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The near-electrode region is important to the design of high-pressure electric arc heaters. An understanding of the detailed physics of the discharge in the immediate region around the attachment to the electrode surfaces, where there is a rapid transition from gaseous to solid-state conduction of electrical current, is key to understanding such issues as electrode erosion and survival. A continuum model for the near-electrode region at high pressure is presented, extending from the flow region all the way into a copper electrode. The gaseous part of the model is treated with a one-dimensional approximation, and the heat conduction in the solid material is analyzed in a three-dimensional reference frame that moves with the arc attachment spot. Experimental data on the near-electrode region of high-pressure arc heaters are very limited, they mainly consist of dimensions of arc tracks and mass-loss measurements. Incorporating a field-emission boundary condition at the cathode spot and a nonequilibrium electron temperature at the anode spot yields results that agree with the data. Nomenclaturewidth of square spot, m CR = current contraction parameter, Eq. (2) d = constant, Eq. (12) C 2 = constant, Eq. (12) c = average molecular speed, m/s d = diameter, mm E = electric field, V/m e = electron charge, C 7 = current, A J = current density, A/cm 2 K = thermal diffusivity, cm 2 /s k = thermal conductivity, W/cm K k B = Boltzmann constant, J/K L c = thickness of current concentration zone, cm m = molecular mass, kg m = mass rate, kg/s n = number density, cm~3 P = pressure, atm Q -collision cross section q = heat flux, MW/cm 2 r = radius, cm T = temperature, K t -time, s U = arc spot velocity, m/s u = dummy variable of integration, Eq. (9) X = normalized coordinate, Uxl2K jc, y, z -Cartesian coordinates, origin on surface of electrode, z normal to surface, + outward for gas phase solution, + inward into copper for solid-state solution; x is + in direction of £/; y is transverse to U, m Y = normalized coordinate, Uy/2K Z = normalized coordinate, Uzl2K AT = temperature rise in copper, K 8 h = nonelastic collision factor e 0 = permittivity of free space, 8.85 X 10~1 2 F/m IJLQ = magnetic permeability of free space, 477 X 1(T 7 H/m or = electrical conductivity, (fl cm)" 1 <£ = work function for copper, eV Subscripts AN = anode a = arc CA = cathode Cu = copper c = current concentration zone cond = condensation Eq = equilibrium e -electron evap = evaporation / = flow affected region g = heavy gas particle s = spot w = wall 0 = reference value

Time-resolved spectral radiance emission measurements of the arc in the AEDC HEAT H1 Facility were successfully performed during the months of April and May of 1991. The HI Arc Heater is a segmented, vortex-stabilized arc heater used for materials testing under high enthalpy and pressure regimes. Emission spectra of the arc were acquired through a joint effort of Sverdrup Technology, Inc. and Calspan Corporation during a series of 15 HEAT HI technology tests in supprt and under the direction of Dr. Orlando Hankins. Data was acquired for steady-state arc currents of 700 to 1200 A at neutral pressures of 40 to 120 atmospheres. Analysis of the spectra has revealed 2 nmher of ztonic mission lines of drgon, nitrogen, and oxygen on top of a continuum emission. The temporal increase of the plasma density, obtained from Stark broadening measurements, and the evolution of the electron temperature determined by comparing measured line intensity ratios to collisional-radiative model calculations will be described.