4 MW Fast Wave Current Drive Upgrade for DIII-D

The DIII-D program has just completed a major addition to its ion cyclotron range of frequency (ICRF) systems. This upgrade project added two new fast wave current drive (FWCD) systems, with each system consisting of a 2 MW, 30 to 120 MHz transmitter, ceramic insulated transmission lines and tuner elements, and water-cooled four-strap antenna. With this addition of 4 MW of FWCD power to the original 2 MW, 30 to 60 MHz capability, experiments can be performed that will explore advanced tokamak plasma configurations by using the centrally localized current drive to effect current profile modifications

Fast Wave Heating and Current Drive in DIII-D Discharges With Negative Central Shear

The noninductive current driven by fast Alfven waves (FWCD) has been applied to discharges in DIII-D with negative central shear. Driven currents as high as 275 kA have been achieved with up to 3 MW of fast wave power with the efficiency and profile as predicted by theory-based modeling. When counter-current FWCD was applied to discharges with negative central shear, the negative shear was strengthened and prolonged, showing that FWCD can help to control the current profile in advanced tokamak discharges. Under some conditions in negative central shear, the plasma spontaneously makes a transition into a regime of improved performance, with a reduction in both the ion and the electron heat diffusivities. Up to 3 MW of fast wave power has been successfully coupled into H-mode discharges with large edge localized modes through use of an innovative decoupler/hybrid power splitter combination

Use of Near-Infrared Detector to Sense RF Antenna Heating

The three antennas used for ion cyclotron heating (ICH) experiments on DIII-D have experienced localized heating of the Faraday shield rods during plasma operations which has resulted in some melting. This melting is of great concern not only because of the damage it does to the rf system's ability to deliver rf to the plasma, but because of its potential to contaminate the plasma during a shot and cast the experimental results from the shot into question. A real-time sensor to detect the temperature of the antennae during plasma operations is described. The sensor uses an avalanche photo diode (APD) with sensitivity from 0.4 to 1.0 {micro}m to monitor the temperature of the antennae. Calculations for the detector sensitivity based on Planck's law are compared with experimental results and detector data taken during plasma operations are presented

Fast wave heating and current drive in tokamak plasmas with negative central shear

Fast waves provide an excellent tool for heating electrons and driving current in the central region of tokamak plasmas. In this paper, we report the use of centrally peaked electron heating and current drive to study transport in plasmas with negative central shear (NCS). Tokamak plasmas with NCS offer the potential of reduced energy transport and improved MHD stability properties, but will require non-inductive current drive to maintain the required current profiles. Fast waves, combined with neutral beam injection, provide the capability to change the central current density evolution and independently vary {ital T{sub e}}, and {ital T{sub i}} for transport studies in these plasmas. Electron heating also reduces the collisional heat exchange between electrons and ions and reduces the power deposition from neutral beams into electrons, thus improving the certainty in the estimate of the electron heating. The first part of this paper analyzes electron and ion heat transport in the L-mode phase of NCS plasmas as the current profile resistively evolves. The second part of the paper discusses the changes that occur in electron as well as ion energy transport in this phase of improved core confinement associated with NCS

The technology of ICRF systems

The technology of ICRF systems has made substantial gains in the past few years. The total power levels have increased from the 5-MW level to greater than 20 MW; the pulse lengths have increased from the 100-msec level to 30 seconds. Recently, fast wave current drive (FWCD) has been demonstrated using phased arrays of ICRF antennas. In order to achieve such large gains, substantial changes and improvements were needed in the level of design analysis, fabrication techniques, and system controls

Comprehensive energy transport scalings derived from DIII-D similarity experiments

The dependences of heat transport on the dimensionless plasma physics parameters has been measured for both L-mode and H-mode plasmas on the DIII-D tokamak. Heat transport in L-mode plasmas has a gyroradius scaling that is gyro-Bohm-like for electrons and worse than Bohm-like for ions, with no measurable beta or collisionality dependence; this corresponds to having an energy confinement time that scales like {tau}{sub E} {proportional_to} n{sup 0.5}P{sup {minus}0.5}. H-mode plasmas have gyro-Bohm-like scaling of heat transport for both electrons and ions, weak beta scaling, and moderate collisionality scaling. In addition, H-mode plasmas have a strong safety factor scaling ({chi} {approximately} q{sup 2}) at all radii. Combining these four dimensionless parameter scalings together gives an energy confinement time scaling for H-mode plasmas like {tau}{sub E} {proportional_to} B{sup {minus}1}{rho}{sup {minus}3.15}{beta}{sup 0.03}v{sup {minus}0.42}q{sub 95}{sup {minus}1.43} {proportional_to} I{sup 0.84}B{sup 0.39}n{sup 0.18}P{sup {minus}0.41}L{sup 2.0}, which is similar to empirical scalings derived from global confinement databases

Investigation of a Light Gas Helicon Plasma Source for the VASIMR Space Propulsion System

An efficient plasma source producing a high-density (approx.10(exp 19/cu m) light gas (e.g. H, D, or He) flowing plasma with a high degree of ionization is a critical component of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) concept. We are developing an antenna to apply ICRF power near the fundamental ion cyclotron resonance to further accelerate the plasma ions to velocities appropriate for space propulsion applications. The high degree of ionization and a low vacuum background pressure are important to eliminate the problem of radial losses due to charge exchange. We have performed parametric (e.g. gas flow, power (0.5 - 3 kW), magnetic field , frequency (25 and 50 MHz)) studies of a helicon operating with gas (H2 D2, He, N2 and Ar) injected at one end with a high magnetic mirror downstream of the antenna. We have explored operation with a cusp and a mirror field upstream. Plasma flows into a low background vacuum (<10(exp -4) torr) at velocities higher than the ion sound speed. High densities (approx. 10(exp 19/cu m) have been achieved at the location where ICRF will be applied, just downstream of the magnetic mirror

Experiments on Ion Cyclotron Damping at the Deuterium Fourth Harmonic in DIII-D

Absorption of fast Alfven waves by the energetic ions of an injected beam is evaluated in the DIII-D tokamak. Ion cyclotron resonance absorption at the fourth harmonic of the deuteron cyclotron frequency is observed with deuterium neutral beam injection (f = 60 MHz, B{sub T} = 1.9 T). Enhanced D-D neutron rates are evidence of absorption at the Doppler-shifted cyclotron resonance. Characteristics of global energy confinement provide further proof of substantial beam acceleration by the rf. In many cases, the accelerated deuterons cause temporary stabilization of the sawtooth (monster sawteeth), at relatively low rf power levels of {approximately}1 MW