Mayuko Koga

Fast ignition integrated experiments with Gekko and LFEX lasers

Based on the successful result of fast heating of a shell target with a cone for heating beam injection at Osaka University in 2002 using the PW laser (Kodama et al 2002 Nature 418 933), the FIREX-1 project was started in 2004. Its goal is to demonstrate fuel heating up to 5 keV using an upgraded heating laser beam. For this purpose, the LFEX laser, which can deliver an energy up to10 kJ in a 0.5–20 ps pulse at its full spec, has been constructed in addition to the Gekko-XII laser system at the Institute of Laser Engineering, Osaka University. It has been activated and became operational since 2009. Following the previous experiment with the PW laser, upgraded integrated experiments of fast ignition have been started using the LFEX laser with an energy up to 1 kJ in 2009 and 2 kJ in 2010 in a 1–5 ps 1.053 µm pulse. Experimental results including implosion of the shell target by Gekko-XII, heating of the imploded fuel core by LFEX laser injection, and increase of the neutron yield due to fast heating compared with no heating have been achieved. Results in the 2009 experiment indicated that the heating efficiency was 3–5%, much lower than the 20–30% expected from the previous 2002 data. It was attributed to the very hot electrons generated in a long scale length plasma in the cone preformed with a prepulse in the LFEX beam. The prepulse level was significantly reduced in the 2010 experiment to improve the heating efficiency. Also we have improved the plasma diagnostics significantly which enabled us to observe the plasma even in the hard x-ray harsh environment. In the 2010 experiment, we have observed neutron enhancement up to 3.5 × 107 with total heating energy of 300 J on the target, which is higher than the yield obtained in the 2009 experiment and the previous data in 2002. We found the estimated heating efficiency to be at a level of 10–20%. 5 keV heating is expected at the full output of the LFEX laser by controlling the heating efficiency.

Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) Project

Since the approval of the first phase of the Fast-Ignition Realization Experiment (FIREX-I), we have devoted our efforts to designing advanced targets and constructing a petawatt laser, which will be the most energetic petawatt laser in the world. Scientific and technological improvements are required to efficiently heat the core plasma. There are two methods that can be used to enhance the coupling efficiency of the heating laser to the thermal energy of the compressed core plasma: adding a low-Z foam layer to the inner surface of the cone and employing a double cone. The implosion performance can be improved in three ways: adding a low-Z plastic layer to the outer surface of the cone, using a Br-doped plastic ablator and evacuating the target centre. An advanced target for FIREX-I was introduced to suit these requirements. A new heating laser (LFEX) has been constructed that is capable of delivering an energy of 10 kJ in 10 ps with a 1 ps rise time. A fully integrated fast-ignition experiment is scheduled for 2009.

Measurement of heating laser injection time to imploded core plasma by using x-ray framing camera.

A simultaneous measurement of imploded core plasma and injection time of heating laser is conducted by using an x-ray framing camera (XFC). The experiments are performed using Gekko XII laser system for implosion of the deuterated polystyrene (CD) plastic shell target and Peta Watt (PW) laser system for heating. The time of PW laser injection is observed as the bright zone in the XFC image. The measured x-ray intensity profiles fit the Gaussian profiles well. The calculations of microchannel plate by using dynode model explain these broadened temporal profiles qualitatively. The peak position of fitted x-ray intensity profile is almost in agreement with the time when the high energy x ray is observed by x-ray streak camera. Moreover, the peak position is delayed corresponding to the delayed setting of PW laser injection time. From these results, it is concluded that we can estimate the heating laser injection time with resolution of the order of 10 ps by using XFC.

Implosion hydrodynamics and heating synchronization measurement using X-ray framing cameras

In fast ignition laser fusion experiments, it is important to know the implosion hydrodynamics and the heating mechanism of imploded core. However, it is difficult to measure the imploded core and heating laser injection at the same time because of their large spectral differences. In this paper, we propose a simultaneous measurement of an implosion process and an injection time of heating laser by using X-ray framing camera (XFC).

Development of the compact electron spectrometer for the FIREX-I Project in Gekko XII

The high energetic electron measurement is one of the most important issues to research the ignition mechanism in the Fast Ignition Realization EXperiment Project. It is also important for the energy spectra with angular distribution because the electron spread is different by the target design. Therefore we have been developed the compact Electron SpectroMeter so as to be installed on different angular potions. We have performed the calibration using L-band LINAC in the Institute of Scientific and Industrial Research, Osaka University. The analyzer has been tested to measure energetic electrons from the aluminum and gold plain targets irradiated by LFEX laser (maximum energy of 10 kJ) up to 800 J. The electron measurement has been performed at the integrated experiments using CD-shell with Au cone. The non-Maxwellian spectrum can be observed when the effective core heating by the electron is occurred.

