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The 'fishbone' fast wave travelling wave antenna was developed for LHD to provide a capability for rotational transform profile control by current drive. The fishbone antenna is equivalent to two combline antennae stacked vertically. The antenna operates around 75 MHz and excites a wavenumber of 14 m−1 when the phase difference between adjacent current straps is 90°. A test of a combline antenna with plasma load on the TST-2 spherical tokamak suggested the possibility that this type of antenna does not need to be installed in the immediate vicinity of the last closed flux surface. Optimization of the design was performed based on measurements on mock-up antennas and model calculations. In the fishbone antenna, controlled excitation of the even mode (with currents in the top and bottom halves of a current strap in the same direction) is necessary. A predominant excitation of the even mode was realized in the LHD fishbone antenna with simulated loading by selecting an appropriate operating frequency.

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.

The electron cyclotron emission (ECE) diagnostic system is installed on the large helical device (LHD). The system includes the following instruments: a heterodyne radiometer, a Michelson spectrometer, and a grating polychromator. A 63.5 mm corrugated waveguide system is fully utilized. Large collection optics and notch filters at the frequency of the LHD electron cyclotron heating (ECH) were developed for this system. In addition to these filters, the rectangular waveguide notch filters, the ECE measurement with the radiometer has been successfully performed during the ECH.

The travelling-wave antenna for heating and current drive in the ion cyclotron range of frequencies is shown theoretically to have loading and wavenumber spectra that are largely independent of plasma conditions. These characteristics have been demonstrated in low-power experiments on the DIII-D tokamak, in which a standard four-strap antenna was converted to a traveling-wave antenna through use of external coupling elements. The experiments indicate that the array maintains good impedance matching without dynamic tuning during abrupt changes in the plasma, such as during L- to H-mode transitions, edge-localized mode activity, and disruptions. An analytic model was developed that exhibits the features observed in the experiments. Guidelines for the design of travelling-wave antennas are derived from the validated model. 11 refs., 14 figs.

Plasma discharges with negative central magnetic shear (NCS) and access to the ballooning second stability regime have been produced on DIII-D with edge plasma conditions similar to those in the standard L mode confinement regime. Confinement enhancement factors up to H identical to tau E/ tau ITER-89P~2.5 are obtained while maintaining the L mode edge. Compared with discharges with monotonic q profiles, highly peaked toroidal rotation (fphi (0) approximately=30-70 kHz), ion temperature (Ti(0) approximately=15-22 keV) and density (ne(0)/ne approximately=2.2) profiles are observed. This regime has yielded the highest DD neutron rates observed to date on DIII-D. Steep pressure gradients drive a large bootstrap current, with up to ~75% total non-inductive current drive. These features make this an attractive advanced tokamak operating regime for further study

Fast Alfven waves radiated from the phased array antenna in the DIII‐D tokamak [L. G. Davis et al. in Proceedings of the 12th IEEE Symposium on Fusion Engineering, Piscataway, 1987 (Institute of Electrical and Electronics Engineers, New York, 1987), p. 991] and used for heating and current drive are studied by employing a B‐loop array mounted on the vacuum vessel wall. The wave propagation direction controlled by the antenna phasing is clearly observed. A small divergence of the rays arising from the anisotropic nature of the fast wave is found. Comparison with a ray tracing code confirms that the ray position calculated by the code is precise up to at least one toroidal turn of the rays. Conservation of ktR, which is a basic assumption in computer codes, is tested. Although the upshift of toroidal wave number kt at small major radius R is confirmed, ktR is not well conserved. The so‐called ‘‘eigenmode’’ is observed and the cause is identified. A mass density interferometer is demonstrated by employing t...

Using the combination of early neutral beam injection, ramping of the plasma current, low electron density and controlled L - H transitions, robust discharges with negative central magnetic shear are reproducibly obtained on the DIII-D tokamak. The effects of these factors on the formation and evolution of the q profile during the initial phase of these discharges are documented. Details of the evolution of the inverted q profile are obtained from measurements of the internal field pitch using a 16-channel motional Stark effect (MSE) system. Time-dependent MSE data are used to directly construct the profile of the toroidal electric field and allow a straightforward calculation of the non-inductive current density profile.

