Kyu-Sun Chung

Dependence of neutron yield on the deuterium filling pressure in a plasma focus device

A Mather-type plasma focus device (32 µF, 4 kJ), called Hanyang University Plasma Focus device, is developed as a prototype device for the irradiation test of neutrons for electric probes and cables to be used in Korea Superconducting Tokamak Advanced Research with pure deuterium gas. The six different lengths of electrode are used in order to see the dependence of neutron yield on the deuterium filling pressure at given system conditions such as capacitance, currents and inductance, after optimizing the focusing condition in terms of electrode length versus filling pressure. The relationship between the pressure and electrode length is to be fitted well by the snow-plow model. The neutron fluence and angular distribution are measured at the angles of 0°, 25°, 60° and 90° by locating the bubble neutron dosimeter at a distance 30 cm from the inner electrode head at each electrode length. The anisotropic factor tends to increase with pressure and the total neutron yield is strongly dependent on the isotropic emission. The maximum neutron yield is estimated to be about 1.6 × 108 (n/shot) at a pressure of 3.4 Torr.

RF-heating and plasma confinement studies in the HANBIT mirror device

HANBIT is a magnetic mirror confinement device. Recently, after finishing the first campaign for the basic system development, it started the second campaign for high-temperature plasma confinement physics study in a mirror configuration. Here, we introduce briefly the HANBIT device and report initial physics experiment results on RF-plasma heating and confinement in the simple mirror configuration. It appears that the discharge characteristics of HANBIT are quite different from those in other mirror devices, and an explanation is presented to clarify the difference.

Initial phase wall conditioning in KSTAR

The initial phase wall conditioning in KSTAR is depicted. The KSTAR wall conditioning procedure consists of vessel baking, glow discharge cleaning (GDC), ICRH wall conditioning (ICWC) and boronization (Bz). Vessel baking is performed for the initial vacuum conditioning in order to remove various kinds of impurities including H2O, carbon and oxygen and for the plasma operation. The total outgassing rates after vessel baking in three successive KSTAR campaigns are compared. GDC is regularly performed as a standard wall cleaning procedure. Another cleaning technique is ICWC, which is useful for inter-shot wall conditioning under a strong magnetic field. In order to optimize the operation time and removal efficiency of ICWC, a parameter scan is performed. Bz is a standard technique to remove oxygen impurity from a vacuum vessel. KSTAR has used carborane powder which is a non-toxic boron-containing material. The KSTAR Bz has been successfully performed through two campaigns: water and oxygen levels in the vacuum vessel are reduced significantly. As a result, KSTAR has achieved its first L–H mode transition, although the input power was marginal for the L–H transition threshold. The characteristics of boron-containing thin films deposited for boronization are investigated.

The Cold and Hot Electron Populations, Temperatures and Their Transports in the Edge Plasma of the ORNL CAPRICE ECR Ion Source

The edge plasma of the ORNL CAPRICE ECR ion source is studied by directly measuring, with electrical probes, its local plasma parameters such as plasma density, temperature and electron energy distribution characteristics at different rf power levels, at various pressures, at various axial magnetic field strengths and different distances from the resonant zone. It is found that the edge plasma can be approximated to be bi-Maxwellian, whose characteristics become more pronounced at the more distant positions from the ECR zone, at lower source pressures and at increased magnetic field strengths. These trends are consistently explained in terms of long-range electron-electron and electron-ion Coulomb scattering collisions that occur during the transport of hot electrons from the ECR plasma to the probe and their frequency dependence on electron temperature and density.

Deduction of the absolute negative ion density by using planar and cylindrical electric probes simultaneously

Analytic formulas are derived for the deduction of the absolute density of negative ions by using the measured electron temperatures and saturation currents of positive ions and negative charges from current-voltage curves taken by planar and cylindrical probes at two different pressures without assuming the sheath potential, sheath velocity, temperatures of positive and negative ions, and effective masses of positive ions. Ratios of ion and electron saturation currents and electron temperatures of two probes and sheath areas of a long thin cylindrical probe at two different pressures are incorporated into two equations with two unknowns for the negative ion density. The procedure to deduce the absolute negative ion density is given.

