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We propose a new scheme for compression of ultrashort laser pulses enabled by Near-Forward Raman Amplification (NFRA) in plasma. In contrast to traditional forward Raman amplification in plasma, the seed pulse in NFRA propagates faster than the pump through manipulation of the spatiotemporal properties of the pump beam. A “π-pulse” solution similar to that of stimulated Raman backscattering amplification is obtained, indicating that NFRA can be used for compression of ultrashort laser pulses. NFRA is numerically demonstrated with a 2D simulation using a front-tilt near-forward pump pulse.

A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.

We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.

Experiments on Stimulated Raman Backscattering (SRBS) in plasma have demonstrated significantly higher energy conversion in a double-pass amplifier where the laser pulses go through the plasma twice compared with a single-pass amplifier with double the plasma length of a single pass. In this paper, the improvement in understanding recent experimental results is presented by considering quite in detail the effects of plasma heating on the modeling of SRBS. Our simulation results show that the low efficiency of single-pass amplifiers can be attributed to Landau damping and the frequency shift of Langmuir waves. In double-pass amplifiers, these issues can be avoided, to some degree, because pump-induced heating could be reduced, while the plasma cools down between the passes. Therefore, double-pass amplifiers yield considerably enhanced energy transfer from the pump to the seed, hence the output pulse intensity.

Periodic plasma density modulations have been observed in a laser spark created by focusing a 5–20 MW laser pulse into nitrogen at 1 to 2 atmospheres. The formation of such structures has been studied by measuring the period as a function of the wavelength λ and the incident angle γ of the laser pulse. The results show good agreement with a surface-instability model. The intensity of the scattered laser light has the same periodicity and is consistent with the measured plasma density profile.

The frequency upshifting of 0.8 μm picosecond laser pulses was demonstrated using the temporal change of the free carrier density in a ZnSe semiconductor crystal. The crystal was ionized by transverse propagating ps pulses. Shifts of up to 1.6 nm were observed, which agree within 25% with the theory.

Details of experiments aimed at achieving low ionization state selectivity in molybdenum are presented. Targets are excited with a 10 J CO2 laser and the resultant vacuum ultraviolet spectrum (300–700 A) has been studied. Combinations of focal spot size, target depth, and target geometries are compared. Simple attenuation of energy is shown not to vary ionization stage composition significantly. Experiments conducted with grazing incidence targets result only in a hot plasma. Modular targets with cooling cylinders of various radii demonstrated good selectivity of the ionization states, but with low absolute signals. Finally, results from combinations of focal spot adjustment and radiative cooling illustrate increased control over desired plasma temperature and density for spectroscopic studies of molybdenum.

In a Raman plasma amplifier, the aim is to create plasma conditions in which Raman backscattering is the fastest growing instability, outrunning all competing effects so that it is possible to amplify and compress a laser beam to unprecedented unfocused intensities by utilizing that instability. However, achieving high efficiencies via this scheme has proven very difficult experimentally. Recent data show the simultaneous occurrence of stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and stimulated electron-acoustic scattering (SEAS). The appearance of SEAS is indicative of strong particle trapping, the existence of which is hard to justify without highlighting the interplay between SRS and SBS.

Raman amplification is a resonant process in which the energy of a long pump pulse is transferred to a short seed pulse by a plasma wave. There has been a significant effort to identify a window in parameter space within which the interaction is expected to be highly efficient and not degraded by competing instabilities or excessive damping. However, experimental results have thus far failed to approach the theoretical limits. Recent amplified signal spectra display a characteristic blue shift, which evolves within the seed pulse duration and suggests that the mechanism responsible for this shift is also limiting amplification in these experiments. We present the evidence and explore different hypotheses for the origins of the shift—namely localization in density minima along the axis of laser propagation induced by an ion acoustic wave that could arise from the Langmuir decay instability, filamentation which could also modulate the plasma density but in the plane transverse to laser propagation, particle...

To optimize the coupling to indirect drive targets in the National Ignition Campaign (NIC) at the National Ignition Facility [E. Moses et al., Phys. Plasmas 16, 041006 (2009)], a model of stimulated scattering produced by multiple laser beams is used. The model has shown that scatter of the 351 nm beams can be significantly enhanced over single beam predictions in ignition relevant targets by the interaction of the multiple crossing beams with a millimeter scale length, 2.5 keV, 0.02−0.05 × critical density, plasma. The model uses a suite of simulation capabilities and its key aspects are benchmarked with experiments at smaller laser facilities. The model has also influenced the design of the initial targets used for NIC by showing that both the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) can be reduced by the reduction of the plasma density in the beam intersection volume that is caused by an increase in the diameter of the laser entrance hole (LEH). In this model, a linea...

