The retrograde motion of arc cathode spots in vacuum

Abstract

Experiments are reported on the retrograde arc spot motion on copper and tantalum cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolutions of <100 ns and <10 µm, respectively. The magnetic flux density amounted to B = 0.4 T and the arc currents to 2-100 A. For times <1 µs random displacement occurs on a time scale <100 ns. At intervals of about 4 µs, jumps of the spot are observed over distances of 50-300 µm in the retrograde direction, thus yielding macroscopic velocities of about 50 m s-1. The jumps are preceded by the ejection of plasma jets in the retrograde direction, having average velocities of about v = 5 km s-1. New spots are formed exactly in the jet direction. The jets are explained by instabilities in the magnetically confined spot plasma, and the spot formation by electric fields E⃗ = v⃗×B⃗ within the jets. The jets are ejected in periods of enhanced plasma production caused by the inner spot processes, i.e., by the dynamics of fragments and cells, having diameters of ⩽20 and ⩽10 µm, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100 K.

Energetics of vacuum arc cathode spots

“…7 Cathode spot processes are essentially of non-stationary nature, and time constants can be well below the microsecond region. 8 Although much progress has been made in determining the elementary processes of cathode spots, much controversy remains due to small spot dimensions, short characteristic times, a broad variety of materials, vacuum conditions and surface states, discharge history, electrical circuit parameters, geometry effects, and the presence, strength, and direction of magnetic fields.…”

Cathodic arcs: Fractal voltage and cohesive energy rule

“…Much research, both theoretically [2][3][4][5][6][7] and experimentally [8][9][10][11][12][13][14] , has been done for decades to understand the spot phenomena that produce non-equilibrium plasmas containing multiply ion charge states. The plasma is mesosonic 15 , i.e., subsonic for electrons and supersonic for ions, with respect to the local ion sound velocity.…”

Влияние Параметров Разряда На Генерацию Ионов Дейтерия В Плазме Сильноточной Импульсной Вакуумной Дуги С Композиционным Катодом Из Дейтерида Циркония

“…Для хорошо очищенных катодов инициирование новых катодных пятен происходит в основном на бруствере кратера (см., например, [22]), образованном вытеснением жидкометаллической фрак-ции при функционировании старого катодного пятна. Как показали наши расчеты, эта фракция практически не содержит дейтерия из-за его термической десорбции.…”