Yulia I. Isakova

The effect of ion current density amplification in a diode with passive anode in magnetic self-isolation mode

The results of a study on gigawatt power pulsed ion beam parameters are presented here. The pulsed ion beam is formed by a diode with an explosive-emission potential electrode, in magnetic self-isolation mode [A. I. Pushkarev, J. I. Isakova, M. S. Saltimakov et al., Phys. Plasmas 17, 013104 (2010)]. The ion current density is 20–40 A/cm2, the energy of the ions is 200–250 keV, and the beam composition is of protons and carbon ions. Experiments have been performed on the TEMP-4M accelerator, set in double-pulse formation mode. To measure the beam parameters, we used a time-of-flight diagnosis. It is shown that the carbon ion current density, formed in a planar diode with graphite potential electrode, is five to seven times higher than the values calculated from the Child–Langmuir ratio. A model of ion current density amplification in a diode with magnetic self-isolation is proposed. The motion of electrons in the anode-cathode gap is simulated using the program CST PARTICLE STUDIO.

Infrared imaging diagnostics for parameters of powerful ion beams formed by a diode in a double-pulse mode

Infrared imaging diagnostics on the parameters of pulsed ion beams of gigawatt power is an effective method for rapid control. It allows for the measuring of energy density distribution at the target, optimization of an ion diode operation and control of the mode of target irradiation. Spatial resolution is of 0.9–1 mm, the sensitivity of a standard thermal camera provides registration of a thermal imprint per pulse at energy densities above 0.05 J/cm2.

Improvement in the statistical operation of a Blumlein pulse forming line in bipolar pulse mode

The paper presents the results of studies on shot-to-shot performance of a water Blumlein pulse forming line of 1-1.2 kJ of stored energy. The experiments were carried using the TEMP-4M pulsed ion beam accelerator during its operation in both unipolar pulse mode (150 ns, 250-300 kV) and bipolar-pulse mode with the first negative (300-600 ns, 100-150 kV) followed by a second positive (120 ns, 250-300 kV) pulse. The analysis was carried out for two cases when the Blumlein was terminated with a resistive load and with a self-magnetically insulated ion diode. It was found that in bipolar pulse mode the shot-to-shot variation in breakdown voltage of a preliminary spark gap is small, the standard deviation (1σ) does not exceed 2%. At the same time, the shot-to-shot variation in the breakdown voltage of the main spark gap in both bipolar-pulse and unipolar pulse mode is 3-4 times higher than that for the preliminary spark gap. To improve the statistical performance of the main spark gap we changed the regime of its operation from a self-triggered mode to an externally triggered mode. In the new arrangement the first voltage pulse at the output of Blumlein was used to trigger the main spark gap. The new trigatron-type regime of the main spark gap operation showed a good stability of breakdown voltage and thus allowed to stabilize the duration of the first pulse. The standard deviation of the breakdown voltage and duration of the first pulse did not exceed 2% for a set of 50 pulses. The externally triggered mode of the main gap operation also allowed for a decrease in the charging voltage of the Blumlein to a 0.9-0.95 of self-breakdown voltage of the main spark gap while the energy stored in Marx generator was decreased from 4 kJ to 2.5 kJ. At the same time the energy stored in Blumlein remained the same.

Statistical analysis of the ion beam production in a self magnetically insulated diode

The paper presents the results of a study on shot to shot variation in energy density of an ion beam formed by a self-magnetically insulated diode with an explosive emission cathode. The experiments were carried out with the TEMP-4M accelerator operating in double-pulse mode: plasma formation occurs during the first pulse (negative polarity, 300–500 ns, 100–150 kV), and ion extraction and acceleration during the second pulse (positive polarity, 150 ns, 250–300 kV). Crucially, it was found that the standard deviation of energy density does not exceed 11%, whilst the same variation for ion current density was 20%–30%, suggesting the presence of neutrals in the beam. This idea is further supported by the fact that ion current density is only weakly dependant on the accelerating voltage and other output parameters of the accelerator (coefficient of determination 0.9). We at...

Analysis of correctness of intense ion beam diagnostics based on the ion-current density

The results of a comparative analysis of the correctness of intense-ion-beam diagnostics, which is based on measurements of the ion-current-density and energy-density amplitudes, are presented. It is shown that when a nanosecond-duration pulsed ion beam is used to modify a surface, the main factor that determines changes in the properties of a treated item is a thermal effect rather than the ion implantation. The analysis of the influence of such factors as the ion-energy variation, the ion-beam composition, accelerated neutrals, the variation in the accelerating voltage, the diagnostics locality, and other factors on the accuracy of controlling the ion-beam impact on a target was performed. It was found that analyzing the stability of the thermal effect of an ion beam on the target on the basis of the amplitude of an ion-current density pulse yields an overestimated standard-deviation value. It was shown that measurements of the energy density provide more accurate and complete information that does not contain systematic errors.

