X-ray Laser Output Research Articles

Enhancement of Gain Coefficient of Li-Like Ion 3d-4f Soft X-ray Laser Oscillation by a Single Resonator

A recombination plasma soft X-ray laser that is associated with Li-like Aluminum ion Al10+ 3d-4f transition (wavelength: 15.47 nm) was investigated to increase the gain coefficient. By optimizing the time interval of the 16-pulse train, higher-density plasma with a low temperature could be generated, and thus, we obtained the gain coefficient of 9.84 cm−1 at the pump laser pulse energy which was as low as 3 J. To further enhance the X-ray laser output energy, the single resonator which was installed by using a Mo/Si multilayer mirror that has a high reflectivity of 0.45 at 15.5 nm, and the X-ray spectrum after the double pass in the lasing medium was measured. As a result, the output intensity was increased to be 1.5 times higher than t=when it was compared with those that were conducted with no X-ray mirror.

Collisional-radiative calculations for the J = 0−1 lasing line of neon-like germanium under anisotropic excitation conditions

A new asymmetry parameter characterizing the differences between the polarized π and σ gain components of the soft-x-ray J = 0–1 lasing line of neon-like ions is calculated in the case of Ge22+ assuming an electron distribution which is a weighted sum of an isotropic Maxwellian and a monoenergetic beam. Using a quasi steady-state collisional-radiative model, we determine in the weak amplification regime the relative populations of the upper M = 0 and lower magnetic sublevels of the lasing line as a function of electron density from 1020 to cm−3. This model includes inelastic and elastic collisional transitions, as well as spontaneous radiative decay between all the 337 M-sublevels arising from the 75 lowest-lying Ge22+ J-levels. The computations were performed for a temperature of the Maxwellian component between and K, a kinetic energy E0 and a fraction f of the beam component in the ranges and , respectively. The basic atomic data, such as level energies, radiative decay probabilities and inelastic collision strengths, were calculated with the flexible atomic code. However, some modifications of this code were made to get the collision strengths for transitions between M-sublevels due to impact with isotropic electrons as well as due to impact with an electron beam in the case of de-excitation. We find that the newly introduced asymmetry parameter may become significant under certain conditions of electron distribution corresponding to relatively low ( K) and E0 (3–6 keV). The results reported here may be useful in the evaluation of the polarization degree of the J = 0–1 x-ray laser output from a germanium plasma in the presence of fast directional electrons.

Ultraviolet laser transverse profile shaping for improving x-ray free electron laser performance

The photocathode rf gun is one of the most critical components in x-ray free electron lasers. The drive laser strikes the photocathode surface, which emits electrons with properties that depend on the shape of the drive laser. Most free electron lasers use photocathodes with work function in the ultraviolet, a wavelength where direct laser manipulation becomes challenging. In this paper, we present a novel application of a digital micromirror device (DMD) for the 253 nm drive laser at the Linear Coherent Light Source. Laser profile shaping is accomplished through an iterative algorithm that takes into account shaping error and efficiency. Next, we use laser shaping to control the X-ray laser output via an online optimizer, which shows improvement in FEL pulse energy. Lastly, as a preparation for electron beam shaping, we use the DMD to measure the photocathode quantum efficiency across cathode surface with an averaged laser rms spot size of $59\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our experiments demonstrate promising outlook of using DMD to shape ultraviolet lasers for photocathode rf guns with various applications.

Method to eliminate the damage aroused by intrinsic prepulse of Blumlein transmission line in capillary discharge soft X-ray laser system

To raise the utilization ratio of the energy power, and meanwhile, guarantee the waveform of the main discharge current to obtain X-ray laser, our capillary discharged Ne-like Ar soft X-ray laser system adopts Blumleim transmission line (BTL) and a 300 kV-Marx generator; and by using our self-developed intrinsic-prepulse elimination device and extrinsic-prepulse generator, we successfully removed the intrinsic prepulse of BTL, and thus eliminated its deteriorative effect towards Z-pinch plasma. By this means, we obtained stable X-ray laser output; and the voltage transmission rate of our system nearly reaches 122%, which provides a guarantee for the miniaturization of the power source and the whole system.

