F. Walsh

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.

Design and characterization of the VULCAN Nd:glass laser to give focused intensities of greater than 1019 Wcm-2

The process of Chirped Pulse Amplification (CPA) as presently implemented on the VULCAN glass laser is capable of delivering 30 J to target with pulselengths in the sub- picosecond regime. Results from various experiments have shown that intensities of 1018 - 1019 W cm-2 have been achieved on target enabling users to carry out important new experiments in X-ray laser research, laser fusion, basic plasma physics and particle acceleration. An EPSRC (Engineering and Physical Sciences Research Council) facility upgrade grant has been awarded to increase the current operational level of 35 TW, to over 200 TW. This upgrade forms Phase I of a two phase upgrade to raise the performance of VULCAN to the Petawatt level. This paper details the design of the upgraded system and describes a new interaction chamber which takes full advantage of the availability of synchronous kJ multi-beam long pulse operation on VULCAN with the CPA capabilities.

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.

Pulse generation and shaping using the ultra-high power laser system -- VULCAN

VULCAN is a multi-beam, multi-terawatt laser facility based on Nd:glass operating at 1053 nm. The system is highly versatile, supplying four experimental areas with laser radiation at a range of pulse durations from 700 fs to 20 ns, at fundamental frequency, frequency doubled, or, as a limited option, frequency tripled wavelengths. Beams are available in a number of geometries dictated by the university based programs, which at present include: cluster; line focus including x-ray laser oscillator/amplifier geometry; backlighting; probing; and chirped pulse amplification (CPA) configurations. The system has eight beams which can deliver synchronized long and short pulses including two beams which can deliver subpicosecond CPA pulses. The CPA capabilities on VULCAN are an integral part of the laser system, not only delivering sub-picosecond pulses, but allowing uncompressed pulses and multi-pulses to be delivered to the target areas synchronized with the nanosecond pulses. This paper describes the system configuration, details the means of pulse synchronization and presents some of the pulse manipulation techniques used on VULCAN to provide the laser requirements for the experimental program.

Research on compact, bright, coherent x-ray sources at RAL

New possibilities for compact, bright, coherent x-ray generation have recently emerged through rapid advances in short pulse, multi-terrawatt (TW) laser technology. We present here our recent developments and progress on high gain recombination x-ray lasers and efficient x-ray harmonics generation from solid surfaces using a 2 ps, 20 J chirped pulse amplification (CPA) beam from the VULCAN Nd-glass laser facility (λ=1.053 μm) at Rutherford Appleton Laboratory (RAL).

Optimization of system design and performance to generate >10 20 Wcm -2

We report focused intensities of > 1019 Wcm-2, with subpicosecond operation of the VULCAN Nd:glass laser at the Rutherford Appleton Laboratory using chirped pulse amplification (CPA) techniques. This paper describes novel aspects of the system including: picosecond and subpicosecond diode pumped oscillators; the use of a regenerative amplifier and system optimization. The ultrashort pulse generated from an additive pulse modelocked LMA oscillator was stretched from 0.5 ps to approximately 200 ps in a double pass grating system and amplified from 1 nJ to 50 J, in phosphate glass amplifiers with a final beam aperture of 150 mm diameter. The stretched pulse was recompressed using a pair of gratings (300 X 150 mm, 1740 lines per mm) and focused using an off-axis parabola to avoid nonlinear effects from transmissive optics. The compressed pulse was monitored using a suite of diagnostics to determine the focusability, pulsewidth, and spectrum. We also describe the current system development program, which is in progress and designed to achieve intensities of approximately 1020 Wcm-2 to target.

35 TW pulse generation for laser-plasma interaction studies

We report the development at the VULCAN Ndglass laser at the Rutherford Appleton Laboratory of chirped pulse amplification (CPA)’ operation at 620 fs to give 35 TW in a single large aperture beam, with focused intensities on target in excess of W9 Wcm-’. A new oscillator was developed that used additive pulse mode-locking of diode-pumped NdLMA in a Michelson cavity arrangement. Nd:LMA was chosen as it offers a large bandwidth (4.4 nm); the wavelength is centred at 1055 nm, suitable for phosphate glass amplifiers; and LMA has absorption bands at 800 nm making it suitable for pumping by AlGaAs laser diodes. The maximum average output power from the modelocked oscillator was 115 mW at a repetition rate of 87 MHz. A minimum pulse duration of 420 fs was measured (assuming sech’ pulse shape), and a time-bandwidth product of 0.32 indicating that the pulses were near transform limited. Previous experiments using CPA with the WLCAN laser system’ found that at a total 8-integral of 2, self-phase-modulation produced additional nonlinear chirp at energies >10 J, which reduced the re-compressibility of the pulse. For the work reported here, additional bandwidth associated with the 420 fs pulse produced an increase in the stretched pulse to -200 ps. This, coupled with optimisation of the amplification system and a reduction in optical losses, reduced the Bintegral to 4 , and enabled a re-compressed pulse of 620 fs to be obtained. We report a technique to optimise the parallelism of the compression gratings, whereby two CW alignment beams at wavelengths of 1053 m and 1061 nm were injected into the compmsor. A retro-mirror was placed after the second grating to IP turn the beams, enabling the grating angle to be fine-tuned by observing co-linearity between the input and exit beams. The grating separation was oplimised by taking a series of test shots at low energy. The compressed pulse was monitored with a single shot autwomlator, and a minimum located. Following this optimisation the focal spot was less than three times the diffraction limit at 515 km diameter. We present the simultaneous measurement of pulse length, bandwidth, beam focusability, and energy. Figure l a shows the autocomlator trace for a recompressed pulse, giving a pulse length of 620 fs (assuming sech2 profile), and Fig. l b shows the spectrometer output giving a bandwidth of 1.53 nm. The energy on the gratings for this shot was 44 J, but due to grating diffraction efficiency and mirror losses, this was reduced to 22 J on target, giving a power to target of 35 swmomnno~m 6 h l J J o m

Experiments of high gain C VI x-ray lasing in rapidly recombining plasmas

Recent experimental results of high gain C VI x‐ray lasing in rapidly recombining plasmas are described. 7 μm diameter carbon fiber targets of up to 5 mm length were irradiated at intensities between 3×1015∼1×1016 W/cm2 by a 2 ps, 20 TW chirped pulse amplification (CPA) beam from the VULCAN Nd‐glass laser (λ=1.053 μm) at RAL. The gain length product on the 18.2 nm Balmer α transition of C VI ions was measured to be 6.5±1. The ratio of intensities of resonance lines of H‐like and He‐like ions in the rapidly recombining plasmas was used as a useful diagnostic of initial conditions for high gain operation of the C VI recombination x‐ray lasing.