P.A. Norreys

High contrast multi-terawatt pulse generation using chirped pulse amplification on the VULCAN laser facility

High power (8 TW), ultra-short (2.4 ps), pulses have been generated using chirped pulse amplification techniques on the VULCAN Nd: glass laser. A novel oscillator was developed as a driver producing ≈ ps pulses at 105 nm. The oscillator output was stretched prior to amplification and compressed at an aperture of 150 mm. The contrast ratio obtained was ≈ 106, which is suitable for laser plasma interaction studies.

Measurements of the hole boring velocity from Doppler shifted harmonic emission from solid targets

The fast ignitor scheme for inertial confinement fusion requires forward driving of the critical density surface by light pressure (hole boring) to allow energy deposition close to the dense fuel. The recession velocity of the critical density surface has been observed to be v/c=0.015 at an irradiance of 1.0×1019 W cm−2 at a wavelength of 1.05 micron, in quantitative agreement with modeling.

X‐ray spectroscopic studies of hot, dense iron plasma formed by subpicosecond high intensity KrF laser irradiation

The time‐integrated x‐ray emission from a hot, dense iron plasma has been recorded. The iron plasma was created when a target with a 1000‐A‐thick iron layer buried beneath 1000 A of plastic was irradiated by a 300 fs pulse of 249 nm laser light at an intensity of approximately 1017 W cm−2. Two models have been used to construct a synthetic x‐ray spectrum. The first employs detailed, spectroscopically accurate atomic data and the second uses a local thermodynamic equilibrium opacity model. The detailed model shows fairly good agreement with experiment whereas the opacity model only shows agreement in the gross features.

Using low and high prepulses to enhance the J=0−1 transition at 19.6 nm in the Ne-like germanium XUV laser

Abstract We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where ≈20 mm long germanium slab targets were irradiated at ≈1.6×10 13 W cm −2 using ≈0.7 ns (1.06 μm) pulses from the VULCAN glass laser. Prepulses were generated at fractional power levels of ≈2×10 −4 (low) and ≈2×10 −2 (high) and arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse. For both the low and high prepulses the output of the 3p-3s, J =0–1, line at 19.6 nm was enhanced such that the peak radiant density (J/st) for this line became greater than that for the normally stronger J =2−1 lines at 23.2 and 23.6 nm. The J =0–1 line, whose FWHM duration was reduced from ≈450 ps to ≈100 ps, delivered ≈6× more power (W) than the average for the combinned J =2−1 lines, whose FWHM duration was ≈500 ps for both levels of prepulse. The higher prepulse was more effective, yielding ≈2× more radiant density and ≈7× more power on both the J =0–1 and J =2−1 transitions compared to the low prepulse case. The most dramatic observation overall was the ≈40× increase of power in the J =0–1 line for the high prepulse (≈2%) case compared with the zero prepulse case. These observations, coupled with measurements of beam divergence and beam deviation through refractive bending, as well as general agreement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and (c) to relax the plasma density gradients, particularly in the J =0–1 gain zone.

Time-resolved measurements of the angular distribution of lasing at 23.6 nm in Ne-like germanium

Abstract The time dependence of the angular distribution of soft X-ray lasing at 23.6 nm in Ne-like germanium has been measured using a streak camera. Slabs of germanium have been irradiated over ≈ 22 mm length × 100 μm width with three line focussed beams of the SERC Rutherford Appleton Laboratory VULCAN laser at 1.06 μm wavelength. The laser beam sweeps in time towards the target surface plane and the divergence broadens with time. The change of the peak intensity pointing and the broadening of the profile with time are consistent with expectations of the time dependence of refraction and divergence due to density gradients in the plasma.

High-gain x-ray lasing at 11.1 nm in sodiumlike copper driven by a 20-J, 2-ps Nd:glass laser

Evidence of high gain pumped by recombination has been observed in the 5g-4f transition at 11.1 nm in sodiumlike copper ions with use of a 20-J 2-ps Nd:glass laser system. The time- and space-integrated gain coefficient was 8.8 +/- 1.4 cm(-1), indicating a single-transit amplification of ~60 times. This experiment has shown that 2 ps is the optimum pulse duration to drive the sodiumlike copper recombination x-ray lasing at 11.1 nm.

Energy transport in plasmas produced by a high brightness KrF Raman laser

Abstract A high brightness KrF Raman laser generating 60 GW, 40 ps pre-pulse free pulses has been focused to an 8 μm focal spot at 9x10 16 W cm -2 . Axial and lateral transport of energy has been studied by analysis of X-ray images of the focal spot and from X-ray spectra emitted by a thin layer of aluminium beneath a variable thickness of CH polymer on planar targets. The results show strong lateral transport of energy and reduced axial penetration of heating.

X‐ray and particle diagnostics of a high‐density plasma by laser implosion (invited)

A series of laser fusion implosion experiments of plastic hollow shell targets was performed by using the Gekko XII glass laser in order to achieve the required fuel areal density for ignition. Introducing random phase plates to improve illumination uniformity, high‐density compression of more than 600 times deuterium liquid density has been achieved. The implosion dynamics and symmetry were observed with a spatially resolved x‐ray streak camera and an x‐ray multiframing camera. The three‐dimensional emission profile of the laser‐heated plasma was reconstructed from the x‐ray images by use of computed tomography and was compared with the laser illumination profiles. The areal density of the imploded core was measured by the neutron activation of a silicon tracer, the secondary reaction method, and the knock‐on proton method. Although the measured density and areal density were consistent with those from 1‐D hydrodynamic simulation, experimental neutron yields were significantly lower than those predicted by the simulation for convergence ratios larger than 20. This suggests that better implosion uniformity is required to create a hot spark.A series of laser fusion implosion experiments of plastic hollow shell targets was performed by using the Gekko XII glass laser in order to achieve the required fuel areal density for ignition. Introducing random phase plates to improve illumination uniformity, high‐density compression of more than 600 times deuterium liquid density has been achieved. The implosion dynamics and symmetry were observed with a spatially resolved x‐ray streak camera and an x‐ray multiframing camera. The three‐dimensional emission profile of the laser‐heated plasma was reconstructed from the x‐ray images by use of computed tomography and was compared with the laser illumination profiles. The areal density of the imploded core was measured by the neutron activation of a silicon tracer, the secondary reaction method, and the knock‐on proton method. Although the measured density and areal density were consistent with those from 1‐D hydrodynamic simulation, experimental neutron yields were significantly lower than those predicted ...

Characteristics of rapidly recombining plasmas suitable for high-gain X-ray laser action

Recombining plasmas produced by picosecond laser pulses are characterized by measuring ratio of intensities of resonance lines of H- and He-like ions in the plasmas. It is found that the rapidly recombining plasmas produced by picosecond laser pulses are suitable for high-gain operation.

Intensity ratio of resonance lines as a diagnostic of initial conditions suitable for XUV laser action in recombining plasmas

Abstract The time varying ratio of intensities of resonance lines of H- and He-like ions in laser irradiated cylindrical fibre targets is analysed using a hydro/atomic physics code. Correspondence between the ratio and gain on the Balmer α transition of H-like ions in the adiabatically cooling and recombining plasma is found. The line ratios and absorbed energies corresponding to maximum gains for carbon, oxygen and fluorine plasmas are calculated. A useful diagnostic method is proposed for infering the absorbed energies in the plasmas and for monitoring the production of plasmas which should give maximum gain during adiabatic expansion.