Reliable operation of discharges with negative central magnetic shear has led to significant increases in plasma performance and reactivity in both low confinement, L‐mode, and high confinement, H‐mode, regimes in the DIII‐D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. Using neutral beam injection early in the initial current ramp, a large range of negative shear discharges have been produced with durations lasting up to 3.2 s. The total noninductive current (beam plus bootstrap) ranges from 50% to 80% in these discharges. In the region of shear reversal, significant peaking of the toroidal rotation [fφ(0)∼30–60 kHz] and ion temperature [Ti(0)∼15–22 keV] profiles are observed. In high‐power discharges with an L‐mode edge, peaked density profiles are also observed. Confinement enhancement factors up to H≡τE/τITER‐89P∼2.5 with an L‐mode edge, and H∼3.3 in an edge localized mode (ELM)‐free H mode, are obtained. Transp...

Current drive experiments have been performed on the DIII-D tokamak using fast waves, electron cyclotron waves, and neutral injection. Fast wave experiments were performed using a 4-strap antenna with 1 MW of power at 60 MHz. These experiments showed effective heating of electrons, with a global heating efficiency equivalent to that of neutral injection even when the single pass damping was calculated to be as small as 5%. The damping was probably due to the effect of multiple passes of the wave through the plasma. Fast wave current drive experiments were performed with a toroidally directional phasing of the antenna straps. Currents driven by fast wave current drive (FWCD) in the direction of the main plasma current of up to 100 kA were found, not including a calculated 40 kA of bootstrap current. Experiments with FWCD in the counter current direction showed little current drive. In both cases, changes in the sawtooth behavior and the internal inductance qualitatively support the measurement of FWCD. Experiments on electron cyclotron current drive have shown that 100 kA of current can be driven by 1 MW of power at 60 GHz. Calculation with a Fokker-Planck code show that electron cyclotron current drive (ECCD) can be well predicted when the effects of electron trapping and of the residual electric field are included.

Since fast Alfven waves propagate across a plasma at the Alfven speed, the plasma mass density can be determined through interferometry. In previous measurements on the DIII-D tokamak,1 fast waves (∼100 MHz, ∼5 W) were launched from an antenna at the outer midplane, but detection of the signal was hampered by poor sensitivity of the receiving antenna, which was mounted behind protective graphite tiles on the inner wall. We modified several graphite tiles to act as more sensitive receiving antennas. At lower frequencies (∼25 MHz), fast waves can reflect from the ion–ion hybrid cutoff layer. The position of this layer is sensitive to the ratio of hydrogen to deuterium in the plasma. Receiving antennas on the outer wall will measure the hydrogen concentration through reflectometry. Launching other frequencies may yield impurity density ratios as well. These techniques may be useful for measuring relative densities if D, T, and α particles in burning plasmas.

The power deposition profiles for different poloidal and toroidal launch angles have been determined by modulating the ECH power and measuring the electron temperature response. The peak of the measured power density follows the poloidal steering of the ECH launcher, and perpendicular launch gives a narrower deposition profile than does oblique (current drive) launch. The difference in wave refraction between X-mode and O-mode allows positive identification of an unwanted O-mode component of the launched beam.

The 110 GHz gyrotron installation on the DIII-D tokamak has been upgraded to three tubes in the megawatt class with plans for further upgrades. The latest addition uses a diamond output window. The report describes the installation, plans, and experimental results to date.

The physics of non-inductive current drive and current profile control using the fast magnetosonic wave has been demonstrated on the DIII-D tokamak. In non-sawtoothing discharges formed by neutral beam injection (NBI), the radial profile of the fast wave current drive (FWCD) was determined by the response of the loop voltage profile to co, counter, and symmetric antenna phasings, and was found to be in good agreement with theoretical models. The application of counter FWCD increased the magnetic shear reversal of the plasma and delayed the onset of sawteeth, compared to co FWCD. The partial absorption of fast waves by energetic beam ions at high harmonics of the ion cyclotron frequency was also evident from a build up of fast particle pressure near the magnetic axis and a correlated increase in the neutron rate. The anomalous fast particle pressure and neutron rate increased with increasing NBI power and peaked when a harmonic of the deuterium cyclotron frequency passed through the center of the plasma. The experimental FWCD efficiency was highest at 2 T where the interaction between the fast waves and the beam ions was weakest; as the magnetic field strength was lowered, the FWCD efficiency decreased to approximately half of the maximum theoretical value.

