A. E. Dangor

Neutron production from picosecond laser irradiation of deuterated targets at intensities of

Neutron fluxes of up to were measured when planar deuterated targets were irradiated with 1.3 ps FWHM (full width at half maximum) laser pulses at a wavelength of 1054 nm and focused intensities up to . The neutron energy spectra are consistent with an angularly dispersed beam target interaction, whereas a thermonuclear source is considered unlikely.

Optical and x-ray observations of carbon and aluminium fibre Z-pinch plasmas

This paper reports investigations of a Z-pinch plasma formed from carbon and aluminium fibres by a pulsed power generator delivering 100 kA with a rise time (10 - 90%) of 55 ns. For carbon fibres the breakdown was observed to proceed from the cathode and was coincident with a pulse of hard x-rays consistent with an electron beam of about 4 kA and > 50 keV. For aluminium fibres plasma emission was observed to be uniform over the whole length of the fibre and there was no hard x-ray signal. A radial expansion with a velocity of of the plasma column was seen for both fibres. During the expansion, at about 4 ns for carbon fibres and later for thicker fibres, m = 0 perturbations distributed along the length of the column became evident with a scale length which increased with time as the column expanded. Also for carbon fibres, two groups of bright spots separated by about 10 ns emitting optical and x-ray radiation were observed; the second group was more intense, lasted longer and showed bifurcation. Coincident with the second group was an electron beam of 16 keV for and 7 keV for carbon fibres. Bright spots were also evident for aluminium fibres but these were in a single group, lasted for a much longer time and were accompanied by emission from the anode consistent with 7 keV electron beam. Analysis of time-integrated x-ray pinhole images gave temperature of the bright spots between 75 - 135 eV for carbon fibres and 80 - 100 eV for carbon fibres. The very intense bright spots observed were found to coincide with density islands which were observed later in time but only for carbon fibres. The density in these islands was measured by interferometry to be greater than . After 110 ns plasma was not observable for carbon fibres; for carbon and aluminium fibres, the plasma was observed for longer times.

Characterization of wire x pinches driven by a microsecond-long capacitive discharge

The use of a simple capacitive discharge as a driver for an x-pinch soft x-ray source is demonstrated. The 30 kV, 4 kJ capacitive discharge had a quarter period of 1.2 μs, peak current of 320 kA, and current rise of 2.5×1011u200aAu200as−1. X-pinch x-ray emission was characterized by pinhole photography and solid-state detectors. Soft x-ray emission (800 eV–4 keV) was observed in both single and multiple bursts, with yields from 180 mJ for aluminum to 1.5 J for tungsten wire x pinches. X-ray emission from x pinches was higher than z-pinch emission from the same materials using the same power source. Hard x-ray emission (>8 eV) from the x pinch was lower with the long pulse capacitive discharge than with a 360 kV pulsed power driver delivering 100 kA peak with a rate of current rise of 2.2×1012u200aAu200as−1. Visible photography and laser-based schlieren photography showed that the x pinch was asymmetric about the crossing point of the wires. This asymmetry is due to the influence of electron beam generation at this point.

Z-pinch discharges in aluminum and tungsten wires

A series of experiments on Z-pinch plasmas, driven by a pulsed power generator that delivers 160 kA with a rise time (10%–90%) of 65 ns are reported. Tungsten wires of various diameters were used and results are compared with 15 μm diameter aluminum wire. The expansion of the pinch is studied as a function of wire diameter and material. Schlieren observations show that the coronal plasma of various diameters of tungsten wires expands with the velocity of (9.4±1.0)×103u200am/s. The aluminum pinch expands at least a factor of 2 faster. The m=0 perturbations appear at about 8 ns for the aluminum compared with 20 ns for the tungsten pinch. The wavelength and diameter of the perturbations increase with time for both types of wires, and relatively faster for the aluminum pinch. The short wavelength perturbations (∼200 μm) persist for a longer time for larger diameter tungsten wires. Bright spots are seen to appear after 60 ns from the current start for tungsten wires, whereas for aluminum wires, bright spots appear...

Optical probing of fiber z-pinch plasmas

An experimental study of optical probing of a dense z-pinch plasma using the MAGPIE (mega-ampere generator for plasma implosion experiments) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)] is reported. The generator was operated with a peak current of 1.1 MA rising in 150 ns (10%–90%). The loads were 33 μmu2009diam carbon fibers. Faraday rotation was used to investigate the distribution of the current flowing in the plasma. A measurable Faraday rotation angle was observed only in a time window from 50 to 60 ns after the current start, due to the fact that this effect depends on a combination of the magnetic-field strength and electron number density. A new type of self-referencing cyclic radial shear interferometer was used to evaluate the plasma density profiles which are necessary for the reconstruction of the current distribution. It was calculated that ∼110u2009kA was flowing in the plasma at 52 ns after the current start. Shadowgraphy was used to study the dynamics of the plasma and to ...

