A.G. MacPhee

Transient Strain Driven by a Dense Electron-Hole Plasma

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

Measurements of direct drive laser imprint in thin foils by radiography using an x-ray laser backlighter

In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. We have used an x-ray laser as an XUV backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and spectral dispersion at both 0.35 pm and 0.53 pm irradiation, and induced spatial incoherence at 0.53 pm irradiation. We also demonstrate measurements of the modulation due to a single mode optical imprint generated by a narrow slit interference pattern, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process. 8 refs., 10 figs.

The effects of multi-pulse irradiation on X-ray laser media

Abstract Multipulse irradiation with 100 ps pulses of stripe Germanium targets is shown to enhance by up to several orders-of-magnitude the output of Ne-like Ge lasing on the J = 0–1 line at 196 A compared to single pulse pumping. Various pre-pulse and multipulse configurations have been experimentally investigated for irradiances of ≈ 4 × 10 13 W/cm 2 with a 1.06 μm wavelength pumping laser. The ionisation balance measured by a KeV crystal spectrometer (KAP crystal) has been found to not affect the X-ray laser output. Good agreement between the experimental results and a fluid code incorporating atomic physics, gain and X-ray beam ray tracing is obtained. The code results show that the enhanced X-ray laser output is produced by multipulse irradiation reducing the electron density gradients in the gain region and simultaneously increasing the gain region spatial size. These changes reduce the effect of refraction on the X-ray laser beam propagation.

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.

Gain saturation of the Ni-like X-ray lasers

Abstract Driven by a double 75 ps pulse with 2.2 ns separation, saturated operation of nickel-like Ag, In, Sn, and Sm X-ray lasers have been demonstrated with only 75 J drive energy on target. The variation of X-ray laser output with target length is found to fit well to a simple model for an amplified spontaneous emission (ASE) laser including saturation. Small signal gains of ∼10 cm −1 , effective gain length products of ∼18, and saturation irradiance of (1–5)×10 10 W/cm 2 are measured for these lasers using a fitting procedure.

Spatial coherence of x-ray laser emission from neonlike germanium after prepulse

The time-integrated spatial coherence of neonlike germanium x-ray laser radiation has been studied with a new dispersing coherence diagnostic. Angle-dependent spatial coherence data are recorded by sampling the diverging beam at each lasing wavelength in several directions simultaneously. Measurements of the spatial coherence, and hence effective source sizes, relevant to the output beams from double-slab targets for the J=2–1 spectral lines at wavelengths 28.6, 23.6, and 23.2 nm and for the J=0–1 line at 19.6 nm show differences, which indicate different conditions in the plasma volume amplifying these emissions. Targets are pumped by subnanosecond pulse drivers, with and without a prepulse, but 19.6 nm emission is detected only in the prepulsed case. The differences are discussed in terms of the time evolution of the spectral lines.

Travelling wave chirped pulse amplified transient pumping for collisional excitation lasers

Abstract A ∼3 ps travelling wave chirped pulse amplified pulse at 6×10 14 W cm −2 superimposed on ∼300 ps background pulses is shown to be an efficient method to pump transient collisional excitation X-ray lasers in both Ni-like and Ne-like ions. Measurements of X-ray laser output as a function of plasma length are fitted with results of an amplified spontaneous emission model of the laser output taking account of travelling wave pumping effects. A small signal gain coefficient ∼42 cm −1 and a effective gain length product of ∼18 are measured for the Ni-like Sn laser at 120 A. Simulations from a hydrodynamic and atomic physics code (EHYBRID) coupled to a ray trace code show that a spatially averaged small signal gain ∼65 cm −1 can be obtained in Ne-like Ge provided the optimum pumping pulse arrangement is used.

Measurement of single mode imprint in laser ablative drive of a thin Al foil by extreme ultraviolet laser radiography

The temporal development of laser driven single mode perturbations in thin Al foils has been measured using extreme ultraviolet (XUV) laser radiography. 15, 30, 70 and 90 μm single modes were imprinted on 2 μm thick Al foils with an optical driver laser at 527 nm for intensities in the range 5×1012 to 1.5×1013 W cm−2. The magnitude of the imprinted perturbation at the time of shock break out was determined by fitting to the data estimated curves of growth of the Rayleigh–Taylor instability after shock break out. The efficiency of imprinting is independent of perturbation wavelength in the parameter range of this experiment, suggesting little influence of thermal conduction smoothing. The results are of interest for directly driven inertially confined fusion.

Optimization of double pulse pumping for Ni-like Sm x-ray lasers

We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 A, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model.

A study to optimise the temporal drive pulse structure for efficient XUV lasing on the J = 0–1, 19.6nm line of Ge XXIII

Abstract Short pulses of 100 ps FWHM duration at 1.06 μm wavelength are used as the pump source for driving the J = 0–1, 19.6nm, Ne-like germanium X-ray laser. Different combinations of short pulses are investigated and quantitatively compared. Configurations investigated include a single pulse, double pulses at 400 ps and 800 ps separation, single pulses with prepulses and double pulses with prepulses. Data are presented in the form of integrated energy measurements, and supported by modelling. The most efficient short pulse configurations are shown to be orders of magnitude more effective than pumping with nanosecond duration pulses.