D. J. Hoffman

Global ICRF system designs for ITER and TPX

The design of feed networks for ICRF antenna arrays on ITER and TPX are discussed. Features which are present in one or both of the designs include distribution of power to several straps from a single generator, the capability to vary phases of the currents on antenna elements rapidly without the need to rematch, and passive elements which present a nearly constant load to the generators during ELM induced loading transients of a factor of 10 or more. The FDAC (Feedline/Decoupler/Antenna Calculator) network modeling code is described, which allows convenient modeling of the electrical performance of nearly arbitrary ICRF feed networks.

Methods of calculating selected geometrical effects in the design of ICRH antennas

Abstract This paper discusses the effects of realistic antenna geometry on the design and performance of ion cyclotron resonance heating (ICRH) antennas and presents methods to calculate these effects and to modify performance predictions that are based on idealized antenna geometry. It emphasizes the engineering aspects of antenna design, such as the electrical characterization of the antenna for inclusion into the power distribution network, as well as detailed analysis of the Faraday shield structure. The analysis of the Faraday shield includes the calculation of the shields power transmission and dissipation properties (rf load), its effect on the strap phase velocity and characteristic impedance, and the rf heat distribution for the purpose of thermal analysis. The finite antenna length and its interaction with the recessed cavity and reduced phase velocity are presented in terms of an effective antenna length. Calculation of interstrap coupling with slotted septa separating the straps is presented, including a discussion of the basic trade-offs between directionality, loading, and circuit stability in the case of directional phased arrays for fast wave current drive at ICRH frequencies.

The folded waveguide: a high frequency rf launcher☆

Abstract The folded waveguide, an alternative to loop antennas for launching power in the ion cyclotron range of frequencies (ICRF) into plasma devices, operates as a cavity with apertures for coupling RF power to the plasma. The RF field pattern is similar to that of a loop antenna, but with a lower ratio of electric to magnetic field. Power enters from a coaxial line via a sliding contact, whose position matches impedances between the coaxial line and the folded waveguide. The folded waveguide has operated at 1 MW and promises high power density. Calculations indicate a factor of 4 increase in power handling capability over a comparable loop antenna. The possible use of the folded waveguide on several tokamaks is discussed.

Fast Wave Current Drive Antenna Performance on DIII‐D

Fast wave current drive (FWCD) experiments at 60 MHz are being performed on the DIII‐D tokamak for the first time in high electron temperature, high β target plasmas. A four‐element phased‐array antenna is used to launch a directional wave spectrum with the peak n∥ value (≂7) optimized for strong single‐pass electron absorption due to electron Landau damping. For this experiment, high power FW injection (2 MW) must be accomplished without voltage breakdown in the transmission lines or antenna, and without significant impurity influx. In addition, there is the technological challenge of impedance matching a four‐element antenna while maintaining equal currents and the correct phasing (90°) in each of the straps for a directional spectrum. In this paper we describe the performance of the DIII‐D FWCD antenna during initial FW electron heating and current drive experiments in terms of these requirements.

Results of Folded Waveguide Tests on RFTF

Experiments with the 80‐MHz prototype folded waveguide on the Radio‐Frequency Test Facility (RFTF) at Oak Ridge National Laboratory have achieved substantially higher power levels than any previous tests on comparably sized loop antennas. This result, combined with a superior wave spectrum, suggests that the folded waveguide should be capable of coupling several times the power flux of a loop antenna into a tokamak plasma.

Phased operation of the DIII-D FWCD antenna array with a single power source☆

Abstract A phasing and matching system has been designed and implemented for the four-element fast wave current drive (FWCD) antenna array on DIII-D, This system permits phased operation using a single transmitter. Coupled power levels of 1.1 Mw have been reached with relative phasing of ±π/2 and equal magnitudes of current and voltage on all four lines. Use of a single power source requires the achievement of amplitude and phase control at high power, without feedback control of these quantities. The system uses only standard components consisting of transmission lines, unmatched tees, and manually controlled phase shifters and stubs. Phasing, matching, and amplitude control for all four current straps are done using a total of five tuning elements. This simplification is achieved through the use of two resonant loops, each connecting a pair of straps. A tuning algorithm developed for the system produces accurate matching, phasing, and amplitude balance within a small number of shots (≤ 5) in cases where the loading is sufficiently high, that is, when the resonant series load resistance (RSLR) > 2 ω, at values of kQ approaching 1. A coupled transmission line model of the antenna array and resonant loops has been created and used to determine changes in resistive and reactive loading, as well as changes in coupling between array elements during plasma shots. The design and modeling of this system and the operating experience are reviewed.

Floating potential measurements in the near field of an ion cyclotron resonance heating antenna

Large variations in the plasma potential can lead to large sheath potentials at the surface of the Faraday shield of an ion cyclotron resonance heating (ICRH) antenna. This sheath can accelerate ions to energies where sputtering is significant, resulting in impurity generation. The time‐varying floating potential is being measured in the near field of an ICRH antenna by using the Radio Frequency Test Facility (RFTF) at Oak Ridge National Laboratory. The antenna used is a resonant loop antenna that has a two‐tier Faraday shield with a layer of graphite on the outer tier. The electron cyclotron heated plasma in RFTF is produced by a 10.6‐GHz klystron. The magnetic field at the antenna is ∼2 kG, with a plasma density of ∼1010 cm−3 with an electron temperature of ∼6 eV. Ionization by rf power from the antenna produces a local plasma of ∼1011 with an electron temperature of 12 to 20 eV. The rf floating potential is measured by using a capacitively coupled probe that is scanned in front of the antenna, parallel...

Development of a folded waveguide antenna for ICRF heating in the large helical device

Abstract We report results of folded waveguide antenna experiments with high power, up to 1 MW, which include the dependence of plasma loading resistance on the plasma density and how much r.f. power can be delivered. We propose its application to the large helical device, which is a heliotron-torsatron device scheduled to produce its first plasma in 1998. The folded waveguide antenna launches an ion Bernstein wave to heat electrons via the Landau damping process. Preliminary experiments show a sharply defined wavenumber spectrum along the magnetic field line, which suggests core plasma electron heating. Direct electron heating is superior to fast wave minority heating because it avoids the orbit losses of deeply trapped high energy ions.

The technology of fast wave current drive antennas

Abstract The design of antenna arrays for fast wave current drive (FWCD) involves several issues and trade-offs in addition to the usual considerations in the design of fast wave antennas for plasma heating. One feature of a well-designed FWCD antenna is that it will function efficiently for plasma heating when phased symmetrically. Most of the antennas in use today on major fusion experiments were designed solely for heating and are generally not suitable for current drive. New RF systems capable of addressing FWCD are being installed on a number of machines, including DIII-D and the Joint European Torus (JET).

Spectral shaping and phase control of a fast‐wave current drive antenna array*

The requirements for antenna design and phase control circuitry for a fast-wave current drive (FWCD) array operating in the ion cyclotron range of frequencies are considered. The design of a phase control system that can operate at arbitrary phasing over a wide range of plasma-loading and strap-coupling values is presented for a four-loop antenna array, prototypical of an array planned for the DIII-D tokamak (General Atomics, San Diego, California). The goal is to maximize the power launched with the proper polarization for current drive while maintaining external control of phase. Since it is desirable to demonstrate the feasibility of FWCD prior to ITER, a four-strap array has been designed for DIII-D to operate with the existing 2-MW transmitter at 60 MHz. 3 refs., 6 figs.