The high power operation of gyrotrons exceeding their nominal powers has been carried out for ECRH at the plug region in the GAMMA 10 tandem mirror device. The highest recorded value of the axial ion confining potential for a hot ion mode plasma has been obtained. The mean energy Teff of the axially flowing electrons driven by the plug ECRH increases to 3 keV according to the increase in the heating power up to 260 kW. The maximum field-aligned potential difference ΔΦ has reached 5.5 kV and the scaling law between ΔΦ and Teff has expanded.
Laser-induced shock waves can generate very high pressures in condensed phase materials and are very suitable for the equation of state (EOS) studies under high pressures where conventional technologies can not attain. Characteristics of shock compression and conventional experimental shock compression technologies are briefly reviewed for understanding the relatively new technology, laser-induced shock wave compression. Generation and properties of laser induced shock waves are explained with focused interests on equation-of-state.
Laser-induced shock waves have been used to study the extreme state of various materials (equation of state;EOS), achieving pressures exceeding 100 Mega bars (10 Tera Pascal). Recent developments regarding experimental systems, including diagnostics, have been proved to obtain accurate EOS data with errors of less than several percent. Methods to obtain EOS data are introduced as well as the diagnostics.
High power lasers are nowadays a tool that can be used to determine important parameters in the context of Warm Dense Matter. In this paper, the general issues that have been addressed in the last few years are presented. Recent results concerning water and iron experiments are exposed as a paradigm of laser driven shocks experiments.
Equation-of-State (EOS) experiments, based on an impedance matching scheme, were performed on plastic materials using laser-driven shock waves. The hugoniot of polystyrene, polyimide, and deuterated-polyethylene, each of which are materials of interest in inertial fusion energy studies, were measured. The EOS points were determined with good accuracy in very wide pressure regime.
We performed equation of state measurements of tantalum using laser-induced shock waves. A strong shock wave (> TPa) was generated in tantalum by the direct irradiation of laser beams from the GEKKO⁄HIPER glass laser system (laser wavelength λL = 351 nm, laser pulse width τL= 2.5 ns) of the ILE of Osaka University. A two-step, two-materials target, called a double-step target, was used in this experiment. The target assembly was fabricated from a base plate (aluminum or copper), a standard step (aluminum or copper), and a sample step (tantalum). The shock velocity was measured directly by observation of the emission from high temperature material or the change of the reflectivity of the rear surface of the target resulting from the arrival of the shock wave. Particle velocity and pressure were calculated using an impedance-matching technique. The obtained EOS data showed close agreement within the limits of experimental error with the extrapolation of the shock compression curve obtained by the conventional experimental shock technique.
Flyer acceleration by laser generated plasmas can achieve even higher pressures than direct ablation methods. The principles of acceleration and high pressure generation were reviewed. Numerical simulations and some experimental results proved that the flyer method is promising in equation of state studies under extremely high pressures, where direct irradiation methods cannot produce well defined shock compression states. Future possibilities of symmetric impact experiments by“cold flyers”are discussed.
We have developed laser-driven shock compression and sample recovery techniques for exploring novel materials. Strong shock waves are generated by hypervelocity impacts of mini-flyer accelerated by pulsed laser ablation. We have measured the mini-flyer velocity using a time-resolved velocimeter (VISAR), with the impact pressures estimated by means of the impedance match solution method. Through sample recovery techniques, the high-pressure phase transitions and dynamic stability of cubic diamond and hexagonal BN are investigated and compared in terms of shock compression durations.
Ultrashort pulsed electron beams and X-rays were generated by femtosecond intense laser field and used for a time-resolved measurement of matter under shock compression. Time-resolved electron shadowgraphy measurement was performed involving the infrared picosecond ablation of a copper film in order to investigate the expansin of plasma into a vacuum. The electron shadowgraphy clearly showed shock wave in the plasma and a dense plume having an expansion speed of 970 and 110km⁄s, respectively, for picosecond laser irradiation. Time-resolved X-ray diffraction was also performed to probe the shock wave in single-crystal silicon under picosecond laser irradiation, and the temporal and spatial evolution of the strain profiles in the silicon crystal was measured.
An intense, short-pulsed laser beam can accelerate a small flyer as fast as LEO (low earth orbit) satellite velocity. Using laser-accelerated flyers, we performed hyper-velocity impact tests as a simulation of orbital debris impact. Using a high-speed framing camera, we succeeded in observing the deformation and fracture behavior of a CFRP (carbon fiber reinforced plastics) target. After the impact experiments, we investigated the damages to the target using an optical microscope, and observed delaminations as well as cracks along the carbon fibers using a scanning electron microscope (SEM). These results indicated the following impact fracture mechanism of CFRP laminates : (1) Spallations are caused by reflected tensile waves and the fracture surfaces similar to the crack-opening mode I are created. (2) Spalling cracks propagate along the direction of the carbon fibers and produce fracture surfaces of shear mode II or mixed-mode I⁄II. (3) Carbon fibers are kinked and broken by tension at the center of the spalling layer.
The current status of equation-of-state(EOS) study using lasers is summerized. Absolute measurement of EOS data is a key issue in the future studies. Isentropic compression technology is also important in several aspects.
A laser shock wave is generated by the irradiation of a 10-ns pulsed laser beam using a plasma confinement target assembly. Nanosecond time-resolved fluorescence spectroscopy is performed on rhodamine-6G dye in an ethanol solution. The fluorescence spectra show a red shift under shock compression. The observed data demonstrate that the fluorescence of rhodamine-6G dye is suitable for use as a pressure scale for laser shock compression at the examined pressure range.