A new approach to fast wave antenna array design based on the traveling wave antenna has been successfully demonstrated on the JFT-2M tokamak. A traveling wave antenna is powered though a single feed and the power flow from element to element is only via mutual reactive coupling. A combine is a particular type of traveling wave antenna, in which only the fed element and the element at the downstream end of the array are connected to vacuum feed troughs, while the intermediate elements are terminated with reactances inside the vacuum chamber. A twelve element combine for operation at 200 MHz was designed and fabricated at General Atomics, and installed and operated on the JFT-2M tokamak. The full output power of a single transmitter, 0.2 MW, was coupled to tokamak discharges with very little conditioning required. The input impedance of the combine was well matched to the transmission line impedance for all loading conditions, including vacuum (no plasma), Taylor discharge cleaning plasmas, and ohmic, L- and H-mode tokamak discharges with neutral beam heating without any adjustment of tuning elements.

Fast Alfven waves radiated from the phased array antenna in the DIII‐D tokamak and used for heating and current drive are studied by employing a B‐loop array mounted on the vacuum vessel wall. The wave propagation direction controlled by the antenna phasing is clearly observed. A small divergence of the rays arising from the anisotropic nature of the fast wave is found. Comparison with a ray tracing code confirms that the ray position calculated by the code is accurate up to at least one toroidal turn of the rays. Conservation of Rkt which is a basic assumption in computer codes is tested. Although the upshift of toroidal wavenumber kt at small major radius R is confirmed, Rkt is not well conserved. A mass density interferometer is demonstrated by employing the extraordinary fast wave.

The physics of electron heating and current drive with the fast magnetosonic wave has been demonstrated on DIII-D, in reasonable agreement with theoretical modeling. A recently completed upgrade to the fast wave capability should allow full noninductive current drive in steady state advanced confinement discharges and provide some current density profile control for the Advanced Tokamak Program. DIII-D now has three four-strap fast wave antennas and three transmitters, each with nominally 2 MW of generator power. Extensive experiments have been conducted with the first system, at 60 MHz, while the two newer systems have come into operation within the past year. The newer systems are configured for 60 to 120 MHz. The measured FWCD efficiency is found to increase linearly with electron temperature as {gamma} = 0.4 {times} 10{sup 18} T{sub eo} (keV) [A/m{sup 2}W], measured up to central electron temperature over 5 keV. A newly developed technique for determining the internal noninductive current density profile gives efficiencies in agreement with this scaling and profiles consistent with theoretical predictions. Full noninductive current drive at 170 kA was achieved in a discharge prepared by rampdown of the Ohmic current. Modulation of microwave reflectometry signals at the fast wave frequency is beingmore » used to investigate fast wave propagation and damping. Additionally, rf pick-up probes on the internal boundary of the vessel provide a comparison with ray tracing codes, with dear evidence for a toroidally directed wave with antenna phasing set for current drive. There is some experimental evidence for fast wave absorption by energetic beam ions at high cyclotron harmonic resonances.« less

First observations of a traveling wave antenna (TWA) in a tokamak are reported. For a fractional bandwidth (δω/ω) of about 20%, TWA experiments using a four‐element antenna in DIII–D showed less than 1% reflected power and excellent stability of the phase velocity of the launched wave—even during giant ELMs. The power radiated into the plasma varied from about 40% during ELM‐free H‐mode to almost 90% during L‐mode and ELMing H‐mode. Theoretical modeling validated by these low power observations is used to predict the plasma load resistance of a high power combline being built for JFT‐2M.

Coherent density fluctuations induced by rf electric fields have been successfully observed using an upgraded reflectometer system on DIII‐D. The method utilizes spatially localized X–mode reflectometry (65–73 GHz) measurements to detect coherent density fluctuations driven by the fast wave (FW) electric field. These fluctuations at the FW frequency (60–120 MHz on DIII‐D) are detected using heterodyne techniques. The directional nature of the FW launch has been experimentally verified at a position close to the FW antenna; the reflectometer signal is enhanced for a fast wave launch directed towards the detection position. Large spatial and temporal signal variations are observed, which may be related to the detailed internal FW electric field structure.

