Moo-Hyun Cho

Design and construction of the KSTAR tokamak

The extensive design effort for KSTAR has been focused on two major aspects of the KSTAR project mission - steady-state-operation capability and advanced tokamak physics. The steady state aspect of the mission is reflected in the choice of superconducting magnets, provision of actively cooled in-vessel components, and long pulse current drive and heating systems. The advanced tokamak aspect of the mission is incorporated in the design features associated with flexible plasma shaping, double null divertor and passive stabilizers, internal control coils and a comprehensive set of diagnostics. Substantial progress in engineering has been made on superconducting magnets, the vacuum vessel, plasma facing components and power supplies. The new KSTAR experimental facility with cryogenic system and deionized water cooling and main power systems has been designed, and the construction work is under way for completion in 2004.

A lower hybrid current drive system for ITER

A 20 MW/5 GHz lower hybrid current drive (LHCD) system was initially due to be commissioned and used for the second mission of ITER, i.e. the Q = 5 steady state target. Though not part of the currently planned procurement phase, it is now under consideration for an earlier delivery. In this paper, both physics and technology conceptual designs are reviewed. Furthermore, an appropriate work plan is also developed. This work plan for design, R&D, procurement and installation of a 20 MW LHCD system on ITER follows the ITER Scientific and Technical Advisory Committee (STAC) T13-05 task instructions. It gives more details on the various scientific and technical implications of the system, without presuming on any work or procurement sharing amongst the possible ITER partners(b). This document does not commit the Institutions or Domestic Agencies of the various authors in that respect.

Microwave-excited atmospheric-pressure microplasmas based on a coaxial transmission line resonator

We report the design, fabrication and characterization of two microwave-excited microplasma sources based on coaxial transmission line resonators (CTLR). The sources are capable of generating electric fields of ~106 V m−1 at 900 MHz and 2.45 GHz. These devices can self-ignite helium or argon discharges in a wide pressure range including atmospheric pressure. The gas temperature in an argon discharge open to atmospheric air is ~400 K. Using air as a dielectric, the working gases can be passed through the CTLR, resulting in the formation of plasma jets suitable for surface treatments. The device efficiency on transferring the input power into the plasma is 50–85% depending on the gas used. No thermal damage or electrode erosion has been observed in the devices.

Decomposition of volatile organic compounds and nitric oxide by nonthermal plasma discharge processes

Two types of nonthermal plasma processes such as pulsed corona discharge and dielectric barrier discharge were investigated for the decomposition of volatile organic compounds and nitric oxide. The performance equation of the plasma reactor was derived with the assumption that the decomposition rate of the pollutant is directly proportional to the concentration of the pollutant and the discharge power. From this model equation and the experimental decomposition data, the apparent decomposition rate constants of various organic compounds and nitric oxide were determined. Alkene and substituted alkene were found to have much larger decomposition rate constants than aromatics and substituted alkane compounds, which implies that the decomposition of the derivatives of aromatics and alkanes requires more energy. To verify the validity of the model derived, the experimental data in the present study and in the literature were compared with the calculation results, and relatively good agreements were achieved between them.

The design of the KSTAR tokamak

Abstract The Korea Superconducting Tokamak Advanced Research (KSTAR) Project is the major effort of the Korean National Fusion Program (KNFP) to develop a steady-state-capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. Major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 Tesla, and plasma current 2 mA with a strongly shaped plasma cross-section and double-null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beam, ion cyclotron waves, lower hybrid waves, and electron–cyclotron waves for flexible profile control. A comprehensive set of diagnostics is planned for plasma control and performance evaluation and physics understanding. The project has completed its conceptual design phase and moved to the engineering design phase. The target date of the first plasma is set for year 2002.

Magnetic and collisional effects on presheaths

The effects of both magnetic field and collisions on presheath properties are experimentally investigated in plasmas with electron temperatures much greater than ion temperatures. Measurements of plasma potential in collisionless plasmas show presheath thicknesses at boundaries oblique to magnetic field to be approximately (Cs/ωci)sin ψ, where Cs is the ion sound speed, ωci is the ion gyrofrequency, and Ψ is the angle between the magnetic field and the normal to the wall boundary. Measurements of plasma potential in collisional plasmas find presheaths consisting of two distinctive regions. With ion–neutral collision mean‐free path λn<(Cs/ωci)sin ψ, the presheath region next to the sheath has collisional characteristics and a thickness of approximately (0.5–0.6)λn. The corresponding presheath region adjacent to the bulk plasma has magnetic characteristics and a thickness of approximately (0.5–0.9)(Cs/ωci)sin ψ. Equipotential contours in the collisional region of this presheath are found to be parallel to t...

The virtual cathode as a transient double sheath

The two‐dimensional plasma potential measurements are given of a space‐charge dominated double sheath near a hot cathode. Laboratory data show that a virtual cathode is a self‐consistent solution only for a transient cathode‐plasma system. Slow charge exchange ions get trapped in the potential dip that forms the virtual cathode and eventually destroy it.

Emissive probe current bias method of measuring dc vacuum potential

It is experimentally demonstrated that, with proper current bias, emissive probes can accurately measure dc electric potential in a vacuum. A comparison is made of the accuracy and time response of this ‘‘vacuum current bias’’ method with two other emissive probe techniques, the inflection point in the limit of zero emission, and the floating potential of a strongly heated probe.

Temporal evolution of collisionless sheaths

We have performed experimental measurements to determine the temporal evolution of Langmuir sheaths near an electrode to which a negative step bias is applied in a collisionless argon plasma. The plasma was produced by a hot‐filament discharge in a multidipole device. Plasma potential data were obtained using emissive probes with two different techniques: time resolved sampling and time averaged techniques. The sheath is found to initially form close to the electrode, to extend to a maximum separation, and to contract to a steady‐state value. The time scale required to reach a steady state is close to the time scale of the presheath relaxation. Characteristics of sheaths in rf plasmas are also measured using a parallel‐plate plasma capacitor. It is observed that the plasma potential profile has significant variation with frequency, even for frequencies as low as 1 kHz which are far below the ion plasma frequency (∼1 MHz).

ECH pre-ionization and assisted startup in the fully superconducting KSTAR tokamak using second harmonic

This letter reports on the successful demonstration of the second harmonic electron cyclotron heating (ECH)-assisted startup in the first plasma experiments recently completed in the fully superconducting Korea Superconducting Tokamak Advanced Research (KSTAR) device whose major and minor radii are 1.8 m and 0.5 m, respectively. For the second harmonic ECH-assisted startup, an 84 GHz EC wave at 0.35 MW was launched before the onset of the toroidal electric field of the Ohmic system. And it was observed that this was sufficient to achieve breakdown in the ECH pre-ionization phase, allow burn-through and sustain the plasma during the current ramp with a low loop voltage of 2.0 V and a corresponding toroidal electric field of 0.24 V m−1at the innermost vacuum vessel wall (R = 1.3 m). This is a lower value than 0.3 Vm−1 which is the maximum electric field in ITER. Due to the limited volt-seconds and the loop voltage of the Ohmic power system, the extended pulse duration of the ECH power up to 180 ms allowed the plasma current to rise up to more than 100 kA with a ramp-up rate of 0.8 MA s−1.