G.I. Dolgachev

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.

A Modernized PC-20 Facility for Studying the Characteristics of a Plasma-Opening Switch

Abstract-A modernized PC-20 facility with a plasma opening switch (POS) is described. It contains a four-module voltage pulse (Marx) generator (MXG) connected via a high-voltage feedthrough to a POS. The energy stored in the MXG is increased by a factor of 12.5 and amounts to 240 kJ at a maximum voltage of 1 MV. At such a voltage, the POS current amplitude is 320 kA and the current rise time is 2 μs. The breakdown strength of the high-voltage insulator is raised to a significant degree. The modernized facility was used in experiments in which the maximum accessible POS parameters (the obtained voltage, passed charge density, etc.) were evaluated. A voltage of up to 3.5 MV was obtained in the first experiments at a MXG voltage of 0.84 MV and a current of 300 kA.

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.

The improvement of pulse power scheme for Baikal project

The researches on a laboratory source of powerful soft x-ray radiation are considered. This project develops by cooperation from Kurchatov Institute, TRlNITI, Efremov Institute and VNIITF. The compression of fast liners is considered as the most perspective source for production of x-ray pulses at MJ range. The project is based on inductive stores of energy, which as a result of consecutive transformations generate an electrical pulse with parameters necessary for compression of liners.

Plasma opening switch conduction phase adjustment

Plasma Opening Switch conduction phase duration adjustment has a practical importance for several POS applications. They are POS synchronization and increase of energy and current density coming through the POS. The report represents the results of both experiments and simulations on POS conduction phase duration on experimental test-bench.

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.

Shaper of a current pulse of megaampere level with rise time of 100 ns

The paper presents the method of shaping the current pulse of submegampere level with the rise time of 100 ns. At this a device on the basis of an explosive magnetic generator can be the source of the original current of microsecond duration. A low-inductive foil electrically exploded current opening switch designed at VNIIEF and a vacuum discharger developed at the RSC KI under “Baikal” program are used to shape the current pulse with required parameters. To test the proposed method of the current pulse shaping, the configuration of a series of explosive experiments has been worked out. The helical explosive magnetic generator with the explosive current opening switch is used as the energy source. In the experiments it is proposed to get the current pulse with the amplitude 4–5 MA and rise time of ∼100 ns in the load of ∼10 nH. The results of the experimental checking of the experiment configuration under laboratory conditions on the facility with capacitive energy storages will be given. The results of the first explosive experiment will be described and analyzed.

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.