C.P. Moeller

High power experiments of remote steering launcher for electron cyclotron heating and current drive

Abstract High power (0.5 MW) radiation tests of a remote steering launcher for an electron cyclotron heating and current drive (EC H&CD) system were carried out to study the characteristic of the rf beam steering and transmission. The remote steering launcher consists of a 4.632 m corrugated square waveguide (45.7×45.7 mm2) and a movable flat mirror which is placed at the input of the waveguide. Measurements were made in the range 0–10° at a frequency of 170 GHz. The measured radiation pattern is clearly a Gaussian-like distribution, which is in agreement with the theoretical prediction and the low power measurements. A steering capability of up to 10° was experimentally confirmed. Transmission and radiation of 0.5 MW 3.0 s and 0.2 MW 10.0 s pulses were successfully demonstrated over a steering angle range of 0–10°. Transmission losses in the launcher waveguide over 0–10° were measured calorimetrically to be less than 5%. No arcing was observed during the experiments and no damage to the waveguide was found after the experiments were finished. We have confirmed that remote steering is a possible alternative scheme for EC H&CD beam steering.

Second harmonic electron cyclotron pre-ionization in the DIII-D tokamak

Second harmonic 60 GHz electron cyclotron (EC) pre-ionization using low field side (LFS) X-mode launchers has been effective in producing target plasmas for startup of the DIII-D tokamak with electron densities comparable to fundamental LFS pre-ionization plasmas using the same EC system. A visible Bremsstrahlung array showed that breakdown occurred at the 2nd harmonic resonance location and after a few milliseconds the EC driven plasma filled the entire vessel, independent of the resonant location, which was varied from near the inner wall to the centre of the torus. The power threshold for ionization of ~0.4 MW was observed in DIII-D for these 2nd harmonic pre-ionization experiments. An orbit following code calculated that cold (0.03 eV) electrons could be heated to energies above 20 eV where the ionization of the neutral deuterium gas can occur. Scaling from DIII-D to ITER indicates that electron cyclotron heating (ECH) 2nd harmonic pre-ionization and initial plasma formation is possible if ITER operation at reduced toroidal field is desired, but additional experiments are required to extrapolate the entire ECH start-up scenario from DIII-D to ITER.

The Electron Cyclotron Resonant Heating System on the DIII-D Tokamak

Abstract In the DIII-D electron heating and current drive installation, up to six gyrotron microwave generators in the 1-MW class at pulse lengths up to 5 s have been operated simultaneously. The frequency for all the gyrotrons is 110 GHz, corresponding to the second harmonic of the electron gyrofrequency at 2 T. The peak generated power has been >4 MW with peak injected power slightly greater than 3 MW. The radio frequency (rf) generators are located remotely and are connected to the tokamak by up to 100 m of evacuated circular corrugated waveguide carrying the HE1,1 mode with overall transmission efficiency, including coupling to the waveguide, of up to 75%. Ancillary equipment for polarization control, beam switching, power monitoring, control of launch direction, and system protection has been developed. The system has been used to support a wide variety of physics experiments, including control of magnetohydrodynamic modes, current density profile modifications, basic plasma heating and current drive, transport studies, and rf-assisted start-up. The gyrotron complex is being upgraded by the acquisition of additional tubes with 5- to 10-s pulse length capability.

Performance and Upgrades for the Electron Cyclotron Heating System on DIII-D

The electron cyclotron heating (ECH) system on the DIII-D fusion reactor consists of six 110-GHz gyrotrons with 6 MW installed power for pulses limited administratively to 5 s in length. The transmission coefficient is better than -1.1 dB for four of the transmission lines, which is close to the theoretical value. A new depressed collector gyrotron was recently installed and is injecting up to 720 kW of power into DIII-D during 2013 tokamak operations. Three of the four dual waveguide launchers, which can steer the RF beams ±20° both poloidally and toroidally, were used for real-time neoclassical tearing mode control and suppression with increased poloidal scanning speed up to 60°/s and positioning accuracy of the beams of ±2 mm at the plasma center. The ECH capabilities on DIII-D are being steadily updated, leading to increased experimental flexibility and high reliability of the system. In the past year, the ECH system reliability reached 87% for 2352 successful individual gyrotron shots into DIII-D. Planning is under way for the addition of two new depressed collector gyrotrons, one at 110 GHz, 1.2 MW and another at 117.5 GHz, 1.5 MW generated power, both of which are in the test stage at Communications and Power Industries.

Development of a plasma current ramp-up technique for spherical tokamaks by the lower hybrid wave

Spherical tokamaks (STs) have the advantage of high beta capability, but the realization of a compact reactor requires the elimination of the central solenoid (CS). The possibility of using the lower hybrid wave (LHW) to ramp up the plasma current (Ip) from zero to a high enough level required for fusion burn in ST is examined theoretically and experimentally. Excitation of a travelling fast wave (FW) by the combline antenna installed on TST-2 was confirmed by a finite element analysis, but efficient current drive requires excitation of the LHW, either directly by the antenna or by mode conversion from the FW. The analysis using the TORLH full-wave solver indicates that core current drive by LHW is possible in the low-density, low Ip plasma formed by electron cyclotron heating (ECH). It is important to keep the density low during Ip ramp-up, and the wavenumber must be reduced as Ip increases. Initial results from TST-2 demonstrate that RF power in the LH frequency range (200 MHz) can achieve initial Ip formation, and is more effective than ECH for further ramp-up of Ip. Ip ramp-up to over 12 kA was achieved by combining ramp-up of the externally applied vertical magnetic field and ramp-up of the RF power. The significant asymmetry observed between co-current drive and counter-current drive is attributed to the presence of RF driven current. An optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability is being designed. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low-aspect-ratio reactor.

