D. D. Maslennikov

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.

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.

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.

Increase of plasma opening switch conduction phase duration

A creation of the Plasma Opening Switch with a conduction time up to tens of microseconds has a significant importance for the BAIKAL research program which plans using a high current generator with long duration prepulse. To increase the conduction phase various scenarios of POS gap filling with plasma are planned. This includes in series connection of plasma guns. The results of experiments aimed to increasing the charge coming through the POS during the conduction phase are reported at 200 kV Marx generator voltage, 120 kA drive current. And 1 µs current rise time. The prepulse was created by a specially designed pulse forming system.

A calorimetric dosimeter of high-power pulses of hard X rays with a lead absorber

A simple small calorimetric dosimeter for proximate measurements of the dose of high-power pulse bremsstrahlung γ-radiation was developed. The dose was measured by the heating of a lead absorber. The dosimeter was tested on a generator of a high-current relativistic electron beam with an exposure dose rate of up to 5 × 109 R/s with a total dose per pulse of up to 800 R. The maximum possible measured dose is evaluated as 3 × 105 R at a sensitivity threshold of ∼20 R under our experimental conditions.

A pulsed generator of X-ray bremsstrahlung with a high peak exposure rate

A new pulsed generator of X-ray bremsstrahlung based on an inductive energy storage and a plasma opening switch is described. The radiation parameters are as follows: the peak exposure rate is as high as 5 × 109 R/s, and the dose is as high as 800 R.

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.

Dynamics of plasma filling the gap of a module of the plasma opening switch of the MOL facility

The Baikal program aimed at the creation of a superhigh-power generator for experiments in the field of inertial thermonuclear fusion includes the development of a MOL facility for modeling the main principles of the generator structure. It is suggested to use a six-module plasma opening switch (POS) in an external magnetic field as the output power sharpener. The scheme of upgraded plasma guns of the module of the MOL facility’s POS and the results of studying the dynamics of filling the POS gap with plasma transversely to an external magnetic field are presented. It is shown that, in the presence of a long (∼40 μs) prepulse in this module, a POS cathode-anode closure with plasma along the force lines of the external magnetic field occurs within a time interval between the first turning-on of the plasma guns and the POS operation, which is hazardous for its operation. To eliminate this threat, it is proposed to increase the POS length to ∼70 cm. On the basis of the results obtained, a simpler and more reliable solution of the prepulse problem is proposed.

Experimental verification of the technique for calculating a plasma opening switch and its matching to the load

A technique for calculating a plasma opening switch in an external magnetic field and its matching to a load the impedance of which increases with time was verified experimentally. The experiments were performed in the RS-20 facility both in the absence of a load and with various inductive loads. The amplitude of the voltage pulse at the input of the plasma opening switch was 0.36–0.84 MV, the current amplitude was 280–320 kA, and the pulse duration was ∼2 μs, whereas the corresponding parameters of the output pulse were 0.8–3.2 MV, 0–240 kA, and ∼100 ns.

Matching of a multimodule plasma opening switch to a liner load

One of the key problems of the Baikal project, intended to create a superpower pulsed generator for ICF experiments, is that of matching a multimodule plasma opening switch (POS) to a liner load. An intermediate inductance or a separating discharger is proposed to be used as a matching element between the POS and the load. An analysis is made of the effect of both versions of the matching system on the synchronization of the POS modules and the energy transfer from the inductive storage to the load. Methods for optimizing the matching element are examined. It is shown that the POS modules can be synchronized and the inductive storage energy can be efficiently transferred to a low-impedance load. A multigap vacuum discharger with a point anode and plane cathode is to be used as a separating discharger. Such an electrode system make it possible to concentrate the electric field at the point anode and to substantially enhance the electric strength of the inter-electrode gap. Results are presented from experimental studies of vacuum breakdown in such an electrode system with a gap length of about 1 mm.