1. Consider some of the principal results of the experimental investigations of an arc in a vacuum under conditions in an MHD channel required for further analysis. The most reliable and accurate characteristic of the arc, the cathode potential drop, uk, has been determined with an error not exceeding 1 70for solid and film cathodes made of many different materials [25, SI33]. It was determined that the voltage at the arc undergoes temporal oscillations. However, its minimum value remains at a constant level, which is assumed to be the cathode potential drop, uk. It depends weakly on the current on solid cathodes and is completely independent of current on film cathodes, where the cathode drop is a function of the thickness of the film. The cathode potential drop is somewhat lower for a copper film on a copper substratum (uk= 11 V) than for solid copper cathode (uk« 15 V) [25, 31-33], Only the effective nearelectrode potential drop was measured under conditions in MHD generators in the presence of constricted discharges. Depending on conditions in an MHD generator channel, the effective near-electrode potential drop, consisting of a sum of potential drops in the space charge zone, ionization zone (analog of uk) and the spreadout zone may reach several hundred volts [2, 4, 10, 12, 13, 34-37]. No direct measurements of ukare available under these conditions. Indirect, calorimetric data on water cooling of electrodes indicate that the potential drop decreases somewhat with increasing current and is on the order of 17 V [13, 37].
2. Experimental data on erosion rates of various metal cathodes (parameter G) for arc discharges in vacuum and in air were acquired in the range of currents of 10-103A [25, 27, 30, 38-46]. The principal result of these measurements is the determination of two characteristic regions as a function of erosion by the discharge current. At low discharge current (I0< 100 A), the cathode material erosion rate at room temperature depends linearly on the current and is relatively low. According to various data, the electron transfer coefficient lies within the range of 104-106g/C. At high currents (between 102and 103A) the rate of cathode material erosion in vacuum increases exponentially, and is on the order of 103-10-1g/s, or 104g/C.
3. Erosion of electrodes operating in a constricted discharge mode, under conditions in an MHD facility, has been investigated [1, 11, 13, 47-49]. It should be noted that the erosion rate of cooled copper electrodes of an MHD facility (T0* 500 K) is considerably lower than in vacuum (or in air), varying between 107-106g/C at a current I0= 5-102A. As the electrode temperature increases to T0^ 1000 K, the cathode erosion rate may increase to 104g/C.
4. One of these is the method of autographs [50-56], based on the relationship between the diameter of the current-conducting area and the width of the trace caused by thermal interaction of the spot with the cathode surface. The current density determined by using this method for various metals lies between 104-108A/cm2and reportedly [56] may even be as high as 1011A/cm2.
5. Another method of determining j, frequently referred to as the optical method, involves high speed photography of the luminescent region of the cathode spot and identification of its dimensions with the current conducting channel [57-59]. This method makes it possible to investigate simultaneously the dynamics of variation of the external form of the cathode spot, and to determine the displacement rate along the surface of a solid cathode [39, 40].