A.G. Ushakov

Limiting Parameters of the Plasma Opening Switch

Implementing programs for nuclear fusion research and X-ray generation requires the creation of superpower generators based on plasma opening switches (POSs) capable of commutating currents as high as several tens of megaamperes at output voltages of up to 5 MV and higher. The physical mechanisms limiting the POS voltage are investigated. It is shown that, as the generator voltage Ug increases, the voltage multiplication factor k = UPOS/Ug (where UPOS is the POS voltage) decreases. An explanation for such a dependence is proposed, and the maximum value of the POS voltage is estimated. A POS design that enables operating in the above current and voltage ranges is considered. The design is based on applying an external magnetic field to the POS interelectrode gap, increasing the initial generator voltage, and decreasing the linear (along the perimeter of the outer electrode) density of the charge passing through the POS during the conduction phase.

Microsecond plasma opening switches in externally applied magnetic field

The plasma opening switch (POS) is a key element producing nanosecond high-voltage pulses in many pulsed power systems using inductive energy storage. For POS experiments with microsecond conduction times described in the paper, an erosion regime is considered to be critical at the opening and power delivery phases. The technique to enhance the magnetic insulation of the POS gap and to increase the switching voltage using externally applied magnetic field is discussed. Examples of use of the applied field for microsecond sub-MA POS to achieve switching voltages up to 3 MV are reported. Further development of POS schemes at megaampere currents is discussed. A design of the new six-section POS is proposed for the 3.7 MA facility prototyping the 1/24 of the future concept of the multimegajoule Baikal generator.

Study of repetitive plasma opening switch generator technology

Repetitive plasma opening switch (POS) research for X-ray and electron beam generators for commercial applications is reviewed. This research was started on the RS-20 generator in 1991 with submicrosecond conduction times, 100 kA currents, and MV voltages. In the experiments wall-plug to electron beam production efficiency was increased from 10-50%. Maximum voltage obtained at POS was 3 MV starting from 0.8 MV at the Marx generator. POS operation was improved using an applied external magnetic field for the opening phase. Diode current and total generator efficiency were increased by combining the POS and diode system in one technology unit. For the opening phase of the high-impedance, repetitive POS, a set of conditions based on early POS experiments in the Kurchatov Institute were proposed. Repetitive POS generators have obtained peak X-ray dose rates of 1 MGy/s and electron dose rates of 10 GGy/s. This may be important for applications as pulsed irradiation may cause more efficient sterilization at lower doses. An X-ray generator based on repetitive POS technology at 5 MV and 200 kW in the electron beam is proposed.

High-Voltage Closing Switches Using Vacuum Electric Breakdown for Pulsed-Power Applications

In our previous research, a high-voltage vacuum closing switch has been proposed to synchronize the output pulses delivered from multiple parallel plasma opening switch modules to the load in a scheme of power conditioning of a generator of high-power nanosecond pulses. However, the breakdown voltage for each stage of the closing switch has to be increased to 150-300 kV to make the whole concept feasible. This paper considers experimental research of the electric breakdown of 1-mm vacuum gaps formed between the plane cathode and the sharp high-voltage anode to achieve higher breakdown voltages. Two electrode arrangements described in this paper include a point-to-plane system at 5 kA and a multichannel disk-shaped switch at 50 kA. Both systems were studied at voltages up to 170 kV, current densities up to 5 kAmiddotcm-2 , and submicrosecond pulse durations. A concept of a new closing switch capable of operating at 3 MV, 50 ns, and 0.3 MA using multiple sharp-anode vacuum gaps stacked in series is proposed on the basis of the research results

Efficiency study of repetitive POS-in-magnetic-field technology

The report describes possibilities to make existing repetitive plasma opening switch (RPOS) technology competitive with existing accelerator X-ray radiating systems. The use of an external magnetic field for magnetic insulation of the RPOS system allows to increase the achievable voltage in RPOS and diode and the total efficiency of the system from the wall plug to X-ray irradiation. The experiments on an RS-20 pulsed power generator with the use of an external magnetic field allows one to raise the RPOS voltage from 2-2.2 to 2.8-3 MV. Another option is to combine the RPOS and diode in one unit to avoid energy losses when switching current to the diode with a wall plug to e-beam efficiency of 50-70%. On this basis, a concept of a 5 MV RPOS X-ray generator is proposed, capable of producing 200-300 kW in an e-beam.

