Discharge Feature and Acceleration Mechanism of Magnetoplasmadynamic Thrusters

为研究轴向磁场对准稳态磁等离子体动力推力器性能和放电特性的影响，了解其加速机理，对外加磁场的准稳态磁等离子体动力推力器进行了研究。主电源脉冲形成网络（PFN）能够在8 KV下存储62 KJ，提供最大27 KA的单次非可逆准稳态电流，脉冲宽度为0.6 msec。在推进器点火的地方，有一个长5.75米、直径0.6米的真空槽，每次放电前，真空槽的真空度约为10^<-3> Pa。脉冲轴向磁场由一个小匝线圈施加，该线圈与一个独立于主放电电路的PFN相连。H_2、N_2+2H_2和Ar混合气体的推力特性表明，H_2和N_2+2H_2混合气体的推力随轴向磁场强度的增大而增大，但在H_2和Ar较低的放电电流水平下，每种气体的推力都有一个最优的轴向磁场强度，此时每种气体的推力都能达到最大。随着轴向场强的增加，各气体的放电都倾向于向上游发生，导致放电室内的高温分布较宽。推测轴向磁场对推力器性能和电弧特性的影响是由于-J_rxB_z的旋转运动，即涡流加速和热化增强。

Quasisteady magnetoplasmadynamic thrusters with applied magnetic fields were studied to clarify the influence of axial magnetic fields on the thruster performance and the discharge feature and to understand the acceleration mechanism. The main power-supplying pulse forming network (PFN), which was capable of storing 62 KJ at 8 KV, delivered a single nonreversing quasisteady current of maximum 27 KA with a pulse width of 0.6 msec. A vacuum tank 5.75m in length and 0.6m in diameter, where the thruster fired, was evacuated to some 10^<-3> Pa prior to each discharge. Pulsed axial magnetic fields were applied by a few-turn coil, which was connected with a PFN independent of the main discharge circuit. An increase in axial field strength raised the discharge voltages at constant discharge currents below the limiting current with H_2, mixture of N_2+2H_2 and Ar. The thrust characteristics for H_2 and mixture of N_2+2H_2 showed that there was the optimum axial field strength with which the maximum thrust was achieved for each gas, although at low discharge current levels for H_2 and Ar the thrusts increased with axial field strength. The discharges for all gases were inclined to occur more upstream with an increase in axial field strength, resulting in broad high-temperature distributions in the discharge chamber. It was inferred that these effects of axial magnetic fields on the thruster performance and the arc feature were due to rotating motion of -J_rxB_z, that is, swirl acceleration and enhanced thermalization.