Why Is the Mach Probe Formula Expressed as R=Jup/Jdn=exp [KM∞]?

The normalized drift velocity of flowing plasmas can be deduced using a Mach probe, which has two directional probes at opposite sides. The relationship between the ratio (R) of upstream ion saturation current density (Jup) to downstream current density (Jdn) and the normalized drift velocity (M∞=Vd/√Te/mi) of plasma has generally been fitted into an exponential form as R=Jup/Jdn=exp [KM∞], where K is a calibration factor depending on the magnetic flux density, collisionality, viscosity, and ion temperature of plasmas. Without going into detailed theories for various conditions of plasmas and probes, a simple explanation is given in terms of decaying current density in the downstream region. Existing theories and experiments of Mach probes in magnetized and unmagentized flowing plasmas are summarized along with key physics and comments.

Heat and radiation effect on the degradation behaviors of polymeric liquids

Abstract We propose a mathematical model based on continuous-mixture kinetics to describe the degradation mechanism of polymeric material. Chu’s distribution function is employed to represent molecular-weight distributions of decomposed polymers. Calculated results agree fairly well with experimental observations for polymer degradation. The radiation damage is demonstrated as a number of chain scissions caused and G value for the scission that is defined as a number of radiolysis events caused by the absorption of 100 eV of radiation.

Honeycomblike large area LaB6 plasma source for Multi-Purpose Plasma facility

A Multi-Purpose Plasma (MP(2)) facility has been renovated from Hanbit mirror device [Kwon et al., Nucl. Fusion 43, 686 (2003)] by adopting the same philosophy of diversified plasma simulator (DiPS) [Chung et al., Contrib. Plasma Phys. 46, 354 (2006)] by installing two plasma sources: LaB(6) (dc) and helicon (rf) plasma sources; and making three distinct simulators: divertor plasma simulator, space propulsion simulator, and astrophysics simulator. During the first renovation stage, a honeycomblike large area LaB(6) (HLA-LaB(6)) cathode was developed for the divertor plasma simulator to improve the resistance against the thermal shock fragility for large and high density plasma generation. A HLA-LaB(6) cathode is composed of the one inner cathode with 4 in. diameter and the six outer cathodes with 2 in. diameter along with separate graphite heaters. The first plasma is generated with Ar gas and its properties are measured by the electric probes with various discharge currents and magnetic field configurations. Plasma density at the middle of central cell reaches up to 2.6 x 10(12) cm(-3), while the electron temperature remains around 3-3.5 eV at the low discharge current of less than 45 A, and the magnetic field intensity of 870 G. Unique features of electric property of heaters, plasma density profiles, is explained comparing with those of single LaB(6) cathode with 4 in. diameter in DiPS.

Determination of plasma flow velocity by mach probe and triple probe with correction by laser-induced fluorescence in unmagnetized plasmas

Plasma flow velocity was measured by Mach probe (MP) and laser-induced fluorescence (LIF) methods in unmagnetized plasmas with supersonic ion beams. Since the ion gyro-radius was much larger than the probe radius, unmagnetized Mach probe theory was used to determine plasma flow in argon RF plasma with a weak magnetic field (<200 G). In order to determine flow velocities, the Mach probe is calibrated via LIF in the absence of the ion beam, where existing probe theories may be valid although they use different geometries (sphere and plane) and analyzing tools [particle-in-cell (PIC) and kinetic models]. For the comparison of the average plasma flow velocities by MP and LIF, the supersonic ion beam velocity was measured by LIF and then incorporated into a simple formula for average plasma velocity with provisions for background plasma density and beam-corrected electron temperature (Te) measured by a triple probe.

Response to ‘‘Comment on ‘An analytic treatment of the bounded and free presheaths with arbitrary viscosity in magnetized flowing plasmas’ ’’ [Phys. Plasmas 1, 2864 (1994)]

The comments of Hutchinson (Ref. 1) on the author’s titled paper are discussed. The one‐dimensional model of free presheaths does include viscosity and not merely convective momentum transport. Mathematical arguments are given to support this assertion. (AIP)