fluences of 1–10 MJ/m2 and greater over 0.1–1 msec pulse durations are expected on the surfaces of plasma-facing components in large tokamaks during a plasma disruption. The formed vapor plasma (the boundary layer) absorbs a large fraction of the incident energy, and thus acts as a self protecting layer (vapor shield). Carbon materials (pyrolytic graphite and other graphite grades)) are used as plasma-facing components, and tungsten and refractory materials are potential candidates. The experimental test facility SIRENS has been used to expose carbon and tungsten materials to heat fluences between 0.2 and 6 MJ/m2 for 100 µs duration to characterize the performance of such materials under typical heat loading conditions.

Studies of material surface erosion exposed to plasmas in an electrothermal launcher

Summary form only given. The experimental electrothermal launcher device SIRENS has been operated to measure the erosion of material surfaces subjected to high-heat flux from a high-density, low-temperature plasma (1-3 eV) with a strong applied magnetic field. High-heat flux erosion studies have been performed on Lexan, graphite, copper, brass, stainless steel, and aluminum samples. Initial analysis of the samples indicates a strong axial erosion dependence for metallic surfaces as opposed to uniform ablation for insulators. Surface conditioning through multiple exposure to repetitive discharges showed a decrease in the rate of ablation for a Lexan insulator and an increasing erosion rate for metallic surfaces. Externally applied parallel magnetic fields of up to 10 T were applied in experiments with Lexan samples to demonstrate the magnetic vapor shielding phenomenon. The result indicates a threshold for the onset of the magnetic vapor shielding effect. For the field above the threshold, a reduction of the surface ablation of the sample was observed, implying a reduction of the energy transport through the vapor shield with applied field