I. V. Romanov

Hard X-ray emission in laser-induced vacuum discharge

The dynamics of fast laser-induced vacuum discharge, with a rather small value of amplitude of current (≤ 10 kA), as well as the voltage and energy of the capacitor bank (≤ 20 kV and 20 J, respectively), have been investigated. It has been experimentally demonstrated that the initiations conditions determined by the energy and duration of the laser radiation, fundamentally determine the dynamics of the discharge. Two types of space and time separated plasma instabilities are revealed. It was found that the first of instabilities occurs at the initial stage of the discharge and is caused by a pinch structure, which takes place in front of a cathode jet extending in vacuum. The second type of instabilities arises at the top or recession of the current and is accompanied by the generation of hard (energy ≥100 keV) bremsstrahlung X-ray radiation from the anode area. The excess energy of the hard components of radiation over the potential of the current source is associated with the effects of plasma-erosive breaking.

Micropinches in laser induced moderate power vacuum discharge

Studies of x-ray emission from a laser induced vacuum discharge of moderate storage energy (<20 J) are examined. It is found that micropinches in the initial stage of the cathode jet expansion into the interelectrode gap are formed. Hot plasma of the micropinches emits a soft x-ray radiation and beams of accelerated electrons moving toward the anode. It is found that these phenomena occur just when both amplitude of the discharge current and energy of the initiating laser pulse lie in the specified ranges of values. The position of the micropinch within the interelectrode gap and also the intensity of the x-ray radiation and beams of the accelerated electrons emitted from the micropinch are variable over a wide range of values through changes in energy of the laser pulse and/or amplitude of the discharge current. The experimental results are compared with the data of the particle-in-cell simulations and a fair agreement is found between them.

Characteristics of moderate current vacuum discharge triggered by multipicosecond and nanosecond duration laser pulses

A comparative study of the characteristics of moderate-current (∼10kA), low-energy (⩽20J) vacuum discharge triggered by multipicosecond and nanosecond duration laser pulses is performed. Temporal profiles of the x-ray emission, discharge current, and anode voltage measured in vacuum discharge created between a planar titanium cathode and a conical point-tip anode are observed to be quite different for the two regimes of the laser pulse duration. While cathode plasma jet pinching is clearly observed in the discharge created by low-energy (∼5mJ), 27 ps full width at half-maximum (FWHM) laser pulses, a feeble pinching occurred for 4 ns (FWHM) laser pulses only above a threshold energy of ∼250mJ. In addition to the multiple K-shell x-ray pulses emitted from the titanium anode up to 100 ns, evidence of a much harder x-ray component (hν>100keV) is also seen in the discharge triggered by picosecond laser pulses.

Laser-driven high-current-density pulsed electron emission from lead zirconium titanate ferroelectric ceramic

Laser driven high current density pulsed electron emission is observed from a lead zirconium titanate (PZT, Zr/Ti: 53/47) ferroelectric ceramic disk using second harmonic of neodium: glass laser (wavelength ∼527 nm, pulse duration ∼3 ns, laser fluence ∼5–40 mJ/cm2) in the presence of a dc extraction field. Apart from electron current pulses of ∼100 A/cm2 [full width at half maximum (FWHM) ∼400 ns] obtained from laser irradiation of the surface towards the positive end of the polarization vector, electron current pulses of ∼30 A/cm2 (FWHM ∼1.1 μs) are also emitted from laser irradiation of the opposite surface. Dependencies of various characteristics of the electron pulse on laser fluence in the two configurations are presented.

Investigation of a laser driven vacuum diode X-ray source of photon energy [similar]5 keV

An investigation of a monochromatic point X-ray source of photon energy ∼5 keV was carried out. The source was set up using a laser produced Al plasma as a cathode and a point-tip Ti anode. Optimum parameters of the diode were determined from experimental measurements of X-ray intensity dependence on laser pulse energy, applied accelerating voltage, and distance between target (cathode) and anode, using Nd:glass laser pulses (FWHM 30 ns) of 2 mJ to 4 J energy. Electron temperature characterization of the target plasma was also performed from the XUV emission spectrum (5–80 A). Parameters of X radiation in Ti K -shells are (1) spectral brightness of ∼10 20 photons/cm 2 -sec-sr-keV, (2) spatial size ∼300 microns, and (3) X-ray pulse duration less than 20 ns.