The traveling wave antenna for heating and current drive in the ion cyclotron range of frequencies is shown theoretically to have loading and wavenumber spectrum which are largely independent of plasma conditions. These characteristics have been demonstrated in low power experiments on the DIII-D tokamak, in which a standard four-strap antenna was converted to a traveling wave antenna through use of external coupling elements. The experiments indicate that the array maintains good impedance matching without dynamic tuning during abrupt changes in the plasma, such as during L- to H-mode transitions, edge localized mode activity, and disruptions. An analytic model was developed which exhibits the features observed in the experiments. Guidelines for the design of traveling wave antennas are derived from the validated model.

We propose use of a fast Alfven wave interferometer and reflectometer as a tokamak diagnostic to measure the plasma mass density, D–T species mix profile, and density fluctuations. Utilizing the property that the phase velocity of the fast wave propagating across the magnetic field is the Alfven speed with thermal correction, this fast wave interferometer on the DIII-D tokamak was successfully used to obtain the line integrated density. Since the position of the ion–ion hybrid cutoff in tokamaks is uniquely determined by the species mix ratio and wave frequency, the reflectometer arrangement finds the species mix profile. The inversion method of reflectometry is discussed. The multiple chord interferometer also measures the mass density fluctuation profile.

The first experimental test of the ergodic magnetic limiter concept is being conducted on the TEXT device. Modular coils have been installed on TEXT allowing the external generation of resonant helical field perturbations. Preliminary results are presented.

The resonant ion-beam---driven electrostatic ion-cyclotron instability is identified. Measured dispersion relation and onset versus beam energy and density agree with numerical calculations based on a theory which includes beam-acoustic terms. After amplitude saturation, velocity-space diffusion of the beam ions is observed.

A lower-hybrid instability with ion cyclotron harmonics is observed to be resonantly driven by an ion beam injected obliquely to the confining magnetic field, in agreement with a linear, warm-plasma theory. Quasilinear velocity-space diffusion of the beam is observed. (AIP)

The size of a magnetic island driven by an error field or a helical coil depends strongly upon the plasma response aiming at the maintenance of equilibrium. Sample calculations for tokamaks within the context of a linear theory show that the island size is greatly augmented when the tearing mode is close to a marginally stable state.

The Ergodic Magnetic Layer Experiment on ohmically heated Text demonstrates that a small resonant helical field (m/n = 7/2 or 7/3) with fractional amplitude r/BT of about 10−3 creates a stable ergodic magnetic layer and substantially modifies the boundary heat flow. Field line tracings accurately map the observed perturbed limiter heat load patterns. The level of the intrinsic impurities is reduced, consistent with a lower edge electron temperature, as measured by the electron cyclotron emission detector.

The resonant ion-beam-driven electrostatic ion cyclotron instability is identified. Measured dispersion relation and onset vs. beam energy and density agree with numerical calculations based on a theory which includes beam acoustic terms. After amplitude saturation, velocity space diffusion of the beam ions is observed. (auth)

Deformation of the charged surface of liquid 4He is studied by optical means. Near the dimple crystal formation point we observe saturation effects and a stripe structure exhibiting hysteresis. We compare data to existing theories of nonlinear equilibrium.

A nonlinear theory of the capillary waves accounts for the macroscopic lattice formation on the electron-charged surface of liquid helium observed in a recent experiment. The lattice allows both shear and compressional lattice waves to propagate.

A poloidal quadrupole field is applied to a tokamak discharge in order to study the magnetic field‐line reconnection. Elongation of the poloidal cross‐section of the plasma is observed when the quadrupole field strength is below a critical value. As the quadrupole field is increased above this critical field strength, reconnection occurs and the magnetic axis changes from being the center of elliptic magnetic surfaces (i.e., an ’’O’’ point) to the center of symmetry of hyperbolic surfaces (an ’’X’’ point). The reverse process is also observed.

The detailed identification of two instabilities driven by an ion beam injected parallel to the confining magnetic field, a resonant ion cyclotron instability, and a nonresonant ion cyclotron drift wave driven by the density gradient of the beam, is reported. Experimental results are in good agreement with numerical calculations based on the kinetic Vlasov equations. Quasi‐linear velocity space diffusion of the beam ions by wave‐particle interaction is observed, demonstrating enhanced energy transfer. The relevancy of these results to tokamak neutral beam injection heating is discussed.