H. C. Pant

An x-ray biplanar photodiode and the x-ray emission from magnetically confined laser produced plasma

Development of a single and multichannel biplanar vacuum photodiode for x-ray detection is reported, which has been used to study the x-ray emission from laser produced plasma expanding across an externally applied magnetic field. Two to three times enhancement in x-ray emission has been observed which was found correlated with decrease in size of the x-ray emitting plasma plume (expansion velocity of plasma). Experimental observations were found in close agreement with the analytical model based on an increase in plasma density as a result of plasma confinement in magnetic field. Temporal evolution of x-ray emission indicates that recombination radiation seems to be playing an important role in x-ray enhancement.

Some studies on picosecond laser produced plasma expanding across a uniform external magnetic field

Some characteristics of the picosecond laser produced plasma expanding across an externally applied magnetic field (0.6T) have been reported. Two to three times enhancement in X-ray emission from copper plasma has been observed for the laser intensity range ∼5 X 10 11 to 5 X 10 12 W/cm 2 . The X-ray yield has been found to increase with an increase in the magnetic field intensity. Enhancement in X-ray emission is correlated with confinement of expanding plasma either in external magnetic field or in high ambient gas pressure. Generation of some highfrequency instability and fast ions was also observed in the plasma along with bouncing of plasma near the β = 1 surface where kinetic pressure of plasma equals magnetic pressure.

Scaling laws for a self-generated axial magnetic field in laser-produced plasma

The generation of an axial magnetic field caused by the interaction of standing waves formed by two counterpropagating, elliptically polarized electromagnetic waves with a laser‐produced plasma has been investigated. Study of the variation of magnetization with different powers and wavelengths available from Nd–glass and CO2 lasers shows that induced axial magnetization increases linearly with an increase in power for all wavelengths and increases exponentially with the wavelengths for different laser powers at a particular plasma density. The variation of magnetization with plasma density reveals that the peak value of magnetization occurs below the critical density (nc) for a particular wavelength and power. Scaling laws for the induced axial magnetic field have been given. The poloidal fields arising from such mechanisms are important in the lateral energy and axial energy transport mechanisms. Such a field inhibits lateral energy transport, but axial energy propagation may be increased.

Characteristics of plasma flow from laser irradiated planar thin foil targets

Filamentary structures or jetting in the plasma flow generated from laser irradiated thin foil targets have been observed using an optical probe technique. Laser wavelength, pulse duration, and intensity range used in the experiments were 1.06 μm, 5 ns, and 5×1012 W/cm2, respectively. A 0.53‐μm‐wavelength optical pulse of 50‐μJ energy and 1‐ns duration was used as an optical probe. Optical shadowgrams showed that the filamentary structures or jets in the plasma flow were strongest in the region about 300 μm away from the target surface, and in time, much later than the peak of the main laser pulse. It was also observed that jetting does not affect the stability of the ablatively accelerated thin foils. Jetting was found to be stronger for targets with a higher atomic number. A time‐resolved interferometric study of the plasma produced from targets with high atomic numbers also showed the existence of an abrupt change in fringe shift or plasma density in the region of these jets.

Effect of lateral energy transport on ablation pressure scaling in laser irradiated planar foil targets

Experimental investigations on ablatively accelerated thin plastic foil targets irradiated by a 6J, 5 nsec Nd: glass laser pulse, were conducted using shadowgraphy technique. A 2 nsec, 0.53 µm probe pulse, derived from the main laser was used for recording the foil motion. It was observed that 6 µm plastic foils could be accelerated to a velocity of about 3 × 106 cm/sec for an incident laser intensity of 5 × 1013 W/cm2 and the corresponding ablation pressure was 0.4 Mbar. Ablation pressure (P) scaling against absorbed laser intensity (Ia) was slower (P ∝Ia0.4) for a smaller laser focal spot (30 µm) as compared to the scaling (P ∝Ia0.7) for a larger focal spot (500 µm). This result has been explained considering the loss due to lateral energy transport from the laser plasma interaction region.

Dependence of lateral energy transport on laser irradiance in laser‐induced plasmas

An experimental study of lateral energy transport in laser‐produced plasmas is presented. X‐ray pinhole photography has been used for studying the variation of the plasma diameters at the target surface as a function of the laser intensity. A scaling law has been found for this variation.

Effect of chamber pressure induced space charge potential on ion acceleration in laser produced plasma

The acceleration of ions (∼10 8 cm/s) has been observed in the laser produced plasma expanding across an uniform magnetic field at low laser irradiance (∼5 × 10 12 W/cm 2 ). This acceleration was found correlated with the onset of instabilities in the plasma and decrease in the slope of X-ray emission with laser intensity. A large enhancement in the X-ray emission (E > 2 KeV) from plasma in the presence of magnetic field supports the observation of ion acceleration. An increase in the number of ions was noticed in the pressure range in which enhancement in the self-generated spontaneous magnetic field has already been established. Even scaling of both the variations with chamber pressure during rising part was found in close agreement, which further supports the correlation. The possibility of an external magnetic field in triggering the acceleration and space charge potential in generating a correlation (between ion acceleration and self-generated spontaneous magnetic field) has been discussed.

Laser‐induced ablation pressure in thin gold foils

A study of the variation of the effective ablation pressure with thickness of laser irradiated thin gold foils is presented. Gold foils (2.5 and 12.0 μm thick) were irradiated by a glass laser (1.06 μm wavelength, 5‐ns pulse duration) in 1011–1013 W/cm2 irradiance range. Ablation pressure and its scaling is found to be higher for the 12‐μm‐thick gold target. The results are consistent with other results obtained on the basis of enhanced x‐ray transmission through thin gold foils. Theoretical estimates of the x‐ray transport are carried out to support this.

Effect of lateral energy transport on ion expansion energy scaling in laser‐produced plasmas

Polythene targets have been irradiated at a maximum flux of 1014 W/cm2 with a focused Nd:glass laser. Spatial extent of the lateral energy transport as a function of the incident laser intensity has been determined using pinhole camera imaging with 12‐μm spatial resolution. The effect of lateral energy transport on the plasma expansion energy scaling has been investigated. The scaling is significantly affected at higher intensities for a smaller irradiation spot.

Enhancement of ablation smoothing in laser-irradiated, highZ coated, thin foil targets

An enhanced spatial smoothing of ablative motion of thin plastic foil targets coated with high atomic number ablators such as gold or aluminium, irradiated by a spatially modulated Nd: glass laser beam was observed. Optical shadowgraphy coupled with double foil technique was used to observe the laser-irradiated foil motion. Laser irradiance used for the experiments was in the range of 1011–1013 watts/cm2. A 60–80% enhancement in the smoothing was observed for a laser beam modulation (width 75–150 µm) at the target surface.