Grazing incidence pumping of an 18.9 nm Ni-like Mo X-ray laser

Results of an experiment investigating the grazing-incidence pumping of a Ni-like Mo X-ray laser are presented. The X-ray laser was produced with a 600 mJ, 500 ps background pulse and a 250 mJ, 240 fs main pulse. X-ray laser output was measured using a flat-field spectrometer modified to record both the first- and second-order output of the molybdenum lasing emission. Time-resolved data was obtained using an Axis-Photonique PX1 X-ray streak camera coupled to the first order output of the flat-field spectrometer. It is shown that saturated X-ray laser output is produced with energy up to 150 nJ, and pulse duration of typically 1.7 ps for a range of grazing angles and delays between the background and main pulse. The experimental data is interpreted in terms of a model taking account of the expected ASE variation of X-ray laser output.

Time-resolved measurements of the angular distribution of the Ni-like Pd soft-x-ray laser emission

The temporal history of the x-ray laser output has been measured in dependence of the emission angle of the x-ray laser with respect to the target plane. The results show both the exit time (the time at which the x-ray laser pulse is leaving the plasma) and the pulse duration to depend on the angle of emission. Simulations of the gain in Ni-like Pd combined with ray tracing through the plasma were used to model the dependence of the time of emission on the deflection angle of the individual rays within the laser emission cone. The simulations are in reasonable agreement with the measurements.

Optimal main pulse angle for different preplasma conditions in transient collisionally pumped x-ray lasers

The effects of the incidence angle of the main pump (MP) pulse in non-normal pumping geometry and the influence of the MP duration are investigated experimentally and theoretically for a transient collisionally pumped (TCE) x-ray laser in Ni-like Zr at 45 degrees and 72 degrees incidence angle on the target. The way they transfer to the x-ray laser output depends on the preplasma conditions, most notably on the average ionization distribution at the arrival of the MP. Moreover, contrary to previous grazing incidence pumping results, it is found that the shortest attainable MP maximizes the output. Modeling of the experimental results is performed with EHYBRID code. The results are important for scaling high repetition-rate non-normal incidence pumped lasers to sub- 10nm wavelengths.

Absolute timing measurements of the Ni-like Pd and Sn soft-x-ray lasers

The absolute time of emission of the x-ray laser output with respect to the arrival of a 100-ps pump pulse has been measured with the aid of a calibrated timing fiducial. The results show the x-ray laser to appear up to 60 ps (80 ps) before the peak of the pump pulse in the case of the Sn (Pd) x-ray laser, which is in good agreement with simulations obtained from the LASNEX and CRETIN codes. The pulse duration was found to be {approx}45 ps for both the Sn and the Pd x-ray lasers.

Experiments and Simulations of Short-Pulse Laser-Pumped Extreme Ultraviolet Lasers

Recent experimental work on the development of extreme ultraviolet lasers undertaken using as the pumping source the VULCAN laser at the Rutherford Appleton Laboratory is compared to detailed simulations. It is shown that short duration (/spl sim/picosecond) pumping can produce X-ray laser pulses of a few picosecond duration and that measurement of the emission from the plasma can give an estimate of the duration of the gain coefficient. The Ehybrid fluid and atomic physics code developed at the University of York is used to simulate X-ray laser gain and plasma emission. Two postprocessors to the Ehybrid code are utilized: 1) to raytrace the X-ray laser beam amplification and refraction and 2) to calculate the radiation emission in the kiloelectronvolt photon energy range. The raytracing and spectral simulations are compared, respectively, to measured X-ray laser output and the output of two diagnostics recording transverse X-ray emission. The pumping laser energy absorbed in the plasma is examined by comparing the simulations to experimental results. It is shown that at high pumping irradiance (>10/sup 15/ Wcm/sup -2/), fast electrons are produced by parametric processes in the preformed long scalelength plasmas. These fast electrons do not pump the population inversion and so pumping efficiency is reduced at high irradiance.

Quantitative simulations of short pulse x-ray laser experiments

EHYBRID fluid and atomic physics code simulations are compared with experimentally measured Ne-like nickel x-ray laser output at 23.1 nm and emission from nickel and germanium plasmas produced by line focus irradiation into a pre-formed plasma with 1.2 ps pulses of 1.06 µm wavelength and peak irradiance 7 × 1015 W cm−2. A particle-in-cell (PIC) code is used to simulate the laser energy absorption and resulting electron energy distribution in the experiment. It is shown that some absorbed laser energy is distributed to fast electrons that do not produce collisional excitation of the population inversion.