Coronal plasma behavior of the Z pinch produced from carbon and cryogenic deuterium fibers

A series of fiber pinch experiments has been carried out on the MAGPIE (mega-ampere generator for plasma implosion experiments) generator (1.8 MA, 150 ns) [Mithell et al., Rev. Sci. Instrum. 67, 1533 (1996)] to study the temporal evolution of the coronal plasma. Analysis of schlieren photographs, axial streak images and gated x-ray photographs gives the radial and axial motion of the coronal plasma. The influence of a current pulse (prepulse) of 30 kA applied 200 ns before the main discharge was also studied. Radial expansion velocities of 5.5×106u2009cm/s for carbon fiber shots without prepulse and 3.6×106u2009cm/s for carbon fibers with prepulse were measured. Axial wavelengths (λz) of dominant instabilities in the corona were between 0.05 and 0.2 cm corresponding to ka∼10–20. Comparison of the results obtained with carbon fibers with and without current prepulse and cryogenic deuterium fibers are presented.

Feasibility study of high harmonic generation from short wavelength lasers interacting with solid targets

Abstract The generation of the third and fourth harmonics from the interaction of a 1 ps, ultraviolet (UV), krypton fluoride (KrF) laser with a solid surface is investigated. The conversion efficiency is seen to increase linearly with Iλ2, with a transition from specular harmonic emission to emission into 2π steradians occurring between 1015 and 1016 W cm−2 μm2. The diffuse emission is strongly dependent on the incidence angle of the laser, with the peak in emission at around 30° being consistent with measurements for resonance absorption. Finally, the conversion efficiencies are found to be in agreement with particle-in-cell (PIC) simulations including appropriate density scalelengths.

The response of the Fabry-Pérot interferometer to rapid changes in optical length

The response of the Fabry-Perot etalon to changes in optical length is analysed. The optical length is initially fixed and then allowed to change at a constant rate. The analysis is therefore applicable to experimental situations such as plasma refractive index measurements or spectral line profile investigations. Two time constants of interest, td and tc, are obtained. td is the time lag between changes in the optical length and subsequent changes in the output intensity; it is equal to either the single or double transit time within the cavity, depending on the method by which the optical length is changed. tc is the time during which transients occur and is related to the energy decay time within the cavity. Under certain conditions at high frequencies, the output exhibits harmonics and the necessary and sufficient conditions for this to occur are established. A physical explanation for the generation of these harmonics is given and the relevance of these harmonics to the experimental use of the Fabry-Perot etalon at high frequencies is discussed.

Large magnetic fields generated by z-pinch flux compression

Experiments to generate large magnetic fields by flux compression have been performed in two z-pinch configurations: (i) a novel plasma liner developed from a thin cylindrical surfactant film and (ii) a conventional gas pinch formed in a uniform cylinder of gas. The apparatus, experiments, and the results obtained are described in this article. A comprehensive investigation of the gas pinch shows that an optimum set of conditions exists, for given discharge parameters, for obtaining the highest magnetic field. The scaling of the attainable peak magnetic field with discharge parameters is obtained experimentally and compared with simplified models of the compression. Using a current of 500 kA with τ1/4=2u2009μs, an initial magnetic field of 0.3 T has been compressed to 38 T, demonstrating a compression ratio of over 120.

Kaufman-type xenon ion thruster coupling plasma: Langmuir probe measurements

This paper details an investigation into the physics of the external plasma in a Kaufman-type electrostatic ion thruster operating with xenon gas. The external plasma, which is located outside the hollow cathode between the keeper and the anode electrodes, was investigated with a single Langmuir probe. Detailed scans give a peak temperature of eV at the orifice of the keeper decreasing to eV at the anode. The temperature is essentially constant over the external plasma volume. The radial electron density distribution peaks on the axis at decreasing radially to at the boundaries of the plasma; these values have not been corrected for any perturbation of the plasma caused by the presence of the probe. The velocity distribution of the electrons was studied using the second derivatives of the probe characteristics assuming a collisionless sheath. It was shown that the electrons have a Maxwellian distribution. This allowed essentially independent calculations of the electron temperature and plasma potential distribution to be made. These measurements were taken in the stable `spot mode discharge. Observations in the less stable `plume mode were also made. However, electrical noise due to plasma instabilities masked the probe signal rendering the analysis inaccurate.