Axial motion of collector plasma in a relativistic backward wave oscillator

Abstract

In this paper, it is proposed that plasma formed at the collector may drift back to the cathode and cause pulse shortening of the relativistic backward wave oscillator. Theoretical analysis shows that the axial drift velocity of plasma ions can be up to 5 mm/ns due to the presence of space charge potential provided by an intense relativistic electron beam. Particle-in-cell simulations indicate that the plasma electrons are initially trapped around the collector surface. With the accumulation of the plasma ions, a large electrostatic field forms and drives the plasma electrons to overcome the space charge potential and enter the beam-wave interaction region along the magnetic field lines. As a result, the beam current modulation is disturbed and the output microwave power falls rapidly. The plasma ions move in the beam-wave interaction region with an average axial velocity of 5–8 mm/ns. After the plasma ions reach the diode region, the emitted current at the cathode rises due to the charge neutralizations by the ions. The impedance collapse leads to further decrease of the microwave power. In experiments, when the diode voltage and beam current were 850 kV and 9.2 kA, and the collector radius was 2.15 cm, the output microwave power was 2.4 GW with a pulse width of less than 20 ns. The ion drift velocity was estimated to be about 5 mm/ns. After an improved collector with 3.35 cm radius was adopted, the pulse width was prolonged to more than 30 ns.

Current loss of magnetically insulated coaxial diode with cathode negative ion

“…In this model, the movements of neutral particles and their mutual interactions are neglected for simplicity. [29,30] Based on the electron impact-ionization model, [29][30][31] the cathode plasma including ions and electrons are generated when an electron beam propagates through the neutral gas layer and ionizes the gas. Similarly, the cathode negative ions (H − ) are produced when the backscattering ions (H + ) bombard and ionize the residual neutral gas according to the ion impact-ionization model as the dominant negative ion production mechanism.…”

“…7(c) and 8(c). The average negative ion velocity that depends on the space potential across the A-C gap can be calculated by ig-noring the weak influence of the magnetic field as follows: [31]…”

“…[19,20] However, we do not observe an apparent impedance collapse except in the case of 0.2 T. The cathode negative ions and the anode plasma may be the main factors leading to the current loss. [20] The effect of the anode plasma from the anode tube wall except for that formed at the collector [31] will be studied further.…”