Vladimir A. Gribkov

High rep rate high performance plasma focus as a powerful radiation source

Basic operational characteristics of the plasma focus are considered from design perspectives to develop powerful radiation sources. Using these ideas we have developed two compact plasma focus (CPF) devices operating in neon with high performance and high repetition rate capacity for use as an intense soft X-ray (SXR) source for microelectronics lithography. The NX1 is a four-module system with a peak current of 320 kA when the capacitor bank (7.8 /spl mu/F/spl times/4) is charged to 14 kV. It produces 100 J of SXR per shot (4% wall plug efficiency) giving at 3 Hz, 300 W of average SXR power into 4/spl pi/. The NX2 is also a four-module system. Each module uses a rail gap switching 12 capacitors each with a capacity of 0.6 /spl mu/F. The NX2 operates with peak currents of 400 kA at 11.5 kV into water-cooled electrodes at repetition rates up to 16 Hz to produce 300 W SXR in burst durations of several minutes. SXR lithographs are taken from both machines to demonstrate that sufficient SXR flux is generated for an exposure with only 300 shots. In addition, flash electron lithographs are also obtained requiring only ten shots per exposure. Such high performance compact machines may be improved to yield over 1 kW of SXR, enabling sufficient exposure throughput to be of interest to the wafer industry. In deuterium the neutron yield could be over 10/sup 10/ neutrons per second over prolonged bursts of minutes.

Damage of structural materials for fusion devices under pulsed ion and high temperature plasma beams

Results of experiments are presented on the influence of high-energy pulses on austenitic and ferritic steels carried out using dense plasma foci devices PF-1000 and PF-60 with hydrogen and deuterium as working gas, respectively. Pulsed irradiation of specimens was performed in two regimes: (1) under microsecond hydrogen plasma pulses with power density q=107–109 W/cm2, and (2) under 100-ns deuterium plasma pulses with q=109–1011 W/cm2. Features of damage, phase-structural transformations and compositional changes in these materials under these conditions were investigated.

Application of intense plasma-ion streams emitted from powerful PF-type discharges for material engineering

This paper concerns various applications of powerful ion- and plasma-streams generated by high-current pulse discharges realized within different plasma-focus (PF) facilities. General characteristics of the emitted plasma-ion streams are summarized. The possibility of application of special arrangements, e.g. cryogenic targets, CD2 or metal wires, hydrogen or deuterium getters, special alloy targets, etc, is described. The paper presents results of different experiments oriented on the interaction of the pulsed plasma-ion streams with various material targets placed inside PF-360 device (at IPJ in Swierk) and PF-1000 facility (at IPPLM in Warsaw). Attention is paid to plasma–target interactions and influence of the material targets on emission characteristics of the PF-type discharges. The diagnostics includes current and voltage measurements, optical photography and spectroscopy, x-ray emission observations and fast-neutron measurements (from deuterium discharges). Particular attention is paid to time-resolved spectroscopic studies. The use of pulsed ion- and plasma-streams for modifications of different materials, e.g. those of particular interest for the construction of nuclear fusion reactors, is described.

Enzyme activation and inactivation induced by low doses of irradiation

Activation phenomenon has been observed with two sets of enzymes under the conditions of low dosage irradiation. Activation was registered for angiotensin-converting enzyme under in vitro γ-irradiation (0.662 MeV, pulse duration approx 10s) at dose levels of 1–3 Gy and under X-ray irradiation (approx 9 keV, pulseduration approx 10−9s) at dose levels of 2×10−5 Gy. An activation effect has also occurred for native and recombinant horseradish peroxidase and tobacco peroxidase under γ-irradiation. The phenomenon observed is rationalized in terms of a kinetic model suggesting the existence of at least one activated enzyme conformation induced by radiolysis. The activity oscillations registered in dense plasma focus experiments were rationalized using the same model with the corresponding kinetic equation converted into the form describing the decaying oscillations caused by exciting force. The model analysis is presented.

The experimental and theoretical investigations of damage development and distribution in double-forged tungsten under plasma irradiation-initiated extreme heat loads

Abstract The influence of extreme heat loads, as produced by a multiple pulses of non-homogeneous fl ow of slow plasma (0.1-1 keV) and fast ions (100 keV), on double-forged tungsten (DFW) was investigated. For generation of deuterium plasma and fast deuterons, plasma-focus devices PF-12 and PF-1000 are used. Depending on devices and conditions, the power flux density of plasma varied in a range of 107-1010 W/cm2 with pulse duration of 50-100 ns. Power flux density of fast ions was 1010-1012 W/cm2 at the pulse duration of 10-50 ns. To achieve the combined effect of different kind of plasmas, the samples were later irradiated with hydrogen plasma (105 W/cm2, 0.25 ms) by a QSPA Kh-50 plasma generator. Surface modification was analysed by scanning electron microscopy (SEM) and microroughness measurements. For estimation of damages in the bulk of material, an electrical conductivity method was used. Investigations showed that irradiation of DFW with multiple plasma pulses generated a mesh of micro- and macrocracks due to high heat load. A comparison with single forged tungsten (W) and tungsten doped with 1% lanthanum-oxide (WL10) reveals the better crack-resistance of DFW. Also, sizes of cells formed between the cracks on the DFW’s surface were larger than in cases of W or WL10. Measurements of electrical conductivity indicated a layer of decreased conductivity, which reached up to 500 μm. It depended mainly on values of power flux density of fast ions, but not on the number of pulses. Thus, it may be concluded that bulk defects (weakening bonds between grains and crystals, dislocations, point-defects) were generated due to mechanical shock wave, which was generated by the fast ions flux. Damages and erosion of materials under different combined radiation conditions have also been discussed.

