J.H. Jeong

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.

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.

ECRH-assisted plasma start-up with toroidally inclined launch: multi-machine comparison and perspectives for ITER

Electron cyclotron resonance heating (ECRH)-assisted plasma breakdown is foreseen with full and half magnetic field in ITER. As reported earlier, the corresponding O1- and X2-schemes have been successfully used to assist pre-ionization and breakdown in present-day devices. This contribution reports on common experiments studying the effect of toroidal inclination of the ECR beam, which is ≥20° in ITER. All devices could demonstrate successful breakdown assistance for this case also, although in some experiments the necessary power was almost a factor of 2 higher compared with perpendicular launch. Differences between the devices with regard to the required power and vertical field are discussed and analysed. In contrast to most of these experiments, ITER will build up loop voltage prior to the formation of the field null due to the strong shielding by the vessel. Possible consequences of this difference are discussed.

Status of KSTAR Electron Cyclotron Heating System

An 84-GHz, 500-kW electron cyclotron (EC) heating (ECH) system is under installation for ECH-assisted start-up in the Korea Superconducting Tokamak Advanced Research (KSTAR) facility. An 84-GHz, 500-kW gyrotron, and 1.5-MVA power supply system have been installed at KSTAR, and the initial test of the gyrotron has been carried out with a short-pulse condition of 20 μs and maximum beam parameters of 80 kV and 25 A that generate an output radio-frequency (rf) power of 500 kW. The planned 2-s-long operation with 500-kW rf output power is beginning with a long-pulse test of the gyrotron power supply. The launcher system was fabricated in collaboration with Princeton Plasma Physics Laboratory. It will inject 500-kW rf power into the KSTAR plasma with a highly flexible steering mirror system, allowing toroidal and poloidal beam deposition scans. KSTAR will employ 170-GHz EC current drive (CD) in ITER-relevant experiments such as the suppression of the neoclassical tearing modes and the creation of an electron internal transport barrier. The Japan Atomic Energy Agency will provide a 170-GHz, 1-MW gyrotron on loan in 2008 in accordance with a Korea-Japan fusion collaboration agreement, and it will be used for the 170-GHz, 1-MW ECCD system in 2010. This paper describes the current status of the installation and initial conditioning tests of the 84-GHz gyrotron system as well as the development plan of the 170-GHz ECH and CD system. Also, this paper discusses the CD efficiency and the steering range of the second-harmonic X-mode injection at 170 GHz and 5 MW from an equatorial launcher.

Operation of high power 84-GHz gyrotron for KSTAR ECH system

The initial objective of the KSTAR ECH system is to produce seed plasmas by irradiation of powerful millimeter waves for the main ohmic discharges. A specially designed gyrotron, manufactured by CPI, has specifications of 500 kW, 84 GHz, and 2- second pulse duration. It adopts the state-of-arts technologies of a diode gun, a depressed potential collector, an internal mode converter, and a diamond window. As a part of the final acceptance tests of the gyrotron, we constructed a test and including a test modulator to confirm its operating conditions. With short-pulses of 20 microseconds, we produced the output peak power of 500 kW at 84 GHz. The output power distribution is confirmed to be a Gaussian by IR images on targets. In this experimental study, we describe details of the gyrotron, the test stand, and experimental results.

COMMISSIONING OF KSTAR 84-GHz ECH TRANSMISSION SYSTEM

Abstract An 84-GHz electron cyclotron heating (ECH) system has been installed to assist plasma start-up by preionization in the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 84-GHz ECH system consists of a 500-kW gyrotron, a transmission line, and an antenna system. The wave power is transmitted from the gyrotron to the antenna through an evacuated corrugated circular waveguide of 31.75-mm inner diameter and six miter bends, which include a pair of polarizer miter bends for polarization control. The maximum permitted vacuum pressure without radio-frequency (rf) breakdown in the 31.75-mm waveguide at 84 GHz, 500 kW was calculated to be ~0.1 torr. The pumping time to reach the vacuum pressure of 1 × 10-3 torr in the KSTAR ECH system was ~2 h by two turbomolecular pumps. The transmission efficiency of ~93% from the output of the mirror optical unit to the torus window was measured using a low-power rf source. The wave polarization by a pair of polarizer miter bends with grooved mirrors was tested using the low-power system, and it showed good agreement with numerical calculations. In this paper, we present the design and commissioning results of the KSTAR 84-GHz transmission line.

Characteristics of global energy confinement in KSTAR L- and H-mode plasmas

We evaluate the characteristics of global energy confinement in KSTAR (?E,KSTAR) quantitatively in three ways; firstly by comparing it with multi-machine scalings, secondly by deriving multiple regression equations for the L- and the H-mode plasmas, respectively, and lastly by comparing confinement enhancement of the H-mode phase with respect to the L-mode phase in each discharge defined as Hexp. The KSTAR database exhibits ?E,KSTAR of ?0.04 to ?0.16?s and of ?0.06 to ?0.19?s in L-mode and in H-mode plasmas, respectively. The multiple regression equations derived by statistical analysis present the similar dependency on PL and higher dependency on Ip compared with the multi-machine scalings, however the dependency on ? in both L- and H-mode plasmas draw the negative power dependency of ??0.68 and ??0.76 for H-mode and for L-mode database, respectively on the contrary to the positive dependency in all multi-machine empirical scalings. It is found that the energy confinement of both L-mode and H-mode of the discharges with Hexp?>?1.5 can be well-predicted by multi-machine scalings, ?E,89L and ?E,92H. Apart from this, the H-mode confinement with 1.5?

Mode Content Study of Propagating Waves Using Burn Patterns in the KSTAR 84-GHz ECH System

Abstract In order to transmit a wave efficiently in an electron cyclotron heating (ECH) system, it is important to suppress mode conversion loss caused by coupling in the matching optics unit and misalignment in the transmission line. To understand the cause of mode conversion loss, it is necessary to analyze the mode content in an oversized circular corrugated waveguide. For mode analysis of the propagating wave in the corrugated waveguide, several methods based on the phase-retrieval process and the iterative process are suggested. But, in the Korea Superconducting Tokamak Advanced Research 84-GHz ECH transmission line, a well-known method using burn patterns was used for better coupling of the output beam from the gyrotron onto the axis of the corrugated waveguide by adjusting a large ellipsoidal mirror in an L-shaped chamber, a so-called L-box. During the adjustment of the mirror in the L-box, evidence of the existence of higher modes other than HE11 was found. For the mode content study, the radiation intensity distribution was measured using thermal paper as a function of the distance along the waveguide at a high power level. The mode content of the wave was estimated by comparing the measured burn patterns and calculated patterns at different locations. This paper describes the results of mode content estimation using burn pattern images as a function of the mode mixture ratio.

A Lower Hybrid Current Drive System for ITER and High Power CW Klystron Development

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 currently planned procurement phase, it is now under consideration for an earlier delivery. An LH program has been initiated under EFDA, following the ITER STAC recommendation, to provide a pre‐design document including the conceptual design, costing, possible procurement allocation, WBS and R&D needs.