The presentations focusing on the physics and applications of heating and current drive in the electron cyclotron range of frequencies are summarized.
Electron Cyclotron Heating and Current Drive
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An overview of the objectives of ECH and ECCD in toroidal high perfonnance confinement systems for steady-state power generation was presented by T. Luce. This paper stressed the sound theoretical basis for understanding and predicting the effects of electron cyclotron waves and their flexibility in supporting steady-state operation in both tokamaks and stellarators. Three primary roles are envisioned for EC effects in steady-state systems: access to the burning plasma state, support of the toroidal current in tokamaks, and control of the operating point. Access to the operating conditions includes pre ionization and bum-through and plasma heating to fusion temperatures, a role that ECH is expected to largely fill on ITER. For noninductive current support, all auxiliary current drive systems are too inefficient energetically to support all the plasma current of a tokamak reactor, but with large bootstrap current fractions this can be done by ECCD while preserving adequate energy multiplication. ECCD, because of its localizability, can be used to optimize the current profile to provide improved confinement and stability. Active control of the reactor operating point may be necessary if small perturbations tend to grow. Also, ECCD has been demonstrated to be effective at suppressing neoclassical tearing modes and to a lesser degree sawteeth instabilities, and this is important for maintaining high perfonnance operation.
Preionization and startup
The first stage of operation of any tokamak is the preionization and startup phase. Until recently, pre ionization has usually not required ECH, but with the advent of superconducting tokamaks, ECH startup assist has proven necessary. Fully superconducting tokamaks have severe limits on the loop voltage, but it was reported at EC16 that second hannonic ECH was successful in KSTAR and EAST tokamaks in improving the startup capabilities, following up on previous work on the copper machines DIII-D and IT-60U and others. This is a positive results for ITER, should it have initial operations at half its nonnal toroidal field so that the 170 GHz ECH system would interact at the second hannonic.
Work on KSTAR at 110 GHz addressed the pre-fill gas pressure, the mode- X-mode or O-mode- of the injected power, and the vertical magnetic field . It was found, as expected, that the X-mode power was more effective at startup than O-mode power. Breakdown at the 3rd harmonic of the X-mode was unsuccessful. Injecting the EC beam at 10 deg counter to the plasma current direction was most beneficial to the startup, but 20 deg injection was also successful. Work on ASDEX Upgrade showed that fundamental O-mode was more effective than second harmonic X-mode, but not greatly so, suggesting that a dedicated fundamental breakdown system was not necessary for ITER.
Electron Cyclotron Heating
There was little experimental work reported at EC 16 on the physics of ECH or ECCD. This may be attributed to the mature state of understanding of the physics and modeling capabilities for ECH. A point of contention remains the role of the upper hybrid layer in cases where the ray tracing models neglect any interaction there. Work with a full wave code suggests that under some circumstances this interaction can be unexpectedly important, and experiments are needed to resolve the physics.
The effect ofECH on transport can be important, however. Work on DIII-D showed examples of discharges in which the 'density pump out' effect caused a strong decrease in global confinement, while in other discharges the confinement remained constant when the ECH was added. In ASDEX Upgrade, which has tungsten walls, it was reported that central ECH is needed in nearly all cases to reduce the neoclassical accumulation of metallic impurities to manageable levels, and that the ECH power had to be deposited within the central 20% of the minor radius. This effect was attributed to an outward pinch in low density discharges where trapped electron modes dominate transport. At higher densities where the ion temperature gradient modes dominate, the pumpout effect is not seen. Similar effects were seen with central ion cyclotron heating.