Design of the Ni-like Ag x-ray laser at 13.9 nm

The Ni-like Ag x-ray laser at 13.9 nm (4d→4p, J=0→1) is numerically studied, using a one-dimensional hydrodynamic code coupled with an atomic physics code. The effects of the drive laser intensity, pulse duration, and delay time on the performance of the Ni-like Ag x-ray laser are investigated. A saturated x-ray laser output could possibly be generated for a short plasma length under the optimized drive pulse configuration. The analysis of the plasma conditions, including the evolution of gain in time and space, the electron temperature and the electron density, contribute to further understanding of the process of Ni-like x-ray lasers.

Comparison of simulated and experimental time resolved emission for a Ne-like nickel x-raylaser

X-ray laser output at 23.1 nm and the intensity of resonance line and continuum emissionbetween 0.6 and 1.8 nm emitted from a nickel plasma are simulated using a fluid and atomicphysics program. The simulations are undertaken for the conditions of a recent experimentusing a Nd:glass laser with a ~1.2 ps pulse at 7 × 1015 W cm−2 irradiance pumping a plasma pre-formed by a 280 ps duration pulse at2 × 1013 W cm−2 with peak-to-peak pulse separation usually set at 300 ps. The simulated duration of x-raylasing (~12 ps) agrees with the measured laser duration and the temporal output of continuum andresonance lines is in agreement with measurements made using a streak camera.

Statistical investigations of the beam stability of the double-pass amplified zinc soft X-ray laser at 21.2 nm

At the Prague asterix laser system (PALS) of the Academy of Sciences of the Czech Republic the 1-TW asterix iodine laser is used as a pump source for soft X-ray laser experiments. The prepulse technique was applied which is known to enhance the X-ray laser output at the J=0-1 transition dramatically. Since Zn slab targets were used the laser wavelength was 21.2 nm. A prepulse beam having 1.6 J was preceding the main pulse by 10 ns. The main and the prepulse beam are focused by two different optical systems separately. Implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror the X-ray laser output was 10 times stronger than at single pass amplification in a 3-cm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 120 ps longer than the round trip time in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing 4 mJ energy what is proved to be enough for applications. In this contribution the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the time-integrated output energy are investigated over more than 100 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series -- achieved during one single day -- the corresponding values are given and detailed chi-squared tests characterize the Zn X-ray laser as a robust tool suitable for applications. At PALS soft X-ray laser beam time can be reserved for external research groups.

Beam properties of a deeply saturated, half-cavity zinc soft-x-ray laser

We report what is to our knowledge the first experimental study of beam quality and prepulse effect for a deeply saturated neonlike Zn soft-x-ray laser at 21.2 nm, comparing two distinct regimes of operation: amplified spontaneous emission (in single pass) and double pass. The single-pass x-ray laser output emitted by a 30-mm-long plasma column delivers a smooth, highly symmetric ellipsoidal beam with horizontal and vertical divergence of 3(±0.5) and 5(±0.5) mrad, respectively. With a half-cavity consisting of a Mo:Si flat multilayer mirror, the x-ray laser output is boosted by factor of ∼11, providing a narrowly collimated beam with horizontal and vertical divergence of 3.8(±0.5) and 5.8(±0.5) mrad, respectively. The dependence of the beam parameters on the half-cavity setup and on the level of prepulse is described and discussed.

Analytical and numerical ray tracing of a transient x-ray laser: Ni-like Ag laser at 139 nm

Transient soft-x-ray lasers are generated from a solid target irradiated by two intense pump laser pulses. Amplification is achieved in the plasma column thus produced. Knowledge of the beam propagation is vital for the intensity and quality of the x-ray laser output. In this paper, x-ray laser beam propagation in transient plasmas is studied both analytically and numerically. General one-dimensional formulas are developed for beams in electron density gradient media, including the exponential profile that describes the plasma created from a solid target. The gradient is predicted to limit the amplification length within the maximum gain to <2.6 mm in standard experiments. The result given by the analytical model is confirmed by numerical ray tracing of x-ray laser beams within an amplifying medium as it is defined by the full numerical simulation results of the ehybrid code.

Modeling of the transient nickellike silver x-ray laser

Recent high-temporal-resolution nickellike x-ray laser experiments have yielded important insights into the output characteristics of picosecond-pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics, which is critical to gain generation within the x-ray laser medium. A numerical study of the nickellike silver x-ray laser has therefore been undertaken to complement our experimental results in an attempt to further our understanding of the processes at work in yielding the observed x-ray laser output. High gain coefficients existing with picosecond lifetimes are predicted, which is consistent with the short x-ray laser durations experimentally observed. The late onset of the continuum emission relative to the temporal peak of the x-ray laser output is explained as a sign of high electron density evolution near the target surface.