Implosion and heating diagnostics of fast ignition laser fusion target with ultra-high-speed x-ray imaging

Implosion and heating experiments of Fast Ignition (FI) targets for FIREX-1 laser fusion project have been performed with Gekko-XII and PW/LFEX lasers at the Institute of Laser Engineering, Osaka University. Typical FI target has a hollow cone for guiding the short-pulse heating laser beam at the time of the maximum compression. The cone is mounted so as to in one-side penetrate the shell target. Detailed implosion hydrodynamics, FI heating and core plasma formation of plastic (CD) shell target with gold cone have been clarified by observing those with ultra high-speed imaging x-ray spectroscopy as well as neutron diagnostics. Multi-channel Multi-Imaging X-Ray Streak Camera (McMIXS) was improved for observation of time-resolved x-ray images and time-resolved two dimensional temperature distributions with spatial and temporal resolutions of 20 microns and 24 ps (42 Gfps), respectively. With this instrument, one can observe heating properties of the imploded core such as spatial distribution of the heated region and its temporal evolution. Also 2D-SIXS (Two-Dimensional Sampling Image X-ray Streak camera) coupled with an x-ray imager was improved for time resolved x-ray imaging of the imploded core. Synchronization of the heating beam injection to the implosion dynamics has been monitored with an x-ray framing camera. It was found that the shape of the core is neither spherical nor uniform mainly because of the existence of the cone and moving toward the tip of the cone and interacting with it. Experimental results are compared with two-dimensional hydrodynamic simulations. Target design taking into account of these phenomena is quite important because such core movement and jet formation can affect the condition of the cone.

Extreme Ultraviolet Emission from a Carbon Dioxide Laser-Sustained Oxygen Plasma

Extreme ultraviolet (EUV) spectrum emitted from laser-sustained oxygen plasma in a laser detonation atomic oxygen beam source was investigated. In order to measure EUV spectra, specially designed flat-field grazing-incidence EUV spectrometer was designed. The EUV spectra were recorded on an imaging plate which provides quantitative analysis capability. It was confirmed that EUV emission in the range of 20 –50 nm was included in the emission from laser-sustained oxygen plasma in a laser detonation source. The experimental results clearly indicated that the EUV intensity depends strongly on the translational energy of atomic oxygen. Even though the effect of EUV on the material erosion has not been confirmed, presence of high-energy photon need to be considered for better understanding of the reaction of hyperthermal atomic oxygen in the ground-based facility.

Present status and future prospect of Fast Ignition Realization Experiment (FIREX) Project at ILE, Osaka

Thermonuclear ignition and subsequent burn are key physics for achieving laser fusion. In fast ignition, a highly compressed fusion fuel generated with multiple ns‐laser beams is rapidly heated with a large energy, ps‐laser pulse in prior to core disassembly. This scheme has a high potential to achieve ignition and burn since driver energy required for high fusion gain is predicted to be about one tenth of that needed for the central ignition scheme. In Japan, Fast Ignition Realization Experiment (FIREX) project has been started to clarify the physics of energy transport and deposition in the core plasma and to demonstrate fuel temperature of above 5 keV. After the success, FIREX‐I will be followed by the second phase of the project (FIREX‐II) to demonstrate ignition and burn. LFEX laser, designed to deliver a laser pulse of 10 kJ in 10 ps, are operational and the first phase of FIREX experiments has been stated. A new target is proposed to attain dense compression of fuel and improve laser‐core coupling efficiency by adopting double‐cone structure, a low‐density inner liner, low‐Z outer coating, and Br‐doped fuel shell. In this paper, present status and near term prospects of the FIREX‐I project will be reported together with activities on target designing, laser development, and plasma diagnostics.

Simultaneous Measurement of Imploded Core and Heating Laser Injection by Using X-ray Framing Camera

In fast ignition experiment, it is important for effective heating of imploded core to control the injection time of heating laser synchronized with imploded core formation. However, it is difficult to measure the imploded core and heating laser injection at the same time. In this paper, we propose the simultaneous measurement using X-ray framing camera (XFC). In implosion experiment without heating laser, we observed only 2D thermal X-ray images of imploded core. On the other hand, in implosion experiments with heating laser, not only 2D X-ray images but also bright zones were observed on striplines. This zone is considered to show high energy X-ray from hot electron heated by heating laser. It is considered that the heating laser injection time is estimated from the peak position of this bright X-ray intensity profile. To explain the broad X-ray intensity profile, MCP gain calculation is attempted. The calculated results agree with experimental data qualitatively. Although the detailed X-ray energy measurement is needed, it is considered that we can estimate heating laser injection time with about 10 ps resolution from the peak position of high energy X-ray intensity profile.

Measurement of PW laser injection time to imploded core plasma by using X-ray framing camera

Measurement of PW laser injection time relative to the imploded core plasma by using X-ray framing camera was successfully achieved. The core plasma radii estimated from the X-ray framing camera are consistent with those estimated from X-ray streak camera and well agree with the results of 1D hydrodynamic simulation (ILESTA-1D). This means that the spatial resolution of X-ray framing camera is high enough and reliable to monitor implosion processes. PW laser injection time was observed as the bright zone on the stripline of X-ray framing camera. The measured X-ray intensity peak is consistent with that observed with X-ray streak camera. It is concluded that one can estimate PW laser injection time with a few tens of ps resolution from the peak position of X-ray intensity recorded by X-ray framing camera.