Heating and confinement in H-mode and L-mode plasmas in DIII-D using outside launch electron cyclotron heating

Using the outside launch with vertical polarization, electron cyclotron heating (ECH) experiments in DIII-D were carried out at 60 GHz at fundamental and second harmonic frequencies. The results are consistent with wave absorption theory for accessibility. For fundamental heating with a cut-off layer between the launch location and the resonant surface, good absorption efficiency was obtained. A process for mode conversion at the wall from the extraordinary mode to the ordinary mode is postulated to explain this result. For most efficient heating it was important to avoid the presence of a locked mode. For low density plasmas at the fundamental, the global energy confinement exceeds Kaye-Goldston scaling by about a factor of 1.4. However, heating at the second harmonic agrees numerically with the Kaye-Goldston scaling formula, for ECH alone or for ECH combined with neutral beam injection (NBI), and shows a mass dependence consistent with TE ~ √Ai. The global energy increase was insensitive to the resonance location for qres 1, global energy confinement scales as TE ~ Bt−0.3. Since plasma current and plasma geometry were not varied for these discharges, a scaling with safety factor q is also consistent. Using ECH alone, H-mode confinement was achieved for second harmonic heating in the central core of the plasma at a power threshold of ~0.75 MW for plasma parameters Ip = 0.5 MA, and e = 1.1 × 1019 m−3. The energy confinement times during the H-mode are similar to Ohmic values with a normalized confinement time τE/Ip ~ 200 ms/MA. For ECH and NBI H-mode plasmas, the transition from the L-mode to the H-mode was correlated with reduced magnetic fluctuations in the divertor region. For ECH H-mode plasmas, transport analysis shows that (1) the H-mode obtained with ECH is accompanied by an improvement of the electron thermal diffusivity χe relative to the L-mode over at least part of the plasma, (2) the ion thermal diffusivity χi is larger than predicted by neoclassical theory for the L- and H-modes, and (3) χe is approximately equal to χi for the ohmically heated discharge and for the L-mode and H-mode discharges heated by ECH discussed in the paper.

DIII-D Electron Cyclotron Heating System Status and Upgrades

The DIII-D electron cyclotron heating (ECH) system consists of six 110-GHz gyrotrons with corrugated coaxial 31.75-mm waveguide transmission lines, and steerable launching mirrors. The system has been gradually updated, leading to an increased experimental flexibility and a high system reliability of 91% in the past year. Operationally, the gyrotrons can generate up to a total of 4.8 MW of RF power for pulses up to 5 seconds. The maximum ECH energy injected into the DIII-D is 16.6 MJ. The HE1,1 mode content is over 85% for all the lines, and the transmission coefficient is better than -1.1 dB for all the transmission lines, close to the theoretical value. A new depressed collector gyrotron was recently installed and was injected up to 640 kW of power into the plasma during 2014-2015 tokamak operations. Four dual waveguide launchers, which can steer the RF beams ±20° poloidally and toroidally, are used for real-time neoclassical tearing mode control and suppression. The launchers now have increased poloidal scanning speed and beam positioning accuracy of approximately ±2 mm at the plasma center. Two more gyrotrons are expected to be installed and operational in 2015-2016. The first is a repaired 110-GHz, 1-MW gyrotron that had a gun failure after more than 11 years of operation at DIII-D. The second is a newly designed depressed collector tube in the 1.5 MW class, operating at 117.5 GHz, manufactured by Communications and Power Industries.

Radiofrequency experiments in JFT-2M: Demonstration of innovative applications of a travelling wave antenna

Several innovative applications of a travelling wave (combline) antenna designed for fast wave current drive have been demonstrated for the first time in the JFT-2M tokamak. High energy electrons of at least 10 keV were produced in the plasma core by highly directional fast waves in electron cyclotron heated plasmas. The ponderomotive potential of the beat wave, produced by fast waves at two different frequencies, was directly measured for the first time by a heavy ion beam probe. Plasma production was demonstrated using the wave fields excited by the combline antenna over a wide range of toroidal magnetic fields (0.5-2.2 T).

Millimeter wave polarimeter for characterizing high-power plasma heating systems

We have built a millimeter wave polarimeter which measures wave polarization parameters: the polarization angle, and the ellipticity including field spin direction in an evacuated high-power system. The polarimeter was applied to diagnose the 1 MW level electron-cyclotron plasma heating system at 110 GHz for the DIII-D tokamak. We have observed the time-dependent behavior of the gyrotrons and have characterized and calibrated the high-power transmission system which consists of grooved mirror polarizers, miter bends, switches, and corrugated wave guides. This article describes the principle of operation and the design method of the polarimeter and the examples of measurements.

Transmission lines power measurements for the 110 GHz electron cyclotron heating system on DIII-D and gyrotron operational performance

Operational trends for the six-gyrotron electron cyclotron heating system on DIII-D are presented. Losses in the transmission lines were measured and values close to theoretical were attained for one of the lines with the existing components. Improved alignment and reduction of the number of miter bends in the lines will increase the power transmitted through the waveguide to the DIII-D tokamak.