J.G. Gilligan

Vapor shielding of surfaces subjected to high heat fluxes during a plasma disruption

Abstract An incident high heat flux on a surface can produce a copious amount of ablated vapor. Vapor shielding occurs when the ablated surface material absorbs the incoming energy and reduces the total energy transported to the ablating surface. The effectiveness of the vapor shield in reducing the incident heat flux has been calculated for the thermal dump phase of the plasma disruption. A one-dimensional, time dependent Lagrangian MHD code with the multifrequency group flux-limited diffusion model for the radiative energy transfer is used for the analysis of the energy transport in the ablative vapor shield. Results show that about 9% of the 3 eV blackbody radiation actually reaches the surface of an “iron” wall after a 10 μs pulse. Parametric studies have been done for several different surface materials and incident energy fluxes.

Time-dependent numerical simulation of ablation-controlled arcs

A zero-dimension (0-D) time-dependent code (ZEUS) developed to simulate ablation-controlled arc (ACA) behavior is discussed. The code includes energy transport, equation-of-state, and electrical resistivity models. Particular attention is given to the equation-of-state and the determination of the charged state of multicomponent plasma under local thermodynamic equilibrium (LTE) conditions. The 0-D model is self-consistently solved by the fourth-order Runge-Kutta method. The numerical simulation of ZEUS was compared with both experimental and other theoretical results. Comparison with experimental results demonstrates that the numerical simulation can correctly predict the behavior of ACAs. >

Numerical simulation and experiment of plasma flow in the electrothermal launcher SIRENS

An electrothermal plasma source (ET) may be used as a launcher by itself, or as a preinjector for electromagnetic launchers (railguns) or electrothermal-chemical (ETC) launchers. The characteristics of the injected plasma may affect the performance of the plasma armature (EMs) or the combustion process (ETCs). A 1-D, time-dependent fluid dynamics code, ODIN, has been developed to model the plasma formation and flow in the source and the barrel of the ET launcher SIRENS. The code models the energy transport, particle transport, plasma resistivity, plasma viscosity, and the equation-of-state. The measured mass ion of the ablating liner in the source section is in good agreement with predicted by the code. Comparisons between the measured and predicted pressures inside the barrel are in good agreement. >

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. >

A Study of Plasma Parameters in a Capillary Discharge With Calculations Using Ideal and Nonideal Plasma Models for Comparison With Experiment

A study of the plasma parameters in a capillary discharge was conducted using an experimental electrothermal plasma facility. The experimental results are compared to calculations using ideal and nonideal formulas of the Coulomb logarithm in the plasma electrical conductivity model to determine the nature of the plasma regime. Calculations are compared to the measured ablated mass, the measured electrical conductivity. Other calculated parameters are compared to results from similar and typical discharges. The measured ablated mass falls in between the ideal and nonideal calculations suggesting that the plasma is neither ideal nor nonideal; however, the linear fit of the experimental and calculated values shows divergence in the ideal calculations at higher peak currents. Measured plasma electrical conductivity is close to the ideal model predictions at lower values of the peak discharge current and approaches the nonideal model predictions at higher peak currents; the shape of the measured conductivity follows that of the nonideal model.

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. >

The use of an electrothermal plasma gun to simulate the extremely high heat flux conditions of a tokamak disruption

Disruption damage conditions for future large tokamaks like ITER are nearly impossible to simulate on current tokamaks. The electrothermal plasma source SIRENS has been designed, constructed, and operated to produce high density (> 1025/m3), low temperature (1–3 eV) plasma formed by the ablation of the insulator with currents of up to 100 kA (100 μs pulse length) and energies up to 15 kJ. The source heat fluence (variable from 0.2 to 7 MJ/m2) is adequate for simulation of the thermal quench phase of plasma disruption in future fusion tokamaks. Different materials have been exposed to the high heat flux in SIRENS, where comparative erosion behavior was obtained. Vapor shield phenomena has been characterized for different materials, and the energy transmission factor through the shielding layer is obtained. The device is also equipped with a magnet capable of producing a parallel magnetic field (up to 16 T) over a 8 msec pulse length. The magnetic field is produced to decrease the turbulent energy transport through the vapor shield, which provides further reduction of surface erosion (magnetic vapor shield effect).

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). >

Time‐dependent simulation of weakly nonideal plasmas in electrothermal launchers

Simulation of electrothermal launchers is extended to include nonideal plasma effects. The simulation is compared to experimental data obtained from the SIRENS facility, for the measurable quantity of ablation depth as a function of the input discharge energy [IEEE Trans. Plasma Sci. PS‐17, 386 (1989)]. The plasma model for nonideal plasma is based on the evaluation of a non‐Debye radius due to incomplete screening of the shielding cloud around a charged particle. The consistent and necessary transport and thermodynamic functions for weakly nonideal plasmas are included in the time‐dependent set of governing equations. In this paper a semiempirical scaling law for the fraction of blackbody radiation that is transmitted through the ablating vapor for Lexan insulators under high heat flux conditions is also presented.

Modeling of particulate production in the SIRENS plasma disruption simulator

Abstract Modeling of the complex interplay among plasma physics, fluid mechanics, and aerosol dynamics is critical to providing a detailed understanding of the mechanisms responsible for particulate production from plasma–surface interaction in fusion devices. Plasma/fluid and aerosol models developed for analysis of disruption simulation experiments in the SIRENS high heat flux facility integrate the necessary mechanisms of plasma–material interaction, plasma and fluid flow, and particulate generation and transport. The model successfully predicts the size distribution of primary particulate generated in SIRENS disruption-induced material mobilization experiments.