Propagation of 800 nm, 120 fs laser pulses with intensities of 4×1016 W/cm2 in supersonic gas jets of N2 and H2 is studied using a shear-type interferometer. The plasma density distribution resulting from photoionization is resolved in space and time with simultaneously measured initial neutral density distribution. A distinct difference in laser beam propagation distance is observed when comparing propagation in jets of H2 and N2. This is interpreted in terms of ionization induced refraction, which is stronger when electrons are produced from states of higher ionization potential. Three dimensional particle-in-cell simulations, based on directly solving the Maxwell–Lorentz system of equations, show the roles played by the forward Raman and ionization scattering instabilities, which further affect the propagation distance.

A plasma-based resonant backward Raman amplifier/compressor for high power amplification of short laser pulses might, under ideal conditions, convert as much as 90% of the pump energy to the seed pulse. While the theoretical highest possible efficiency of this scheme has not yet been achieved, larger efficiencies than ever before obtained experimentally (6.4%) are now being reported, and these efficiencies are accompanied by strong pulse compression. Based on these recent extensive experiments, it is now possible to deduce that the experimentally realized efficiency of the amplifier is likely constrained by two factors, namely the pump chirp and the plasma wavebreaking, and that these experimental observations may likely involve favorable compensation between the chirp of the laser and the density variation of the mediating plasma. Several methods for further improvement of the amplifier efficiency in current experiments are suggested.

The enhancement of stimulated Raman backscattering (SRBS) amplification was demonstrated by introducing a plasma density gradient along the pump and the seed interaction path and by a novel double-pass design. The energy transfer efficiency was significantly improved to a level of 6.4%. The seed pulse was amplified by a factor of more than 20 000 from the input in a 2mm long plasma, which also exceeded the intensity of the pump pulse by 2 orders of magnitude. This was accompanied by very effective pulse compression, from 500fsto90fs in the first pass measurements and in the second pass down to approximately 50fs, as it is indicated by the energy-pulse duration relation. Further improvements to the energy transfer efficiency and the SRBS performance by extending the region of resonance is also discussed where a uniform ∼4mm long plasma channel for SRBS was generated by using two subsequent laser pulses in an ethane gas jet.

Calculations of electron impact collision strengths and spontaneous radiative decay rates are made for neon-like krypton (Kr XXVII) for the 2s2 2p6, 2s2 2p5 3s, 2s2 2p5 3p, and 2s2 2p5 3d configurations. From these atomic data, the level populations as a function of the electron density are calculated at two temperatures, 1 x 10 to the 7th K and 3 x 10 to the 7th K. An analysis of level populations reveals that a volume of krypton in which a significant number of the ions are in the Kr XXVII degree of ionization can produce a significant gain in transition between the 2s2 2p5 3s and 2s2 2p5 3p configurations. At an electron density of 1 x 10 to the 19th/cu cm the plasma length has to be of the order of 1 m; at a density of 1 x 10 to the 21st/cu cm the length is reduced to approximately 0.5 cm; and at an electron density of 1 x 10 to the 22nd/cu cm the length of the plasma is further reduced to approximately 1 mm.

In situ relative and absolute calibrations are described for two grazing incidence vacuum monochromators used in our ongoing x‐ray laser experiments. The soft x‐ray sources used in the calibrations are small plasmas produced by a specially constructed portable vacuum spark generator and by irradiating thin (∼100–300 μm diameter) carbon fibers with a high‐power (1–5 GW, 1012–1013 W/cm2) CO2 laser. The absolute instrumental sensitivities have been determined using the method of branching ratios, while the relative calibrations have been done by simultaneously monitoring given ionic lines with each XUV instrument.

A high‐resolution, time‐resolving soft x‐ray multichannel spectrometer (SOXMOS) that permits the simultaneous measurement of emission in two different spectral ranges has been developed and tested extensively for tokamak plasma diagnostics. The basic instrument is a high‐resolution, interferometrically adjusted, extreme grazing incidence Schwob–Fraenkel duochromator. The instrument is equipped with two multichannel detectors that are adjusted interferometrically and scan along the Rowland circle. Each consists of an MgF2 coated, funneled microchannel plate, associated with a phosphor screen image intensifier that is coupled to a 1024‐element photodiode array by a flexible fiber‐optic conduit. The total wavelength coverage of the instrument is 5–340 A with a measured resolution (FWHM) of about 0.2 A when equipped with a 600‐g/mm grating, and 5–85 A with a resolution of about 0.06 A using a 2400‐g/mm grating. The simultaneous spectral coverage of each detector varies from 15 A at the short wavelength limit to 70 A at the long wavelength limit with the lower dispersion grating. The minimum readout time for a full spectral portion is 16 ms, but several individual lines can be measured with 1‐ms time resolution by selected pixel readout. Higher time resolution can be achieved by replacing one multichannel detector with a single channel electron multiplier detector. Examples of data from the PLT and TFTR tokamaks are presented to illustrate the instrument’s versatility, high spectral resolution, and high signal‐to‐noise ratio even in the 10‐A region.

