Jun-hee Choi

The AC Loss Measurement of the KSTAR PF1 Coils During the First Commissioning

The AC loss in a large superconducting magnet coil shows a tendency to be changed after assembly. For the ac loss measurement of the Korea Superconducting Tokamak Advanced Research (KSTAR) superconducting coils after assembly, several current scenarios, trapezoidal pulses and a DC offset sinusoidal pulses, were applied to the PF1 Upper (U) and Lower (L) coils during the commissioning. The measurement was done once before and once after the plasma discharge experiments. All coil data were obtained by the tokamak monitoring system and helium distribution system which were designed to measure temperature, pressure, and mass-flow at both inlets and outlets of the coils. The PF1 coils of the cable-in-conduit conductor type were made of Nb3Sn superconducting strands, whose winding scheme is 20 layers with each layer having 9 turns. Each has 10 cooling channels for the heat removal by the supercritical helium at 4.5 K. For the trapezoidal pulse tests, the current was increased up to 4 kA with several different ramp rates and a 2 kA DC offset and 0.5 kA sine waves with different frequencies from 0.1 Hz to 0.3 Hz were applied to the coil. According to the analyses, the AC loss was slightly decreased for the same condition after 700 plasma shots. It was believe that such a result was due to reduced inter-strand resistances which changed the transverse resistance between the inter-strands. The coupling time constant was estimated to be 32 ms in the trapezoidal tests and 13.6 ms for the DC offset sinusoidal pulses. The former is larger than the latter because of the effect of the jacket eddy current loss due to Incoloy 908 which is a ferromagnetic material. Considering the jacket eddy current losses, the coupling time constant was recalculated and the value estimated to be about 13 ms for all current wave forms during first commissioning.

Qualification Test Results of the KSTAR Superconducting Coils From the Construction to the Commissioning Steps

To achieve the first plasma of the Korea superconducting tokamak advanced research (KSTAR), the KSTAR superconducting coils were tested in advance. As they should operate in excessively low temperature of 4.5 K and high magnetic field environment of 7.5 T, it is crucial to monitor the cryogenic and the structural behaviors of KSTAR device during the commissioning period including a cool-down. The temperatures of the KSTAR toroidal field (TF) coil and the poloidal field (PF) coils were measured during the entire operating period. The mechanical stresses on the TF and PF structures were continuously monitored to check if they go beyond the limiting value calculated through the simulation. The alignment of the KSTAR device was checked by using displacement sensors. The TF coils were successfully supplied with 15 kA DC current for 8 hours, and the maximum 5 kA/s current variation of the PF coils were tested. For the main experiment, the interlock test of the quench detection system for the KSTAR coils was carried out at reduced currents of 1 kA. From these results the quench protection circuit, and the current-flow of the KSTAR superconducting coils proved to be well performed for the first plasma operation.

Experimental measurement of W-band backward-wave amplification driven by external pulsed signals

The experimental implementation of W-band backward-wave oscillator is achieved by using a multilevel microfabrication of interaction circuit including beam tunnel, slow-wave structure, and output transition, on deep reactive ion etched (DRIE) and metal deposited silicon wafers. The interaction circuit shows precise accuracy in full 3 dimensions, and the return loss measurement agrees well with HFSS simulation. Here we describe the experimental observation of W-band backward-wave oscillation and amplification after successful vacuum sealed integration of interaction circuit, electron gun, beam collector, and output window.

Enhanced RF performance in multi-tunnel backward-wave oscillators

We propose an efficient beam-wave interaction circuit employing a multi-tunnel, slow-wave structure for W-band backward-wave oscillators. The tunnel is disposed one of above and below the beam tunnel, which enhances RF characteristics. The interaction circuit is prepared using a deep-reactive ion etched (DRIE), multi-level microfabrication on silicon wafers. The return loss shows strong resonances predicted by finite-element method (FEM) simulations. The multi-tunnel interaction circuit demonstrates almost similar aspect in return loss to the circuit without beam tunnel. A 1.6 times increase in RF output power is estimated from the particle-in-cell calculation, when compared to the case without multi-tunnel structure. Therefore, we conclude that the multi-tunnel, slow-wave structure successfully improves RF performance.

Prospect of GaN light-emitting diodes grown on glass substrates

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates. We found that GaN morphology affected the EL and achieved enhanced EL of GaN-LEDs on glass by identifying the optimal GaN morphology having both high crystallinity and compatibility for device fabrication. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of ~ 9100 cd/m2 at the peak wavelength of 495 nm. The EL color could be adjusted mainly by varying the quantum well temperature. In addition, new growth methods for achieving high GaN crystallinity at a low growth temperature (e.g. ~700°C) were briefly reviewed and attempted by adopting selective heating. We expect that performance of the GaN LEDs on glass can be much enhanced by enhancing GaN crystallinity and p-GaN coating, and evolvement of low-temperature growth of high-quality GaN might even customize ordinary glass as a substrate, which enables high-performance, low-cost lighting or display.

Development of W-band backward-wave oscillator

The precise patterning of periodic slow-wave structures can be successfully accomplished by modern photolithography technology on flat substrates in high frequency regime (>;100 GHz). When the aspect ratio of the structure between in-plane and out-of-plane dimensions becomes higher than unity, however, controlled MEMS (micro-electromechanical systems) technologies are strongly required to achieve accurate depth profiles of slow-wave beam-wave interaction circuits. Here we report a W-band backward-wave oscillator using microfabrication technologies by which a fully 3-dimensional slow-wave interaction circuit is successfully employed on multi-bonded silicon wafers. The return loss measurement of the circuit appears to be very similar to the simulation, which indicates not only the dimensions but also the surface roughness is under control. A more detailed discussion on the design, fabrication, and RF test result will also be included.