[en] The ICRF shunt regulator is a two-thermal, programmable current sink used to protect the ICRF power supplies from ringing and damage caused by sharp-edged RF amplifier plate current pulses. As an alternative to large banks of capacitors, the shunt regulator softens these edges by drawing a sawtooth-shaped ramp of current both before and after the RF amplifier plate current pulse. In this manner, the load current rise-time can be limited to the order of 100mS, thus preventing ringing and possible damage to the ICRF power supply. 3 refs

[en] The specifications of the current crop of high-power, high-voltage field-effect transistors (FETs) can lure a designer into employing them in high-voltage DC equipment. Devices with extremely low on-resistance and very high power ratings are available from several manufacturers. However, our experience shows that high-voltage, linear operation of these devices at near-continuous duty can present difficult reliability challenges at stress levels well-below their published specifications. This paper chronicles the design evolution of a 600 volt, 8 ampere shunt regulator for use with megawatt-class radio transmitters, and presents a final design that has met its reliability criteria

[en] Phased array antennas with high directivity have a variety of applications. One of their applications is in RF heating for magnetically confined plasma fusion research. Among these RF heating schemes, waveguide arrays with careful phase control on each waveguide can act as a phased array antenna to deliver megawatts of power for heating fusion plasmas in the lower-hybrid range of frequencies (1 GHz-10 GHz). In order to achieve compactness, it is common to stack reduced height waveguide together to form the waveguide array. As long as the delivered power does not cause arcing in the waveguide, the waveguide height can be quite small. Due to this confined space in a stack of reduced height waveguides, power detection of the incident and reflected wave in the reduced height waveguide is extremely difficult. A new compact probe, which employs current loops, to monitor the incident and reflected wave from the narrow side of the reduced height waveguide has been developed. Its theory and performance will be reported in this paper

[en] The coupling of the waves launched from a 4.6 GHz lower hybrid (LH) system into PBX-M plasmas has been studied for both L mode and H mode plasmas. The characteristics of the plasma in front of the LH coupler have been measured with a fast Langmuir probe. The reflected power of the coupler has been measured across the transition to the H mode as a function of phase and the distance between the coupler and the separatrix. A transient rise in the LH reflection coefficient was observed near the L-H transition under some conditions. Coupling depends primarily on the electron density in the vicinity of the coupler, and proper positioning of the coupler can compensate for changes in the plasma edge due to H mode transitions. Good coupling can be maintained throughout the H mode. (author). Letter-to-the-editor. 8 refs, 6 figs

[en] Lower Hybrid Current Drive (LHCD) Heating is proposed to be provided for the Tokamak Fusion Test Reactor Advanced Phase (TFTR-AP). The power to be delivered to the plasma is a total additional RF power of 3 MW at 4.6GHz, divided into two systems of 1.5 MW each. The purpose of this article is to describe the Electrical Power System components for the Lower Hybrid Current Drive. 3 refs., 1 fig

[en] This paper reports on the Lower Hybrid Current Drive (LHCD) System which is an RF source for current drive and second stability experiments on the PBX-M tokamak at the Princeton Plasma Physics Lab (PPL). The system is presently in final testing and is scheduled for 1992 operation

[en] The lower hybrid (LH) system is part of an overall heating and current drive system for TPX. The LH system is required to provide off-axis current profile control, efficient bulk current drive at low plasma temperatures, and electron heating. The device and facility will provide 4 MW of generated LH/LHCD power with 3 MW of power to the plasma at 3.7 GHz through one horizontal port. This paper will describe the RF generator system that supplies RF power to the launcher. The LH launcher is described separately in a paper by Paul Goranson of ORNL

[en] A 28 GHz electron cyclotron heating (ECH) and electron Bernstein wave heating (EBWH) system has been proposed for installation on the National Spherical Torus Experiment (NSTX). A 350 kW gyrotron connected to a fixed horn antenna is proposed for ECH-assisted solenoid-free plasma startup. Modeling predicts strong first pass on-axis EC absorption, even for low electron temperature, Te ∼ 20 eV, Coaxial Helicity Injection (CHI) startup plasmas. ECH will heat the CHI plasma to Te ∼ 300 eV, providing a suitable target plasma for 30 MHz high-harmonic fast wave heating. A second gyrotron and steered O-X-B mirror launcher is proposed for EBWH experiments. Radiometric measurements of thermal EBW emission detected via B-X-O coupling on NSTX support implementation of the proposed system. 80% B-X-O coupling efficiency was measured in L-mode plasmas and 60% B-X-O coupling efficiency was recently measured in H-mode plasmas conditioned with evaporated lithium. Modeling predicts local on-axis EBW heating and current drive using 28 GHz power in β ∼ 20% NSTX plasmas should be possible, with current drive efficiencies ∼ 40 kA/MW.

[en] Efforts have concentrated on redesigning the configuration of the Lower Hybrid coupler for the TPX tokamak. Several concerns motivated this redesign: reduce the effect of thermal incompatibility between coupler and rf-window material, reduce weight, reduce the risk of window failure and address the problem of replaceability, increase the reliability by reducing the number of connections and finally, reduce the total cost. The result is a highly compact, light and easily serviceable coupler which incorporates some of the simplicity of the multijunction coupler but preserves the spectral flexibility of a conventional coupler. copyright 1996 American Institute of Physics

[en] Experimental results have shown that the high harmonic fast wave (HHFW) at 30 MHz can provide substantial plasma heating and current drive for the NSTX spherical tokamak operation. However, the present antenna strap design rarely achieves the design goal of delivering the full transmitter capability of 6 MW to the plasma. In order to deliver more power to the plasma, a new antenna strap design and the associated coaxial line feeds are being constructed. This new antenna strap design features two feedthroughs to replace the old single feed-through design. In the design process, CST Microwave Studio has been used to simulate the entire new antenna strap structure including the enclosure and the Faraday shield. In this paper, the antenna strap model and the simulation results will be discussed in detail. The test results from the new antenna straps with their associated resonant loops will be presented as well