The effect of cathode deuteration on the parameters of vacuum-arc plasma

Abstract

A source of ions based on vacuum discharge is the main element of vacuum neutron generators (neutron tubes) [1]. The electrodes of this source-cathode and anode-contain the working gas (hydrogen isotopes) in occluded state. The evolution of deuterium and for mation of its ions takes place upon the initiation of a short time (~1 μs) spark arc discharge between the cathode and anode. This discharge is accompanied by erosion of the surface of electrodes, which leads to the generation of working gas ions and is a desired process, and by evaporation of the electrode base metal, which is a harmful side effect. Optimization of the working characteristics of an ion source-i.e., the specific ion yield (the ratio of the number of working gas ions to energy consumed per pulse of discharge), working life, and stability-requires adequate description of the discharge.The present Letter proposes a model for determin ing the influence of the relative content of deuterium in a zirconium cathode on the properties of vacuum arc discharge plasma. The model is based on the ecton mechanism of the vacuum arc cathode spot operation [2]. According to this mechanism, vacuum arc plasma is formed as a result of the operation of separate cells (explosive emission centers) in a cathode spot of the vacuum arc. At present, the existence of these cells is commonly accepted [2][3][4]. The arc current buildup is accompanied by growth in the number of simulta neously operating cells of the cathode spot. For this reason, the parameters of ion flux are independent or weakly dependent on the arc current up to a kiloam pere level [5].The cell current in a cathode spot amounts to sev eral amperes, and the current density at the base of a plasma jet is on the level of 10 8 A/cm 2 [2, 3], which is characteristic of the explosive electron emission. The possibility of realization of the explosive electron emission conditions in the cathode spot of vacuum arc has been discussed for a long time. However, at present, it can be ascertained that the interaction of dense near cathode plasma with microinhomogene ities on the cathode surface unavoidably leads to the initiation of explosive electron emission [6][7][8].According to the proposed model, the cathode spot of a deuterated zirconium electrode features simulta neous erosion of deuterium and zirconium. It is assumed that the ratio of erosion rates of these compo nents is determined by the degree of electrode satura tion with deuterium and does not change with the time. Most probably, these assumptions are not gener ally valid because of an extremely complicated charac ter of sorption and desorption of hydrogen isotopes and a sharp dependence of the rates of their diffusion and desorption on the temperature. Nevertheless, these premises are quite acceptable at the initial stage of operation of the ion source, when the electrodes are not strongly heated. Thus, cathode spots directly sup ply deuterium to vacuum arc plasma. In addition, we assume that plasma expansion is spherically symmet ric and, hence, a one ...

Model calculation of the charge composition of a plasma in a vacuum-arc discharge with a composite cathode

“…Equa tions (6) were written for Ti n and Al n components with n = 0, 1, …, 4. Despite the simplified approach to describing ionization, these calculations of the com position of the ionized arc plasma give results that are quite consistent with the experimental data [7,10,13] and with detailed kinetic calculation data [14]. This allows us to hope that the proposed simple method will also yield at least qualitatively correct results for a dis charge plasma with a composite cathode.…”

“…In addi tion, the results of Zhirkov et al [2,9] show that ions of various sputtered components have almost identical velocities. The results of our recent model calculations [10] for a three component plasma showed that the drift velocities of ions with even significantly different masses (zirconium and deuterium) exhibit leveling as a result of strong ion-ion interaction in a dense near cathode plasma. For this reason, the proposed model assumes that ions of different components have the same velocity and, hence, the plasma can be described using the two temperature single fluid approximation for ions with effective mass m i = m Ti (1 -x) + m Al x.…”

On plasma jet formation in vacuum arc with composite cathode

“…If due to some treatment the mixture of elements is improved then the ions of the different elements can be produced in one cathode spots. In this case the ions of different mass will have the same average velocity due to very strong ion-ion collision in dense cathode spot plasma [10]. In the framework of the model of [11] the calculated average velocity of Zn-Pb plasma was 0.61*10 6 cm/s for all ion types.…”

Plasma mass-charge composition of a vacuum arc with deuterium saturated zirconium cathode

“…The maximum peak of spectrum is displaced from Zr 3+ to Zr 2+ , and the share of Zr 4+ ions significantly decreases. Such a behavior of the average Zr ion charge under deuteration of cathode was pre dicted in the framework of model [5] based on the ecton functioning mechanism of cathodic spots in a vacuum arc [6][7][8][9]. According to the results obtained within this model, a decrease in the average charge of Zr ions is caused by the additional energy spent on the ionization of deuterium atoms.…”