Experiments, to measure temperature, pressure and shock wave velocity of compressed polystyrene (PS) simultaneously, are first carried out using the GEKKO XII⁄ HIPER laser system of the Institute of Laser Engineering, Osaka University. An optical system is made of Schwarzschild microscope and a biprism, which allowed the image of rear surfaces of a double-step target to be split into two and focused onto a slit of a visible streak-camera. Each image through two different band-pass filters is then recorded by using the streak-camera for spatially and temporally resolved measurement. First, pressure and shock wave velocity are calculated by observing the emissions from shock break out at the each step. Secondly, we measure the emissivities of the ultra-violet (UV) and blue range of spectrum from the shock wave-front passing in PS and decide color temperature from the ratio.
The quenching of the ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The crystalline structure in a recovered iron sample was determined using an electron backscatter diffraction pattern method and an electron diffraction pattern method. A small quantity of the γ phase of iron also existed. Thermodynamic state inside the shock front has to be known because the shock induced phase transition occurs inside the shock front. Therefore, the temperature inside the shock front was calculated using thermodynamic equations. It was found that the ε phase was induced by the shock itself but not the γ phase. The γ phase was suggested to be induced as an intermediate structure between the α-ε transition. The femtosecond laser driven shock may have the potential to quench high-pressure phases which has not been attained using conventional methods.
The GEKKO⁄HIPER-laser driven shock experiments were characterized in detail for studies on equation-of-state (EOS) in ultra-high pressure regime. High-quality shock waves were produced with optically smoothed laser beams. Key issues on EOS measurement with shock waves, the spatial uniformity and the temporal steadiness of shock, and the preheating problem were investigated by measurements of the self-emission and reflectivity from target rear surface. Our experiments and analysis based on impedance matching method were validated by use of double-step targets consisting of two Hugoniot standard metals. Extreme shock waves previously only achieved in nuclear explosion experiments were generated using the laser direct-drive experimental scheme.
The multiscale simulation method for the damage evolution in irradiated materials is presented. It is shown that the bias effect plays an important role in the damage evolution, such as void swelling and others. The bias effects in general can be categorized into two, dislocation bias and production bias, and basic behaviors for these processes are investigated from the viewpoint of (i) the interaction between a dislocation and point defects and defect clusters, and (ii) one dimensional motion of interstitial clusters (bundled crowdions). Not only the atomistic features in a model lattice (in the region of smaller scale), but also the elastic features in the elastic body (in the region of larger scale) are presented. From these fundamental studies the prediction of the damage evolution in materials under irradiation will become available.
The ratio of population densities of C3+ excited levels, C3+(3p2P3⁄2), C3+(n = 6), and C3+(n = 7), indicates that C3+(n = 7) is predominantly excited by electron collision with the ground state of C3+ in attached plasmas. Hence, Doppler broadening of the C IV (n = 6-7) spectral line provides C3+ temperature. Regression analysis of the spectrum results in two C3+ temperature components. The higher C3+ temperature ranges from 50 eV to 150 eV, the lower around 20 eV. From the results of the analyses with plasma and impurity transport codes and a collisional-radiative model, the higher and the lower C3+ temperature correspond to the C3+ temperature of the common flux (divertor) plasma and of the private plasma, respectively. In the inner divertor, C3+ temperature is close to D+ temperature, and the C IV spectral line is predominantly emitted around the inner divertor leg. Therefore, it is concluded that D+ temperature around the inner divertor leg can be measured from the C IV spectral line. On the contrary, it is found difficult to measure the D+ temperature in the outer divertor.
Recent progress in the development of electron cyclotron heating⁄current drive systems in regard to power and pulse duration have allowed the extension of two normalized parameters (toroidal electric field Eφ and wave power density p). In the extended regime, the conventional theory requires consideration of a distorted electron distribution function. The first validation of the ECCD theory in the extended regime having large Eφ and p is presented. Linear calculation has a tendency to overestimate the EC driven current, as normalized parameters for Eφ and p increase. While the EC driven current IEC obtained by a linearized Fokker-Planck calculation (1.1 MA) did not agree with the measured EC driven current (0.74 ± 0.06MA), non-linear calculation of the Fokker-Planck equation considering the effect of Eφ (0.76 MA) shows close agreement with the experimental result. Calculations show that the decreasing effect of Eφ on IEC was stronger than the increasing effect of p on IEC in the experiment.
The energy distribution of neutral particles emitted from STP-3(M) reversed field pinch (RFP) plasma has been investigated by a neutral particle analyzer (NPA) using a two-channel energy detector with the time resolution of 0.5μs over the whole duration of the discharge. NPA data show a high energy tail as well as the bulk ion distribution. It is found that the ion temperatures of bulk and high energy tail component have reached to the maximum before reversed magnetic field configuration, and that the tail temperature increases in the quiet period (QP). Furthermore, we found that the high energy ion density increases in both the relaxation period and the QP. The ion energy confinement time evaluated from magnetic reconnection model and the obtained ion energy spectrum is found to be about 720μs much longer than the electron energy confinement time 60-100μs, when ion heating mechanisms are discussed.
Recent progresses of varying temperature irradiation experiments on microstructure development in austenitic stainless steels using HFIR are summarized. Each irradiation cycle consisted of 0.05 dpa at lower temperature irradiation (225 and 360°C), followed by 0.45 dpa at higher temperature (340 and 520°C). The specimens were irradiated for a total of 8 irradiation cycles, which resulted in 4 dpa. After the irradiation the TEM samples were electro polished and examined by an electron microscope. The microstructure of the samples irradiated at varying temperature condition (225⁄340°C, 360⁄520°C) were compared with those of same alloys continuously irradiated at 340 and 520°C.