Influence of Cathode Diameter on the Operation of a Planar Diode with an Explosive Emission Cathode

This paper presents the results of experimental investigations into the current-voltage characteristics of a planar diode with an explosive emission cathode made from graphite. Studies were performed using a TEU-500 pulsed electron accelerator (350–500 keV, 100 ns, 250 J per pulse). Duration of diode operation, in a mode when electron current is limited by the emissive ability of the graphite cathode, is 15–20 ns. The contribution of the cathode periphery to total electron current appears only as an increase in the emissive surface area due to an expansion of explosive plasma. Investigations of an ion diode with a graphite cathode (plane and focusing geometry) were also carried out. Experiments were performed using a TEMP-4M ion accelerator, which forms two nanosecond pulses: the first negative pulse (150–200 kV, 300–600 ns) followed by the second positive (250–300 kV, 150 ns). Total diode current in the first pulse is well described by the Child-Langmuir law for electron current at a constant rate of plasma expansion, equal to 1.3 cm/μs. It is shown that for an area of flat cathode over 25 cm2, the influence of edge contribution does not exceed measurement error of total diode electron current (10%).

Explosive-Emission Plasma Dynamics in Ion Diode in Double-Pulse Mode

The results of an experimental investigation of explosive-emission plasma dynamics in an ion diode with self-magnetic insulation are presented. The investigations were accomplished at the TEMP-4M accelerator set in a mode of double pulse formation. Plasma behaviour in the anode-cathode gap was analyzed according to both the current-voltage characteristics of the diode (time resolution of 0.5 ns) and thermal imprints on a target (spatial resolution of 0.8 mm). It was shown that when plasma formation at the potential electrode was complete, and up until the second (positive) pulse, the explosive-emission plasma expanded across the anode-cathode gap with a speed of 1.3±0.2 cm/μs. After the voltage polarity at the potential electrode was reversed (second pulse), the plasma erosion in the anode-cathode gap (similar to the effect of a plasma opening switch) occurred. During the generation of an ion beam the size of the anode-cathode gap spacing was determined by the thickness of the plasma layer on the potential electrode and the layer thickness of the electrons drifting along the grounded electrode.

Stabilization of ion beam generation in a diode with self-magnetic insulation in double-pulse mode

The paper presents the results of statistical studies of ion beam generation in different geometry diodes with explosive emission cathode in a mode of self-magnetic insulation. The experiments were carried out using the TEMP-4M pulsed ion beam accelerator during its operation in both unipolar pulse mode (100 ns, 250–300 kV) and bipolar-pulse mode with the first negative (300–600 ns, 100–150 kV) followed by a second positive (120 ns, 250–300 kV) pulse. It is found that the standard deviation of the total energy and energy density of the beam does not exceed 10–11%, while the same shot-to-shot variation in ion current density was found to be 20–30%. The mechanism of the energy density stabilization from pulse to pulse may be associated with the charge exchange between accelerated ions and stationary molecules and formation of accelerated neutrals. We observed a high correlation between the energy density (or total beam energy) and the duration of the first voltage pulse. We performed analysis of Blumlein statistical performance when the Blumlein was terminated with a resistive load and with a self-magnetically insulated ion diode. A characteristic feature of Blumlein operation in the double-pulse mode is an excellent reproducibility of breakdown of the preliminary spark gap, the variation in breakdown voltage is

Applicability of the 1D Child–Langmuir relation for ion diode current calculation

The paper presents the results of the analysis of the influence of curvature of the electrons trajectory in the anode–cathode gap of an ion diode on the diode impedance and evaluation of applicability of the one-dimensional (1D) Child–Langmuir (CL) ratio for calculation of the electron current. Investigations of an ion diode with a graphite anode in self-magnetic insulation mode were carried out. Experiments were performed on the TEMP-4M ion accelerator set in a double pulse mode, with the first negative pulse (150–200 kV, 400–600 ns) followed by the second positive pulse (250–300 kV, 150 ns). The result of this study is that we have determined the boundary conditions for the applicability of 1D CL ratio for calculation of the electron current in the ion diode. It was found that the deviation of the diode current–voltage characteristics from CL ratio will be observed only with a significant change in the acceleration voltage during electron drift or when the electron drift time exceeds the transit time of ions.

Transportation of a pulsed ion beam formed by a self-magnetically insulated diode

Summary form only given. We present the results of a study on transportation of a gigawatt power pulsed ion beam formed by a self-magnetically insulated diode with an explosive-emission graphite cathode. Studies were performed using diodes of different design: a strip planar and focusing diode, and a spiral diode, all operated in double-pulse formation mode with the first negative pulse (300-500 ns, 100-150 kV), followed by the second positive pulse (150 ns, 250-300 kV). For increasing ion beam focusing efficiency and preventing the ion loss from the beam volume during propagation to the target, we used a metal shield installed on the grounded electrode. The shield was made from 1 mm stainless steel foil. We observed that the beam diameter at the focus decreases from 60 mm (without shield) to 40-42 mm (with a shield) which leads to an increase in the energy density by a factor of 1.5-2 being 4-5 J/cm2 at the focus. Use of the shield on the grounded electrode provides decrease in the half-angle beam divergence from 11° to 7.5-8°1. The effectiveness of the metal shield we attribute to both the elimination of magnetic filed in the beam transport region and realization of the transverse neutralization mechanism. Additional studies using a pin hole camera and heat sensitive paper for beam imprinting at the target showed that the divergence angle of the beam propagating inside the shield does not exceed 3°.