Some applications of ECH that were presented include a study of turbulent transport, in which the ECH power was applied at two nearby minor radii. By square-wave modulating the powers with the radial locations out of phase, the electron temperature gradient was modulated with constant total power. This modulated the nature of the turbulence for purposes of validating the turbulence models. Also, third harmonic X-mode was studied on ASDEX Upgrade to allow heating at lower toroidal field and second harmonic O-mode to provide central heating at densities well above the limit for X-mode. This work used wave
reflectors on the centerpost that provided the optimum polarization of the reflected wave. Work on DIII-D showed that H-mode threshold power is lower for ECH than for neutral injection in deuterium discharges, but this effect is much smaller in hydrogen and helium plasmas. Experiments on ASDEX Upgrade, however, showed similar power levels required for the H-transition in ECH and NBI cases. An interesting observation from the HL-2A tokamak is that
an internal transport barrier was found to form when the far off-axis ECH was switched off; no explanation for this effect was known. Interesting results from ASDEX Upgrade showed suppression of high beta disruptions when ECH was applied close to or outside the q=3/2 surface. Experiments with EC heating on T-IO using diagnostics with improved spatial and temporal resolution showed nonuniform rotation and fixed frequency ratios for different mode numbers. Very high highly peaked central electron temperatures, to 15 keV, have been obtained in low density discharges in the LHD device using 1 MW at 77 GHz.
Several initiatives for major new ECH installations were described at ECI6. A major project for a 10 MW 170 GHz ECH system for JET was described in a series of papers. The main objective of this system is support of the JET operational scenarios, but the concept of such a system on the world's largest tokamak was viewed by many participants as a long-overdue demonstration of the readiness of ECH for use as a primary heating system on ITER. If funded, this system is designed to be operational in 2015. The twelve I-MW gyrotrons would be obtained in partnership with the Russian Federation. Extensive modeling was presented, showing that the system would have sufficient power and flexibility to meet its objectives. Another major installation of EC power will take place on the EAST tokamak using 140 GHz 1 MW gyrotrons for a total power of 4 to 5 MW. Also, a 3 MW ECH system using six 0.5 MW gyrotrons at 68 GHz is being developed for HL-2A.
Electron Cyclotron Current Drive
A major advance in the technology of control of neoclassical tearing modes (NTMs) was made in the TEXTOR tokamak. In this work, the mode was detected using electron cyclotron emission that was collected along by the same antennas that launched the EC power in response. This is impressive because the forward EC power is of order 1 MW while the emitted EC radiation is less than a microwatt. Because the ECE collection followed the same path as the EC waves and used several frequencies surrounding the applied frequency, the location and phase of the instability relative to the local current drive location could be unambiguously determined. This information was used to control the
EC steering angles and phase to optimize the control efficiency. The NTM was simulated by an island created by fields from the dynamic ergodic divertor system. Fully automated control was obtained.
Tearing mode control was also demonstrated on other devices. On HL-2A, improvement in the confinement was obtained when heating near the q=2 surface was applied to cause the mode amplitude to decrease and disappear. Use of steerable mirrors to control the NTM was shown on ASDEX Upgrade and DIll-D. Heating was also used to control the sawtooth period in HL-2A, thereby allowing the density to increase.
ECCD experiments on the Heliotron J device showed that very energetic electrons heated by EC waves drive current consistent with the Fisch-Boozer effect. The dependence of the driven current with the magnetic well depth was
as expected from theory.
Electron Bernstein Waves
Angular scanning of the EBW emission has been used in the MAST spherical tokamak to estimate the pitch angle of the magnetic field at the mode conversion location. The radial location of the mode conversion layer is found using data from Thomson scattering. EBW at a range of frequencies can determine the pitch angle profile, providing an alternative to the more usual determination using the motional Stark effect.
EBW heating at 28 GHz was reported on the WEGA stellarator. A very prompt transition into the O-X-B heating condition was obtained as soon as the density threshold was reached, in confirmation of the theory.
Current drive experiments were done using a new phased array antenna system on the QUEST low aspect ratio tokamak. O-mode power at the 30 kW power level generated a strong nonthermal electron distribution and the plasma current of lO kA was fully supported by the RF for 0.8 sec. Systematic simulations of heating and current drive in spherical tokamaks more generally show that EBWs are a viable option for NSTX, NHTX, and MAST.