Observation of micropinch formation in cathode jet of a low-power laser-induced vacuum discharge

The report presents the results from experimental investigation of micropinch formation in the plasma of a vacuum discharge induced by a 6 ns laser pulse of energy J = 0.5–200 mJ (at a storage voltage from 4 to 15 kV and the discharge current range of 6–26 kA, respectively). The discharge gap images were obtained using a pinhole camera in the EUV and soft X-ray ranges of 15–73 eV and 80–284 eV energy. It is shown that micropinch formation in the plasma cathode jet occurs, mainly, in the matter evaporated by the laser pulse at the discharge ignition near the moment when the current derivative reaches the maximum. It is found that the cathode jet may consist of several pinched areas, and each of them has its own structure, and the improvement of the discharge and laser radiation parameters allows us to reach a stable single pinching of plasma. The parameters of the micropinch (the plasma compression ratio, size, and position of the emitting area in the interelectrode gap) as well as the current flow through...

X-ray Source with Photon Energy 5 keV Pumped by Laser

Investigation of a monochromatic point x-ray source of photon energy ~5 keV has been carried out. The source was set up using a laser produced aluminium plasma as a cathode and a point-tip titanium anode. Optimum parameters of the diode were determined from experimental measurements of x-ray intensity dependence on laser pulse energy, applied accelerating voltage, and distance between target (cathode) and anode, using Nd:glass laser pulses (FWHM 30 ns) of 2 mJ to 4 J energy. Electron temperature characterization of the target plasma was also performed from the XUV emission spectrum (5 to 80 A). Parameters of x-ray radiation in Ti K-shells are 1) spectral brightness of ~1020 photons/cm2-sec-sr-keV, 2) spatial size ~300 microns, and 3) x-ray pulse duration less than 20 ns.

Formation and decay of micropinch in a laser initiated vacuum spark

Results of experimental studies of the development and decay of a micropinch produced in a laser-induced vacuum spark are presented. The studies were performed with X-ray pinhole frame camera supplied with a micro-channel plate as a photocathode and the following visualization of the image by a luminous screen. The pinhole was covered with an Al foil of 0.8 micrometer thick that transmitted the X-ray emission in the range of 15-84 eV. The experiments were performed with the initiating laser pulse energy in the range of 0.5-200 mJ at the discharge voltage of 4, 8, 15 kV. It was found that process of the cathodic plasma jet pinching at the specified discharge current was governed just the mass of the laser produced plasma. It was shown also that dependence of the pinch position distance from the cathode against the laser pulse energy has a power-low dependence L ~ Jα where the index α increases with the discharge voltage and ranged from 0.26 to 0.35.

Fast switch with laser initiation

In a low inductance vacuum spark with laser initiation, the switching time dependence on parameters of the initiating laser pulse as well as on the interelectrode gap voltage in a wide range of the values is studied. It is shown that the switching time depends just weakly on the voltage but depends significantly on intensity of the laser pulse, so that at high values of these parameters, the time, in fact, is determined just by the self characteristics of the discharge circuit, i.e. inductance and capacity. This result means that the possible instabilities within the cathode plasma jet are suppressed by means of enhancement of amount of the ablated cathode matter due to the laser irradiation. That permits us to suggest that laser-induced switch with the switching time just of a few nanoseconds can be employed.

Ion Erosion Rate in a High Current Vacuum Spark

Measurements of the ion erosion rate for copper cathode in the low-inductance vacuum spark are presented. The spark peak current was varied from 3 kA up to 13 kA therewith the current rise rate increases from 4middot109 up to 1.5middot1010 A/s. The mean velocity of the cathode plasma flame was obtained by means of the passive collector time-of-flight measurements. It was shown that the velocity increases from 2middot104 m/s up to 8middot10 4 m/s with the discharge enhancement. Mechanical momentum of the plasma flame was measured with a ballistic pendulum, located beyond annual anode in the immediate vicinity. From these data the ion erosion rate was derived at range of peaks of the discharge current mentioned above. This parameter holds to within the accuracy of measurements throughout the range of the current variations. Its value is of 7middot10-5 g/Q and is close to the well-known one that has been obtained earlier in low-current vacuum arcs