Evaluation of surface, microstructure and phase modifications on various tungsten grades induced by pulsed plasma loading

Progress in the field of nuclear fusion requires the development of a new generation of tungsten materials that are expected to meet specific property, lifetime and safety requirements. Pursuing this goal, the new materials must be properly tested in a wide range of conditions including cases where material is brought to the molten stage, such as with large fusion plasma instabilities. In this study, two prospective candidates from the family of dispersion strengthened (DS) tungsten materials, i.e., W-1%Y2O3 and W-2.5%TiC, were subjected to extreme heat loading exerted by the deuterium plasma generator PF6. The study focuses on the interaction of the tungsten matrix with the dispersed particles during material melting. The materials underwent significant changes in microstructure and phase content. Among the most serious was the loss of TiC particles and void formation in W-2.5%TiC and phase change of polymorphic Y2O3 particles in W-1% Y2O3.

Generation of shock waves in materials science experiments with dense plasma focus device

The paper presents a comparison of the results of numerical simulations of the shock wave (SW) produced inside a stainless steel plate by a powerful pulsed stream of fast ions generated in a dense plasma focus device with its experimental observations. A SW was detected for the first time in a materials science experiment directly by means of multiframe nanosecond laser interferometry. This was visualized in experiments with the PF-1000 facility after the SW went out at the rear side of the thin stainless steel plate into the residual gas atmosphere. In particular, the pressure amplitude of the SW measured by the interferometric method was 16 GPa. The observed value is in good agreement with the results of numerical simulations of the SW pressure amplitude.

Physical processes of the interaction of ion and plasma streams with a target surface in a dense plasma focus device

Dynamics of the interaction of powerful streams of high-temperature plasma and fast ions generated in a device of the “Dense Plasma Focus” (DPF) type has been studied for a special case. In these experiments solid conductive targets with the shape of a plate and a tube, respectively, were placed normally and axially with respect to the Z axis of the DPF chamber on its cathode side. The secondary plasma spread out from the target surface has been examined. The shock-wave action upon the flat targets produced by the ion beam has been revealed.

Opportunities afforded by the intense nanosecond neutron pulses from a plasma focus source for neutron capture therapy and the preliminary simulation results

The use of short and powerful neutron pulses for boron neutron capture therapy (BNCT) can potentially increase selectivity and reduce the total dose absorbed by the patient. The biological effects of radiation depend on the dose, the dose power and the spatial distribution of the microscopic energy deposition. A dense plasma focus (DPF) device emits very short (in the nanosecond range) and extremely intense pulses of fast neutrons (2.5 or 14 MeV neutrons—from D–D or D–T nuclear reactions) and x-rays. Optimal spectra of neutrons formed for use in BNCT must contain an epithermal part to ensure a reasonable penetration depth into tissues at high enough cross-section on boron. So the powerful nanosecond pulses of fast neutrons generated by DPF must be moderated. After this moderation, the pulse duration must be shorter compared with the duration of the reaction with free radicals, that is, ≥1 μs. In this work we focus on the development of a detailed simulation of interaction of short-pulse radiation from a DPF with the devices materials and with different types of moderators to estimate the dose power at the cells for this dynamic case. The simulation was carried out by means of the Geant4 toolkit in two main steps: the modeling of the pulsed neutron source device itself;the study of the interaction of fast mono-energetic neutrons with a moderator specific for BNCT.

A single-shot nanosecond neutron pulsed technique for the detection of fissile materials

A novel technique with the potential of detecting hidden fissile materials is presented utilizing the interaction of a single powerful and nanosecond wide neutron pulse with matter. The experimental system is based on a Dense Plasma Focus (DPF) device as a neutron source generating pulses of almost mono-energetic 2.45 MeV and/or 14.0 MeV neutrons, a few nanoseconds in width. Fissile materials, consisting of heavy nuclei, are detected utilizing two signatures: firstly by measuring those secondary fission neutrons which are faster than the elastically scattered 2.45 MeV neutrons of the D-D reaction in the DPF; secondly by measuring the pulses of the slower secondary fission neutrons following the pulse of the fast 14 MeV neutrons from the D-T reaction. In both cases it is important to compare the measured spectrum of the fission neutrons induced by the 2.45 MeV or 14 MeV neutron pulse of the DPF with theoretical spectra obtained by mathematical simulation. Therefore, results of numerical modelling of the proposed system, using the MCNP5 and the FLUKA codes are presented and compared with experimental data.