Repetitive plasma opening switch in the magnetic field

The report represents the results of experimental research into the plasma opening switch (POS) in an applied longitudinal magnetic field. Applying magnetic field on an area of the POS reduced the electron component of the current through the POS on a stage of sharp increase of the resistance. The POS which has been chosen for the experiments is one of the versions which the authors used in the repetitive pulsed power X-ray generator RS-20. The parameters of the the electron accelerator are as follows: electron energy 2.5 MeV; electron beam current 10-20 kA; pulse width 100 ns; and average power 10-20 kW. The scheme of the electron diode which is represented allowed X-ray flow focusing.

Electric strength of a vacuum gap during breakdown in an inhomogeneous electric field for switching high-power pulses

A system for matching the output of a multimodular plasma opening switch (POS) to a “liner”-type load having a small initial impedance is described. The main element of this system is an isolating spark gap that allows synchronization of the POS modules and prevents a repeated POS closure cutting off the inductive storage from the load. A spark gap based on a multigap explosive-emission diode, which was proposed and tested earlier, is insufficiently strong electrically to be used in the Baikal project aimed at the development of a superpowerful pulsed generator. An urgent problem of increasing the electrical strength of a 1-mm-wide vacuum gap, which is a basic element of the aforementioned spark gap, is being solved. It is proposed to use electrodes of the point (anode)-plane (cathode) type that allow the electric field to be concentrated at the point-type anode, thereby increasing the gap’s electrical strength. The results of experiments aimed at the study of vacuum breakdown in this system of electrodes are presented. It is shown that this design allows the gap’s strength to be raised by several times, a long service life of electrodes to be ensured thanks to the low energy-deposition density, and a multichannel (i.e., low-inductance) breakdown to be stably attained. A diagram of the spark-gap prototype is presented, its breakdown characteristics are obtained, and a full-scale scheme of the spark gap is presented.

Air injected high power repetitive electron beam for radiation treatment

Repetitive POS technology recently developed in the Kurchatov Institute may be applicable for various technology uses. Together with existing 2-3 MV 10-50 kW X-ray systems, a new facility operating in electron beam mode was recently developed for radiation sterilization applications. This compact system possesses 0.5-1 kJ/shot in electron beam of 0.5-0.8 MeV depending on the initial Marx charging voltage and may repetitively perform at 1 Hz frequency. Electron beam creates 1-10 kGy/shot in a surface layers of irradiated substances with dose rate at 5 GGy/s.

Anomalous resistivity effect in the plasma opening switch

Summary form only given. We consider the effect of anomalous resistance in the plasma opening switch which plays an important role in the conduction phase before sharp switching. This effect should be taken into account together with EMH and erosion mechanisms. A low frequency turbulence was studied experimentally by observing Stark broadening of hydrogen lines with enhanced spatial resolution implementing a Fabry-Perot interferometer. This plasma diagnostic was similar to that we used in previous experiments. H/sub /spl alpha// line contour (6563 A) observed was compared with modeling of ion-acoustic oscillations with adding of ion micro-fields (Holzmark) together with Doppler broadening and allowed us to estimate the turbulent field and its localizing during the conduction phase. As a result a turbulent electric field in the POS gap was estimated to be 10-30 kV/cm near the anode increasing up to 50 kV/cm near the cathode. These estimations were made at 1-3 eV electron temperature and corresponded to near 1 Ohm POS resistance during the conduction phase.

Dynamics of the plasma injected into the gap of a plasma opening switch across a strong magnetic field

A method is proposed to increase the linear charge density transferred through a plasma opening switch (POS) and, accordingly, reduce the POS diameter by enhancing the external magnetic field in the POS gap. Results are presented from experimental studies of the dynamics of the plasma injected into the POS gap across a strong magnetic field. The possibility of closing the POS gap by the plasma injected across an external magnetic field of up to 60 kG is demonstrated.