Y. S. Hwang

Abstract An efficient and robust ECH (electron cyclotron heating)-assisted plasma start-up scheme with a low loop voltage and low volt-second consumption utilizing the trapped particle configuration (TPC) has been developed in the versatile experiment spherical torus (VEST). The TPC is a mirror-like magnetic field configuration providing a vertical magnetic field in the same direction as the equilibrium field. It significantly enhances ECH pre-ionization with enhanced particle confinement due to

Characteristics of helicon plasmas are examined as a candidate for accelerator ion sources. Helicon plasmas have much higher density with very high power efficiency in a relatively lower magnetic field than electron cyclotron resonance plasmas do. High electron density and high electron temperature are beneficial to extract high ion current density. Low magnetic field favors for low-emittance accelerators. Taking advantage of placing antenna outside of the plasma chamber, source contamination ca

In time-resolved measurement for triton burnup in Korea Superconducting Tokamak Advanced Research (KSTAR) deuterium plasmas, an NE213 liquid scintillation detector was installed and operated during the 2017 KSTAR campaign. The detector is composed of an NE213 scintillator (50 mm in diameter and 10 mm in thickness) and a photomultiplier tube (PMT). The PMT anode signal was processed under a data acquisition system which contains a field programmable gate array circuit and pulse processing softwar

Reconstruction and modeling of the plasma current profiles in a fully pressure driven tokamak have been performed in the Current Drive Experiment-Upgrade. The reconstructed experimental current profile has a significant deviation from that of the calculated neoclassical currents. Satisfactory agreement between the measured and calculated model profiles was obtained by including a helicity conserving current diffusion term in the modeling which created the required self-generated ``seed'' current

At TRIUMF, H− ion sources have been characterized on a teststand to improve the understanding of their performance. Measured beam characteristics such as current, emittance, and e∕H− ratio were correlated with source conditions as a function of relevant plasma parameters. Plasma densities, temperatures, and plasma potentials were measured with a Langmuir probe and correlated with beam properties for different confining magnetic fields and different values of arc power and gas pressure. The mecha

A Thomson scattering (TS) system has been utilized to measure the electron temperature and density of the core region of Versatile Experiment Spherical Torus (VEST). Recently, the laser injection system is successfully upgraded adopting the burst laser with the repetition rate of 1 kHz and the energy of 2 J. Furthermore, improved collection optics with additional polychromators and a 32-channel fast digitizer are prepared to observe the fast time evolution of radial profiles. This improvement is

Acceleration of ion rotation is observed during internal reconnection events (IREs) in the versatile experiment spherical torus. Two IRE discharges with opposite torques, i.e. acceleration or deceleration of ohmic plasmas with intrinsic, counter-Ip rotation, can be generated using different wall conditionings method. When an IRE occurs, acceleration and deceleration of impurity ion rotation as well as well-known ion heating are observed globally via ion Doppler spectroscopy with multiple channel

Magnetic diagnostics are used to reconstruct current density profiles in noninductively driven plasmas in the CDX-U tokamak. The boundary magnetic fields are measured by 32 pickup coils and three flux loops attached at different poloidal positions inside the CDX-U vessel; in addition, internal magnetic probes are also used. Significant eddy currents are measured on the thick aluminum vessel and accounted for in the analysis. Two-dimensional current density distributions and magnetic flux contour

Polarization characteristics of the transmitted beam through the various sizes of metal wire grid meshes are examined experimentally at 118.8 microm. When these results were compared to Chen's waveguide theory, we obtained excellent agreement. In particular, when the incident beam polarization is neither parallel (TM) nor perpendicular (TE) to the wire grid axis, a linearly polarized incident beam can be made elliptic due to the phase difference between the TE and TM modes.

Local helicity injection (LHI) is a non-inductive startup and current drive method via Taylor relaxation for the spherical torus. In achieving Taylor relaxation, it has been suggested that kink instability in 3D helical flux ropes plays an important role. However, the role and occurrence of kink instability during LHI have yet to be validated. Experimentally, determining the kink mode in a flux rope relies on measuring internal information using a probe. However, for LHI, the 3D geometry complic