A comparison of electron-emission equations used in arc - cathode interaction calculations

Abstract

The predictions of a numerical model of the cathode-sheath region of high-pressure arcs using three different equations for the cathodic electron-emission current density are compared for two typical arc - cathode systems. These equations for cathodic electron emission include: (i) the field-enhanced thermionic emission (FEE) equation representative of the electron emission for high temperatures and moderate electric field strengths ; (ii) the Murphy and Good (MG) equation for thermo-field (T-F) emission valid for high temperatures and high surface electric field strengths in the range about - ; and (iii) the Murphy and Good equation for T-F emission enhanced by the presence of a high density of slowly moving ions in the cathode region (the MG+I equation). Results show that, in the case of a metal vapour arc (representative of vacuum arcs and high-pressure arcs on non-refractory cathodes) the use of the MG+I equation is always prescribed due to the formation of the high-local-pressure cathode-spot plasma. For high-pressure arcs on refractory cathodes the results show that, at atmospheric pressure, the generally used FEE equation introduces an underestimation by at least around 20% of the cathodic electron current density compared with the MG+I equation. The same comparison but for an ambient pressure of 50 atm shows that this underestimation becomes considerable.

Electrode Evaporation and Its Effects on Thermal Arc Behavior

“…The exact value of the voltage drop in the non-LTE region is not known. Coulombe and Meunier [21] have used V for a metal vapor arc on a copper cathode. This value is adopted in the present investigation.…”

“…Yokomizu et al [1] measured the total electrode voltage drop, , which is the sum of the voltage drop across the cathode and the anode , in the current range 10-20 000 A. For copper electrodes with as the working gas, their results indicate that the total voltage drop in the two electrode regions is 17.5 V. Considering the value of 15 V used for the cathode [21], we used the approximate value of 2.5 V for the anode. This value and the amount of electrode vapor produced by anode vaporization are not crucial in our investigation since the anode is taken as the downstream electrode.…”

Theoretical and experimental determination of erosion rate due to arcing in electrical contacts

“…The cathode voltage fall is lower that the one taken in copper case, so we use V, = 12.3 V [13,14]. The ionisation potential is well defined [12] Vi = 7.576 V, and for the electron fraction the value generally used is s=0.8 [15][16][17][18].…”

Влияние Усиленной Электрическим Полем Термической Электронной Эмиссии На Температуру Катода С Тонкой Диэлектрической Пленкой В Дуговом Газовом Разряде

“…В газовых разрядах с металлическим катодом (а также в вакууме при наличии на катоде тонкой оксидной пленки) термополевая эмиссия возможна лишь в случае, если напряженность электрического поля у его поверхности превышает 10 8 Vm −1 [7][8][9][10], что может иметь место только при давлении рабочего газа на 2−3 порядка большем атмосферного [11], а при его давлении по-рядка атмосферного -в микронных межэлектродных промежутках (в микроразрядах) [12,13]. В условиях же, характерных для газоразрядных ламп (давление рабочего газа до 10 6 Pa и межэлектродное расстояние, превышающее 10 −3 m), напряженность электрического поля в разряде недостаточно велика, и термополевой механизм электронной эмиссии не играет заметной роли [6,7,14,15].…”