R. J. Taylor

Properties of dc helicity injected tokamak plasmas

Several dc helicity injection experiments using an electron beam technique have been conducted on the Current Drive Experiment (CDX) [Phys. Rev. Lett. 59, 2165 (1987)] and the Continuous Current Tokamak (CCT) [Phys. Rev. Lett. 63, 2365 (1989)]. The data strongly suggest that tokamak plasmas are being formed and maintained by this method. The largest currents driven to date are 1 kA in CDX (qa =5) and 6 kA in CCT (qa =3.5). An initial comparison of discharge properties with helicity theory indicates rough agreement. Current drive energy efficiencies are 9% and 23% of Ohmic efficiency in two cases analyzed. Strong radial electric fields are observed in these plasmas that cause poloidal rotation and, possibly, improved confinement.

CORRELATION REFLECTOMETRY : AN EXPERIMENTAL INVESTIGATION OF MEASUREMENT CAPABILITY

The correlation reflectometer was conceived as a method of determining the interior correlation length of plasma turbulence in a nonperturbing and localized manner. Currently, measurements on different machines have produced different lengths and tendencies. Theories have been used to interpret the data from this diagnostic, but the diagnostic and theories remain unverified by direct validation with other diagnostics. As part of its ongoing research program aimed at the understanding and full utilization of reflectometry, UCLA has initiated a direct comparison between turbulent correlation lengths as measured by reflectometry and Langmuir probes. This investigation is performed on the CCT tokamak at UCLA. Initial results show good agreement between the two diagnostics, with radial coherence lengths in the range 1.0–3.0 cm depending upon the frequency of the fluctuation. A detailed description of the experiment and initial results are presented.

Vacuum photodiode detector array for broadband UV detection in a tokamak plasma

An array of vacuum photodiode detectors has been used to monitor discharge equilibrium, stability, and cleanliness in the Macrotor tokamak. These detectors use the photoelectric effect on small tungsten plates to measure UV emission in the band lambda approximately 200-1200 angstroms, and so are sensitive mainly to impurity line radiation in Macrotor. The response of this system to controlled impurity contamination experiments and to disruptions is described. The design, construction, and background problems associated with these detectors are discussed in detail.

Properties of the radial sheath in H-mode plasma

Radial currents from a tiny electrode (not a limiter) are used to modify the plasma edge in CCT through the 〈j × B) force. The rotation produces a radial electric field which does not exceed vtiB where vti is the ion thermal velocity. By varying the current or the potential of the electrode, a variety of radial sheaths are generated. Up to 5 kV sheath potentials have been produced with no significant impurity effects. Bifurcation, related to plasma E × B rotation and dissipation in the sheath, has been demonstrated. The resulting signatures of the CCT plasma with large radial sheaths are comparable to that of the H-mode plasmas seen on main-line tokamaks. The transport improvements are related to the reduction of radial electrical conductivity and to the improvement of the magnetic surfaces. The latter improvement is seen to penetrate to the core.

Far-infrared imaging of tokamak plasma

A 20-channel interferometer has been developed which utilizes a linear, one-dimensional microbolometer array to obtain single-shot density profiles from the UCLA Microtor tokamak plasma. The interferometer has been used to study time-dependent phenomena in the plasma density profile. Observations of the sawtooth instability clearly show the growth of the m=0 mode from a localized oscillation (r=1 cm) on axis to an oscillation of the entire plasma. Also, measurements during the initial startup phase of the discharge show evidence of hollow density profiles. In addition, a simultaneous measurement of the poloidal magnetic field has been developed which provides 20 channels of polarimetry. Interferometry and polarimetry both use the same imaging system and the spatial resolution of both measurements has been tested using plastic and crystal-quartz test objects. The signal-to-noise ratio for the polarimeter has also proved adequate for the expected Faraday rotation angle (alphamax=7°, Ip=70 kA, n=5×10^13 cm^−3).