The measurement of the poloidal magnetic field in a tokamak plasma from the Zeeman splitting and polarization of the magnetic dipole radiation from heavy ions is discussed. When viewed from a direction perpendicular to the toroidal field, the effect of the poloidal field on the circularly polarized radiation is detectable using a photoelectric polarimeter. The Zeeman splittings for a number of magnetic dipole transitions with wavelengths in the range 2300–9300 A are presented. An imaging polarimeter is proposed that can measure the poloidal magnetic field with space and time resolution.

We describe a powerful picosecond KrF* laser system consisting of a synchronously pumped dye oscillator, a three‐stage dye amplifier, frequency conversion to 248 nm, and amplification through a series of excimer modules. Characteristics are given at each stage of the system. With the first stage of the system complete, a focused intensity of ∼1016 W/cm2 is obtained. Initial visible spectra obtained by interaction with various targets are presented.

Hydrogenlike oxygen and carbon lines from transitions (Δn=1) between levels of high principal quantum number n, with wavelengths above 2000 A have been observed. Observations of such transitions were possible due to charge‐exchange processes during neutral beam injection of hydrogen atoms into the Princeton Large Torus tokamak. The lines are O VIII 2976 A (8–7 transitions), C VI 3434 A (7–6), and C VI 5291 A (8–7). Application of these lines for ion temperature measurements and initial observations of neutral beam vertical distributions in the plasma are presented.

Inversions in the population of excited levels in hydrogen and He II were observed in the FM‐1 spherator. The inversion increases strongly as the ratio of the decay time of the electron temperature to the decay time of the electron density was decreased. Time‐dependent numerical calculations of the populations were in good agreement with the experimental measurements. More general calculations for high‐Z hydrogenlike ions are discussed.

A CO2 laser (∼0.5 kJ energy, 50–80 ns pulse width) was focused onto the end of an axially oriented, thick (35–350 μ) carbon fiber with or without a magnetic field present along the laser‐fiber axis. We present evidence for axial‐to‐transverse enhancement of the C VI 182 A (n=3–2) transition, which is correlated with the appearance of a population inversion between levels n=3 and 2. For the B=0 kG, zero field case, the maximum gain length product of kl≊3 (k≊6 cm−1) was measured for a carbon fiber coated with a thin layer of aluminum (for additional radiation cooling). The results are interpreted in terms of fast recombination due mostly to thermal conduction from the plasma to the cold fiber core.

The measurement of the poloidal magnetic field in a tokamak plasma from the Zeeman splitting and polarization of the magnetic dipole radiation from heavy ions is discussed. When viewed from a direction perpendicular to the toroidal field, the effect of the poloidal field on the circularly polarized radiation is detectable using a photoelectric polarimeter.

Energy loss rates of energetic ions were studied during neutral beam injection experiments on the FM‐1 spherator. Under typical experimental conditions, the injected beam ions slowed‐down on the plasma electrons resulting in electron heating. The measured slowing‐down rate was found to be in agreement with classical predictions.

An analysis is presented of an experimental study of the scattering of microwaves by density fluctuations due to the current‐driven ion acoustic wave instability in a toroidal plasma confinement device, the FM‐1 spherator. It is found that the instability exists only in the collisional regime (where νef≳ωbe) and it is inhibited (i.e., quenched by two orders of magnitude) in the trapped‐electron banana regime (where νef

The idea of using a multi‐Z (e.g., carbon, oxygen) thin plasma column as a medium for soft x‐ray lasing action is presented. A plasma confined by a strong magnetic field is first heated by a CO2 laser, and then cools rapidly by radiation losses. This leads to a level population inversion of hydrogen‐like carbon or oxygen ions. Two computational models are presented. One uses given electron temperature Te(t) evolutions. The other uses Te(t) calculated from an energy balance equation with CO2 laser beam power as a parameter. According to calculations, a total gain of G≳100 is expected for 3→2 and G≳10 for 4→2 transitions (λ=182 A and λ=135 A, respectively) for C VI ions using a CO2 laser beam with power ∼5×1010 W for plasma column heating.