IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. PS-1l, NO. 3, SEPTEMBER 1983 Bouncing Expansion of the Arc-Cathode Plasma in Vacuum Along the Transverse Applied B Field J. L. MEUNIER AND M. G. DROUET Abstract-Measurements of the angular flux distribution of the ca- thodic arc plasma (I < 100 A) subjected to a transverse magnetic field (B < 8.5 X 10-2 T) ae presented. The angular distribution without magnetic field approximately follows the cosine law. Expansion with transverse field is found to deviate strongly from the cosine law, the plasma being confined close to the cathode plane and expanding along the magnetic-field lines. Furthermore, time-resolved photographs of the expansion reveal the predicted pulsating behavior of the expanding plasma. I. INTRODUCTION T HE VACUUM ARC has been studied for-many years with particular attention given to the physics related to the cathode region (see review in [1). Tanberg [2] in 1930 was the first to report that the cathode was a source of plasma and that vapor jets were emitted from the cathode spots of an arc. Since then, numerous studies have been carried out to charac- terize the composition of these jets, their velocities and their angular-flux distribution as a function of the arc current, the electrode material and the background pressure. Several tech- niques have been used to determine the time of flight, the energy of the emitted particles, the recoil, and the temperature of the cathode. Mass spectrometry has revealed that the expanding vapor cloud consists of multicharged metallic ions, neutral metallic atoms, and metallic particles. The high-speed flux (102-104 m/s) is anisotropic; the angular distribution cor- responding to the plasma component approximately follows the cosine law, i.e., an ion flux proportional to the cosine of the angle to the cathode plane normal, while the particle com- ponent lies mostly near the cathode plane. Studies of the influence of an axial or transverse magnetic field (parallel or perpendicular to the arc column) on the be- havior of the erosion products were also conducted and have revealed that an axial field has the effect of reducing the ion current collected at the walls of the chamber and of increasing the fraction of ions moving in the forward direction [3] - [5]. However, no measurement of the angular distribution of the plasma has been reported. The effect of a transverse magnetic field is different; it will tend to confine the cathodic plasma to its source, thus forcing it out of contact with the anode. This will have the effect of raising the arc voltage and lead to a possible extinction of the arc [61, an interesting feature with regards to circuit breakers. In addition, when the transverse field is applied, an alignment of the cathode spots is observed [7]; the spots without field being dispersed originally over the cathode surface. Manuscript received October 29, 1982; revised March 15, 1983. The authors are with the Institut de Recherche d'Hydro-Quebec, Varennes, Quebec, Canada JOL 2PO. Collectors Vacuum' chamber Camera Fig. 1. Vacuum chamber showing the electrodes arrangement and probe positions. The alignment of the spots observed with a transverse field has been attributed [81 to a preferential confinement of the cathodic plasma along the magnetic field lines; the resulting higher plasma density along these lines would lead to a higher probability of initiation of new current-emitting sites in the same direction than in other regions of the cathode surface. The purpose of the present study was to obtain evidence for this magnetic confinement of the expanding cathodic plasma. Photographic recordings of the shape of the expanding plasma with and without magnetic field are presented as well as the angular distribution of the wall current. In addition, high-speed streak photographs of the expanding plasma are obtained which reveal the time-resolved structure of the expansion. II. EXPERIMENT The experiment was performed in a spherical vacuum cham- ber 25 cm in diameter evacuated to 10-6 torr (Fig. 1). The arc, 1 mm long, is ignited by a high-voltage pulse between the two tungsten electrodes, each 2 mm in diameter. The anode is tapered while the cathode is flat and surrounded by a quartz tube in order to prevent the arc foot from moving away from the tip. An arc current of up to 100 A is provided by the dis- charge of a 0.033-F capacitor with a discharge time constant of 20 ms. A thyristor switching of the discharge gives an ap- proximately square shape to the arc current, which lasts from 25 to 400 Ms with a ramp-up of 50 Ms and a ramp-down less than 10 Ms, so that the arc current remains approximately constant during the discharge. A magnetic field ranging from 0093-3813/83/0900-0165$01.00 © 1983 IEEE 165