Hot-cathode preionization studies in CCT

A hot LaB{sub 6} cathode was used to ionize the gas in the vessel of CCT at the start of tokamak discharges. Substantial plasma densities could be obtained in the preionization phase, resulting in reliable breakdown and initiation of q{sub a} {approx equal} 3 discharges at loop voltages of 4.2 V/turn, considerably lower than the 33 V/turn required with no preionization and the 20 V/turn required when a 15 kHz oscillator was the preionization source. When inductive effects were subtracted, the cathode preionization produced a loop voltage attributable to plasma resistance of 4 V/turn, while the oscillator-produced plasma required 12 V/turn. Repeatable cathode-enhanced breakdowns could be obtained at voltages as low as 3.4 V/turn for discharges with higher q{sub a}. With the cathode-enhanced plasma, the initial value of dI{sub p}/d{Phi}{sub OH} is higher than that with the oscillator-produced plasma. The spectrum of visible light emitted from cathode-initiated discharges shows no additional impurities present beyond those seen in a normal plasma. 10 refs., 4 figs.

Heating, current drive and transport properties of large area ICRF couplers

Initially, the large area ICRF couplers were studied in Macrotor in order to minimize the effects of the RF on the edge plasma and on the related transport. Recently, the following three aspects of these couplers have been studied. (1) Reduction of the edge power density by size alone. (2) Formation of a narrow K‐parallel spectrum. (3) The utilization of this powerful technique for fast wave lower hybrid current drive generation. Current drive experiments in Macrotor have resulted in the prediction that a continuously driven tokamak operation can be achieved at a level that will be consistent with neo‐Alcator scaling.

Energy propagation and field structure of LH fast waves in the UCLA continuous current tokamak (CCT)

3D field maps of the lower hybrid fast waves have been measured in the UCLA CCT tokamak in both the current drive and test wave experiments. The waves was found to propagate at a relatively constant small angle (<20°) to the magnetic field lines. When launched from the edge, the plasma filters the antenna spectrum allowing only the component N//≊N⊥ to penetrate. The wave damping cannot be accounted for either by Landau or collisional damping. The measured N⊥/N//≊1 suggests mode scattering between the fast and slow modes. The low N// component however can be excited when launch directly at the center of the plasma using a small dipole antenna. Substantial asymmetry of the propagated wave pattern was observed in both the up‐down and in‐out directions.

Performance of internally sealed IRF antennas in CCT

The internally sealed ICRF antennas in CCT no longer show impurity problems in the steady state or in the pulsed mode. Experience over a 6 month period indicates a minimum of one order of magnitude reduction in impurity generation for comparable input power (50 kW) used in previous experiments. The antenna radiation resistances agree with computer calculation within experimental error. In the Fast Wave Current Drive regime cavity resonances are absent. The power transfer to the plasma is nearly ideal. It is now clear that the power goes to the ions at the edge, where confinement is poor. The absorption mechanism is not understood. Wave penetration to the center is only possible for extremely low (<1012cm3) densities.

Perpendicular ion acceleration during ICRF, LHRF and ECH experiments in toroidal plasmas in the UCLA CCT device

Measurements of the fast neutral flux in the CCT (Continuous Current Tokamak) device in ICRF‐, LHRF‐, and ECH‐produced plasmas show a common feature: fast ion tails (Ei/Ti≊100) with large ratios of perpendicular‐to‐parallel energy (E⊥/E∥≊ 100). Perpendicular fast ion production is particularly strong during fast wave RF operation, regardless of driver frequency, but enhanced ion generation has been observed at ω=nΩi, up to n=7. Superthermal ion tails (Ei/Ti≊20) are also present in high‐harmonic ICRF‐heated tokamak discharges, but the ion distribution function relaxes towards isotropy in pitch angle due to improved ion confinement. In all of the above plasma, the perpedicularity of the fast ion distribution increases with energy.