Measurement of the duration of X-ray lasing pumped by an optical laser pulse of picosecond duration

Measurements of the duration of X-ray lasing pumped with picosecond pulses from the VULCAN optical laser are obtained using a streak camera with 700 fs temporal resolution. Combined with a temporal smearing due to the spectrometer employed, we have measured X-ray laser pulse durations for Ni-like silver at 13.9 nm with a total time resolution of 1.1 ps. For Ni-like silver, the X-ray laser output has a steep rise followed by an approximately exponential temporal decay with measured full-width at half-maximum (FWHM) of 3.7 (±0.5) ps. For Ne-like nickel lasing at 23.1 nm, the measured duration of lasing is ≈10.7 (±1) ps (FWHM). An estimate of the duration of the X-ray laser gain has been obtained by temporally resolving spectrally integrated continuum and resonance line emission. For Ni-like silver, this time of emission is ≈22 (±2) ps (FWHM), while for Ne-like nickel we measure ≈35 (±2) ps (FWHM). Assuming that these times of emission correspond to the gain duration, we show that a simple model consistently relates the gain durations to the measured durations of X-ray lasing.

Study of the stability of beam characteristics of the neon-like Zn X-ray laser using a half cavity

At the Prague Asterix Laser System Center (PALS) the Asterix iodine laser delivering up to 700 J/0.5 ns is used as a pump source for X-ray laser experiments and applications. The prepulse technique was applied which is known to improve the neon-like X-ray laser output at the J = 0-1 transition dramatically. Since Zn slab targets were used the operating wavelength was 21.2 nm. A prepulse having up to 20 J precedes the main pulse by 10 ns. The main beam and the prepulse beam are focussed by two different optical systems separately and their foci are superimposed at the target surface. By implementing a half-cavity set-up for double-pass amplification using a Mo/Si multilayer mirror - which can be used for more than 100 shots - the X-ray laser output was more than 10 times stronger than at the single pass in a 30 mm long plasma. Double-pass amplification was observed to be most efficient when the pump pulse duration was at least 150 ps longer than the round trip time ( ≈ 260 ps) in the half-cavity. Under this fundamental condition the X-ray laser reached saturation in the double-pass regime containing approx. 4 mJ energy which has been proved to be enough for future applications. In this contribution, the X-ray laser features like divergence in two dimensions, the beam quality (symmetry), the pointing angle and the integrated intensity giving an estimation of the output energy are investigated over 110 shots. To characterize the stability of the X-ray laser the shot distribution, the mean value and the standard deviation for these parameters are evaluated. For 18 shots in a series - what was achievable during one day - the corresponding values are given, and a statistical analysis carrying out a chi-squared test characterize the Zn X-ray laser as a robust tool suitable for applications. In the future it is planned to allocate X-ray laser beam time to external research groups.

STUDY OF COMPACT X-RAY LASER PUMPED BY A PULSE-TRAIN LASER

An advanced experiment has been performed to improve X-ray laser output in the recombination plasma scheme. Two 11-mm-long plasmas were produced by laser irradiation so that each axis aligned into a line consisting the double-target configuration. X-ray intensity of the 15.47 nm transition line of the Li-like Al ion was enhanced in the double-target scheme. A half-cavity experiment in the double-target configuration was also performed, and the enhancement of X-ray was also observed. The results in the half-cavity experiment were consistent with an estimation, where the two plasmas having positive gain contribute to amplify X-rays passing through plasmas and reflecting from the multilayer mirror. Measurements have been started to clarify spatial coherence of the lasing X-ray beam by using the dispersing coherent diagnostic system.

Short pulse pumped X-ray lasers

Double laser pulses of duration - 75 ps and short laser pulses ∼ 1 ps superimposed on longer duration background pulses have been shown to efficiently pump lasing in Ne-like and Ni-like ions. For the 75 ps pumping, X-ray laser output without travelling wave pumping is shown to be well-described by a model of ASE output. With 1 ps pumping, the X-ray laser output with different velocity travelling wave pumping is well-fitted with an extension to the ASE model allowing for travelling wave excitation of the gain along the plasma line. The model is used to investigate the production of short (< 1 ps) x-ray laser pulses and the effects of non-ideal travelling wave velocities on the X-ray laser output. Resonance line spectra of emission perpendicular to the plasma line are measured and simulated. It is shown that an accurate opacity model for the more intense Ne-like ions is needed to correctly simulate the spectra.