We are developing a multi-kHz repetition rate high-average power Ti:sapphire regenerative amplifier as a pumping laser of a laser-plasma X-ray source. With an optimally designed ring resonator with a cryogenically-cooled laser rod, the average output power of 54 W before compression was achieved when pumped by a 180-W green laser at 10 kHz repetition rate. The focusability of the output beam was better than two times of the diffraction limit and can be compressed to 82 fs. Possibility of scaling to higher output power is discussed.
Technologies for materializing our cotton-cake like tin target scheme are being developed. With the scheme, we expect to achieve 4% conversion efficiency into 2π sr supported by our experimental data. In order to implement EUVL, EUV power exceeding 100 W is considered to be required to be sent to an illumination box. Large collection solid angle and high conversion efficiency are the mandatory requirements for a source for EUVL. A route to the goal is not yet clear. With our scheme, we can expect EUV exceeding 100 W at the entrance of an illuminator is obtained with a 15 kW YAG laser. Difficulties we encountered when we tried plasma production by shooting droplets of a SnO<sub>2</sub> suspension were preparation of a long life SnO<sub>2</sub> suspension and stable droplet generation with the suspension. In these few years our technologies are highly improved, and we are now able to supply the suspension for several hours without stop, and concentration of a suspension is now increased to as high as 40 wt %. EUV intensity dependence on concentration was studied by shooting a jet of a suspension. We found the EUV intensity saturated at around several at %, which corresponds to several tens wt%, and the EUV intensity comparable to that from a Sn plate was observed. By introducing active synchronization of laser pulses with droplets, we can now shoot droplets running at 10 kHz with a 10 Hz YAG laser with no miss shot. We are now ready to challenge formation of cotton-cake like tin target to demonstrate a very high CE.
Particle-cluster tin target is presented as the solution of a 100W EUV source for EUVL. Theory for maximizing conversion efficiency of a laser-produced plasma is derived and the theory is experimentally confirmed by using a dispersed SnO<sub>2</sub> particles. The EUV intensity 4 times higher than that from a plasma on a solid Sn plate target is observed at the optimized density. The achieved conversion efficiency for dispersed particles is estimated to be as high as 3%/(2π str 2%BW) or higher from the value for a Sn plate of 0.8% measured by using two multilayer mirrors and a calibrated photodiode. Theoretical consideration reveals that larger diameter plasma enables higher EUV power. The particle-cluster can be delivered at multi kHz rep-rate by using water droplet. Experimental confirmation of delivering particles by droplets is also reported.
Debris-free generation of a tin plasma was demonstrated in the cavity-confined configuration. Narrow band emission at 13.7-nm was observed in an emission spectrum of a cavity confined tin plasma. The spectral efficiency was as high as 12% and we found the conversion efficiency could reach 6%/2π str ultimately while lots of works are required to achieve this value. We also confirmed a magnetic field has some effect of stopping a plasma.
We have developed a new technique for high intensity short pulse generation. It consists of a steep pulse generation and a saturated amplification. Stimulated Brillouin scattering by a broad-band oscillator pulse was used to generate the steep Stokes pulse. The Stokes pulse was amplified by discharge type KrF laser amplifier under strongly saturated condition. The pulse shortening by the amplification was confirmed.
Hydrodynamic simulation code that is based on the cubic interpolated pseudo-particle scheme and the model of the equation of state is developed to analyze the hydrodynamic instabilities in the inertial confinement fusion. Ablation structure, shockwave, and the hydrodynamic instabilities by KrF laser were investigated by the 1D and 2D hydrodynamic simulation.
We have developed a simple method of power multiplication. In this method, multi-path forward Raman amplifier is used. By multi-path amplification, it is possible to transfer almost all energy from long duration pump pulse to short Stokes pulse with small numbers of beams. The demonstration experiments of this method have been carried out by using a short pulse Stokes generator, a forward Raman Preamplifier and a forward Raman pulse compressor. Pump light is one of the sequential output pulses from the e-beam excited KrF laser system ASHURA. Short STokes pulse is generated in the Stokes generator filled with a mixture of methane and hydrogen gases as Raman scattering medium. The Stokes pulse is amplified to almost half of pump intensity by the Raman preamplifier also filled with same kinds of mixed scattering medium. The Raman pulse compressor is a 5-path amplifier with total Raman gain of 10. In latest experiments, the output Stokes pulse grew up to 3.2 times pump intensity by Raman conversion of pump power of 70 percent. The waveform of output Stokes was similar shot pulse as initial Stokes light.
A repetition-rate electron beam pumped KrF laser amplifier is being built at the Electrotechnical Laboratory to develop the technologies required for an inertial confinement fusion energy driver. The pulsed power system of the prototype amplifier has already been competed and generates pulses of -300kV, 80ns with a repetition-rate of 1Hz. The high voltage electric short pulses are obtained by step-up pulse transformers, magnetic switches and a water dielectric pulse forming line instead of a conventional Marx generator with gap switches. One of the key technologies with this system is the cooling method of pressure and anode foils in a HIBACHI structure to increase those lifetimes. Our design adopts radiation and conduction as the main cooling processes and allows the foils to reach a high temperature. HAVAR and molybdenum are chosen as pressure and anode foil respectively instead of conventional titanium foils. The maximum temperature of the foils were numerically estimated to show the feasibility of the design.
A new pumping scheme named exploding pumping is proposed for realizing recombination x-ray lasers with high excitation efficiency. In the new scheme, a very thin membrane is employed as a target and it is heated instantaneously before the plasma starts to move by a high peak power sub- picosecond pumping laser. In the scheme, the plasma heating efficiency is improved by being free from heat conduction loss to bulk, and by suppressing the energy loss to hydrodynamic motion. Inertia of the mass delays the start of the plasma motion and gives sufficient time for full ionization. Owing to the extreme thinness of the initial high density plasma, only a few micrometers expansion leads to great reduction of the density and cools down rapidly to produce large gain. Efficient heating of a membrane plasma is confirmed in experimentally observed x-ray spectra and charge collector signals. It is discussed that the most serious problem for realizing water window x-ray lasers is conventional ablation pumping is density gradient which causes refraction of x-rays and limits gain length. The new pumping scheme can solve this refraction problem. The density profile of the expanding plasma in this scheme is fairly uniform because all material expand explosively and because no mass is supplied during the expansion. According to a numerical simulation, 3.34-nm water window of gain length product of 10 will be realized with the 6 J/ 0.3 ps laser irradiation. Longer wavelength x-ray lasers around 13- nm will be realized with a few J/ a few ps pulse pumping. The key technology in the new pumping scheme is suppression of pre-pulse. The effect of pre-pulse is experimentally observed, and means for pre-pulse suppression is discussed.
Experiments are reported in which a high-density aluminum plasma was generated by a 10-picosecond KrF laser pulse. The amplified spontaneous emission (ASE) level of the prepulse was controlled by using a saturable dye. The electron density was estimated from the intensity ration of the Ly-alpha satellite components. It is found that ASE has to be suppressed to less than 10 exp 9 W/sq cm to create a high electron density plasma.