Ciaran Lewis

An ultra-high gain and efficient amplifier based on Raman amplification in plasma

Raman amplification arising from the excitation of a density echelon in plasma could lead to amplifiers that significantly exceed current power limits of conventional laser media. Here we show that 1–100 J pump pulses can amplify picojoule seed pulses to nearly joule level. The extremely high gain also leads to significant amplification of backscattered radiation from “noise”, arising from stochastic plasma fluctuations that competes with externally injected seed pulses, which are amplified to similar levels at the highest pump energies. The pump energy is scattered into the seed at an oblique angle with 14 J sr−1, and net gains of more than eight orders of magnitude. The maximum gain coefficient, of 180 cm−1, exceeds high-power solid-state amplifying media by orders of magnitude. The observation of a minimum of 640 J sr−1 directly backscattered from noise, corresponding to ≈10% of the pump energy in the observation solid angle, implies potential overall efficiencies greater than 10%.

Plasma-based studies with intense X-ray and particle beam sources

The construction of short pulse (<200 fs) tunable X-ray laser sources based on the X-ray free electron laser (XFEL) concept will be a watershed for plasma-based and warm dense matter research. These new fourth generation light sources will have extremely high fields and short wavelengths (∼0.1 nm) with peak spectral brightnesses 10 10 greater than third generation sources. Further, the high intensity upgrade of the GSI accelerator facilities will lead to specific energy depositions up to 200 kJ/g and temperatures between 1 and 10 eV at almost solid-state densities, enabling interesting experiments in the regime of nonideal plasmas, such as the evolution of intense ion beams in the interior of a Jovian planet. Below we discuss several applications: the creation of warm dense matter (WDM) research, probing of near solid density plasmas, and laser-plasma spectroscopy of ions in plasmas. The study of dense plasmas has been severely hampered by the fact that laser-based methods have been unavailable and these new fourth generation sources will remove these restrictions.

DESIGN AND PERFORMANCE OF A NEW LINE FOCUS GEOMETRY FOR X-RAY LASER EXPERIMENTS

A novel optical line focus system using a lens and off-axis mirror combination which allows a high quality line focus to be obtained has been developed for x-ray laser research. By irradiation of fiber and thin foil targets with a high power laser various experimental observations show the excellent performance of this new scheme.

A COMPUTATIONAL INVESTIGATION OF THE NEON-LIKE GERMANIUM COLLISIONALLY PUMPED LASER

The complex problem of a collisionally pumped Ne-like germanium laser is examined through several detailed models. The central model is EHYBRID; a 1 1/2 D fluid code which self consistently treats the plasma expansion with the atomic physics of the Ne-like ion for 124 excited levels through a collisional radiative treatment. The output of EHYBRID is used as data for ray-tracing and saturation codes which generate experimental observables. A detailed description of the models is given. The atomic physics is investigated through a three-level approximation, a steady state collisional radiative treatment and a time-dependent solution within the fluid model. The accurate calculation of the non-steady state ionization balance is identified as a key issue. Time resolved and time integrated output profiles are generated for various experimental configurations, and the effects of saturation and gain narrowing are examined. The agreement with experiment is excellent in virtually every respect.

Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

Picosecond x-ray chronoscopy

Abstract An X-ray/VUV streak camera with a time resolution of ≈20 psec is described. X-ray pulses, from a laser plasma, down to 22 psec half width, separated by 42 psec, are clearly resolved. Photoelectron time-dispersion effects and the absolute sensitivity of the camera are discussed.

THE VARIATION OF MASS ABLATION RATE WITH LASER WAVELENGTH AND TARGET GEOMETRY

Abstract Measurements of laser-driven mass ablation rates are presented for wavelengths of 1.05 μm, 0.53 μm and 0.35 μm. For spherical and plane targets the mass ablation rates scale as I 0.7 and I 0.3 respectively. The difference is attributed to lateral energy loss on plane targets. Values of ablation pressure are derived.

QUASI-MONOCHROMATIC, PROJECTION RADIOGRAPHY OF DENSE LASER DRIVEN SPHERICAL TARGETS

Abstract A diagnostic technique to measure density profiles in dense plasma generated by the compression of shell targets under laser irradiation is considered. The method employs projection of rays from a quasi-point pulsed X-ray source through the object with subsequent dispersion from a flat Bragg crystal. Conditions necessary for achieving crystal rocking curve wavelength limited spatial resolution in all directions are derived. Scaling laws representing the limits of the technique in the presence of refraction effects in the plasma are given. Plasmas with pr ≈ 0.1 g cm -2 can be diagnosed at λ ≈ 2.5 A. Preliminary experimental radiographs have been recorded illustrating the feasibility of the technique.

Characterisation of soft X-ray amplification observed in Ne-like germanium

Abstract Soft X-ray amplification has been observed on five 3p → 3s transitions of Ne-like germanium at 19.6, 23,2, 23.6, 24.7 and 28.6 nm using flat, massive targets irradiated in 50 and 150 μm wide line foci up to 3.2 cm long at intensities of 0.3 → 14 × 10 13 W/cm 2 by 1.06 μm laser light in 1 ns and 0.5 ns pulses. Gain lengths up to gl ≈ 12 were measured for the J =2 → 1 transition at 23.6 nm and the temporal behaviour of four of the Ne-like germanium lasing lines has been investigated for the first time. The duration of the gain was 60 → 80% of the fwhm of the laser pulse, with the gain turning off at the trailing half-maximum of the incident laser intensity. Output powers in excess of 10 kW per line have been observed.

The Langmuir probe as a diagnostic of the electron component within low temperature laser ablated plasma plumes

A Langmuir probe has been used as a diagnostic of the temporally evolving electron component within a laser ablated Cu plasma expanding into vacuum, for an incident laser power density on target similar to that used for the pulsed laser deposition of thin films. Electron temperature data were obtained from the retarding region of the probe current/voltage (I/V) characteristic, which was also used to calculate an associated electron number density. Additionally, electron number density data were obtained from the saturation electron current region of the probe (I/V) characteristic. Electron number density data, extracted by the two different techniques, were observed to show the same temporal form, with measured absolute values agreeing to within a factor of 2. The Langmuir probe, in the saturation current region, has been shown for the first time to be a convenient diagnostic of the electron component within relatively low temperature laser ablated plasma plumes.