Keiichi Takasugi

Heavy Ion Beam Probe for the Study of Plasma Confinement in Nagoya Bumpy Torus

The heavy ion beam probing system has been constructed for the study of plasma confinement in Nagoya Bumpy Torus (NBT-1/1M). Not only the plasma space potential but also the electron density multiplied by a function of the electron temperature nf(Te) can be measured with good spatial resolution. The feedback controlled detection technique and the time resolved fast detection technique are used, depending on the operation mode. The effects of the hot electron beta and the potential on the beam orbit are analyzed. It was confirmed that these effects on the observation points are usually small, and that the hot electron beta can be estimated from the shifts of the primary beam orbits of many cords.

Energy Transfer to Gas-Puff Z-Pinch and X-Ray Radiation

Gas-puff z-pinch plasmas were generated using rare gases (Ne, Ar and Kr). The intensities of XUV and X-ray radiations increased with atomic number Z of the gas. The net energy transferred to the plasma also increased with Z of the gas, which is a direct result of strong radial compression of the plasma column. Multiple ionization plays an important role in the strong compression of the high Z z-pinch plasma.

Simultaneous measurements of plasma potential and density fluctuations in a bumpy torus by a heavy neutral beam probe

A heavy neutral beam probe was developed for simultaneous measurements of local space potential and density fluctuations in a plasma. It was applied to both electron cyclotron heated (ECH) and radio frequency heated (RF) plasmas in the bumpy torus NBT-1M, and the feasibility of this method was successfully demonstrated by observing a low frequency m=1 interchange mode which was rotating poloidally at a frequency near the E×B drift frequency. It was shown that use of this probe results in extension of the range of fluctuation measurement by beam probe technique to relatively low temperature plasmas. It is applicable to a plasma with a strong RF field with which Langmuir probes cannot be used.

Operating Regions of a Gas-Puff Z-Pinch and Its X-Ray Radiation

A z-pinch experiment is carried out with a hollow-shaped Ar gas puff. Three types of discharge are found depending on the delay time of discharge from gas-puff detection, and the pinch time is compared with the measured gas pressure profile. Strong X-ray radiation is emitted from spots, a cloud and the anode surface. The number of spikes in the X-ray signal is correlated with spot images of the X-ray photograph. An X-ray burst is sometimes observed in the short-delay-time operation.

Macroscopic Behavior and X‐ray Radiation Characteristics of SHOTGUN Z‐Pinch Plasma

Gas‐puff z‐pinch plasmas were produced using Ne, Ar, Kr and Xe as pinch materials in the SHOTGUN device. The hardening of x‐ray radiation with atomic number Z of the operating gas was confirmed. It results from higher energy input into the higher Z plasma due to stronger compression, which is attributed to radiation cooling in the contraction phase.The contracting plasma showed nonuniformity along the axis, which results from m = 0 unstable mode. As the nonuniformity develops, the plasma column is divided into several sections, and each of them collapses by turns from the anode. The x‐ray is emitted at several localized spots which corresponds to the nodes of the rippling.

Energy Transfer and X-Ray Radiation Characteristics of a Gas-Puff Z-Pinch

The characteristics of energy transfer and X-ray radiation from an Ar gas-puff z-pinch plasma have been investigated in the z-pinch discharge region. The input energy reached approximately 10% of the bank energy at maximum. The input energy had a little more than square dependence for the pinch current. The average energy distributed to each particle had a decreasing tendency with delay time. The electron temperature of hot spots was around 1 keV, and it increased slightly with delay time. The increase in the spectral intensity of He-like resonance line showed strong compression for large delay times. The hot spots are small in volume, but large amounts of energy are concentrated on them due to the local compression of the pinch column.

Heavy Ion Beam Study of Negative and Positive Potential Formation in Bumpy Torus Plasma

Potential formation is studied in Nagoya Bumpy Torus (NBT-1M) by the use of a heavy ion beam probe. The concentric negative potential well and its positive rim are observed in the standard operation. The position of the rim moves with the second harmonic ECRH zone at the midplane of each mirror section. It is confirmed that the core electron heating is essential for the potential well formation. A stable positive potential is observed near T-M transition with a smooth change from the negative profile.

Development of Concentric Equipotential Surfaces in Bumpy Torus Plasma

Radial profiles of the plasma space potential are measured in Nagoya Bumpy Torus (NBT-1) by the use of a heavy ion beam probe. Asymmetric potential profiles owing to toroidal drift are observed in high pressure operation (C-mode). As the pressure is decreased, toroidal plasma is effectively heated (T-mode), poloidal precessional frequency overcomes the electron collision frequency and the equipotential surfaces becomes concentric inside the hot electron ring.

Hot spot formation in the divergent gas-puff Z pinch

Summary form only given. The divergent gas-puff z-pinch has been devised for the realization of an efficient point radiation source of X-rays. Because of the axial inhomogeneity introduced at the initiation of discharge, the plasma converges to a small region, and the efficient energy input is obtained compared to the conventional z pinch.The experiment was conducted on the SHOTGUN-III z-pinch device. The device comprises a 12-PF capacitor bank, and it is possible to perform both positive and negative discharges. Ar gas of 5 atm was used, and the divergent gas-puff was per formed from the center electrode. X-ray spectroscopic measurement using a Johansson quartz c rystal spectrograph was conducted. Resonance and intercomb ination lines of He-like Ar ions and their satellite lines were o bserved. The spectra are currently being analyzed for plasma parameters. Observation of hot spots by using a pinhole camera with an x -ray CCD was performed. The CCD employed here was mad e by LLG (Laser Laboratorium Göttingen e.V.). As a result of the analysis, the hot spot was spherical with a diameter of ab out 50μm. It was found that one or closely located two hot sp ots were formed reproducibly. Observation of an organism using soft x-ray of 3.95 Å emitte d from the hot spot were made. In the observation of a tiny moth (Cephitinea colonella) the legs and antennae were clearly projected, and the effectiveness of soft X-ray observations of organisms was confirmed.

Emission of High-Energy Ions in the SHOTGUN III Divergent Gas-Puff Z-Pinch Experiment

Ion pinhole measurements of high-energy ions were conducted on the divergent gas-puff z-pinch plasma. Two types of ions, 1.7 - 2.5 MeV and 0.1 - 0.7 MeV, were observed. The former was observed only on the axis. The latter showed quite different characteristics between positive and negative discharges. These ions were considered to be accelerated by inductive electric field generated by the pinch.