Current-drive Experiments Research Articles

Non-resonant destabilization of (1/1) internal kink mode by suprathermal electron pressure

New experimental observations are reported on the structure and dynamics of short-lived periodic (1, 1) “fishbone”-like oscillations that appear during radio frequency heating and current-drive experiments in tokamak plasmas. For the first time, measurements can directly relate changes in the high energy electrons to the mode onset, saturation, and damping. In the relatively high collisionality of Alcator C-Mod with lower hybrid current drive, the instability appears to be destabilized by the non-resonant suprathermal electron pressure—rather than by wave-particle resonance, rotates toroidally with the plasma and grows independently of the (1, 1) sawtooth crash driven by the thermal plasma pressure.

Simulations of the lower-hybrid antenna in the Madison Symmetric Torus reversed-field pinch

Due to constraints inherent to a reversed-field pinch plasma configuration, an unusual launch structure—the interdigital line—was used for lower-hybrid current-drive experiments in the Madison Symmetric Torus. The antenna design and performance were analyzed using an array of codes (including RANT3D/AORSA1D-H, Microwave Studio and VORPAL). It was found that the voltage phasing was not the intended one. As a result, the parallel-wavenumber spectrum of the launched wave peaks at a value lower than desired, making the accessibility marginal. Further simulations demonstrated that the error can largely be corrected by either lowering the antenna operating frequency or shortening the length of the resonators.

Modification of the collective Thomson scattering radiometer in the search for parametric decay on TEXTOR

Strong scattering of high-power millimeter waves at 140 GHz has been shown to take place in heating and current-drive experiments at TEXTOR when a tearing mode is present in the plasma. The scattering signal is at present supposed to be generated by the parametric decay instability. Here we describe the heterodyne detection system used to characterize the newly discovered signal measured at TEXTOR, and we present spectral shapes in which the signal can appear under different conditions. The radiation is collected by the receiver through a quasi-optical transmission line that is independent of the electron cyclotron resonance heating transmission line, and so the scattering geometry is variable. The signal is detected with 42 frequency channels ranging from 136 to 142 GHz. We demonstrate that the large signal does not originate from gyrotron spurious radiation. The measured signal agrees well with independent backscattering radiometer data.

Equilibrium evolution in oscillating-field current-drive experiments

Oscillating-field current drive (OFCD) is a proposed method of steady-state toroidal plasma sustainment in which ac poloidal and toroidal loop voltages are applied to produce a dc plasma current. OFCD is added to standard, inductively sustained reversed-field pinch plasmas in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. Equilibrium profiles and fluctuations during a single cycle are measured and analyzed for different relative phases between the two OFCD voltages and for OFCD off. For OFCD phases leading to the most added plasma current, the measured energy confinement is slightly better than that for OFCD off. By contrast, the phase of the maximum OFCD helicity-injection rate also has the maximum decay rate, which is ascribed to transport losses during discrete magnetic-fluctuation events induced by OFCD. Resistive-magnetohydrodynamic simulations of the experiments reproduce the observed phase dependence of the added current.

Single-Mode Excitation in High-Power Gyrotrons by Controlling Gun Perveance

Present-day gyrotrons intended for plasma heating and current-drive experiments operate in high-order modes. Therefore, in such gyrotrons, in the process of voltage rise, as a rule, higher frequency parasitic modes are excited prior to the desired mode. This excitation can be avoided when electron beams with lower current density at intermediate voltages are used. Then, when the voltage passes through the region of excitation of high-frequency parasitic modes, the beam current can be made lower than the start current of those modes. At the same time, when the voltage reaches the excitation zone of the desired mode, the current can be high enough for exciting the desired mode. Such lowering of the beam current at intermediate voltages can be realized by reducing the ratio of the space-charge-limited to the temperature-limited emission. The method is illustrated by an example showing how the excitation of a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23, 6</sub> -mode can be avoided in a gyrotron designed for operation at the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22, 6</sub> -mode. Applicability of this method to millimeter-wave megawatt-class gyrotrons is discussed. The method proposed has additional advantages for gyrotrons operating in power-modulated regimes which are used for suppression of neoclassical tearing modes in large-scale tokamaks and stellarators.

The Temperature in the RTP Tokamak under Feedback Control by Regulating the Output of a 110-GHz Gyrotron

In addition to wave properties like polarization, harmonic number, high- or low-field side launching, and launch angle, absorption of electron cyclotron waves depends strongly on plasma parameters, such as the temperature. Therefore, we have introduced a new way of operating our 110-GHz gyrotron for electron cyclotron heating and electron cyclotron current-drive experiments. In the Rijnhuizen Tokamak Project (RTP), a system for feedback control of the plasma temperature by regulating the output of the gyrotron has come into operation recently. Other parameters, i.e., the Shafranov parameter λ = βpol + li/2 - 1, and the loop voltage can be under feedback control as well. The control system is described and some test results are given.

A system to measure suprathermal electron distribution functions in toroidal plasmas by electron cyclotron wave absorption

A two-chord, four-beam suprathermal electron diagnostic has been installed on TdeV (B&amp;gt;1.5 T, R=0.86 m, a=0.25 m). Resonant absorption of extraordinary mode electron cyclotron waves is measured to deduce the chordal averaged suprathermal electron distribution function amplitude at the resonant momentum. Simultaneously counterpropagating beams permit good refractive loss cancellation. A nonlinear frequency sweep leads to a concentration of appropriately propagating power in a narrow range of time of flight, thus increasing the signal-to-noise ratio and facilitating the rejection of spurious reflections. Numerous measurements of electron distribution functions have been obtained during lower-hybrid current-drive experiments.

Continuous Monitoring and Data Acquisition System for Steady-State Tokamak Operation

Long discharges have been demonstrated by lower hybrid current-drive experiments on some tokamak devices. Discharges of longer than 1000 s are also planned for the International Thermonuclear Experimental Reactor (ITER) and Tokamak Physics Experiment (TPX) projects. In the case of long-time or steady-state operation, it is important to monitor the plasma parameters continuously and change the operational conditions during the discharge to maintain the plasma current. However, a conventional data acquisition and analysis system cannot follow these operations because it must show the results after each pulse. A new system that can continuously monitor and support steady-state operation is necessary. A new system is developed in which the signal flow is divided into branches, and one series of processing is made to switch alternately among the groups in every regular desired interval. An application of this system has been demonstrated on a 1-h discharge by TRIAM-1M. 9 refs., 6 figs.

Review of current drive theory: selected topics

Two themes in current drive theory in tokamaks are reviewed, both relevant to the progression of tokamak experiments toward the reactor regime. The author reviews the understanding of the physics of tail electrons. These electrons are capable of carrying enormous RF-driven electric current, and, in the course of current-drive experiments worldwide not only has the current drive effect been demonstrated, but the underlying physical description of these tail electrons has been established. Anticipating the presence of the energetic alpha particles that result from D-T reactions in a reactor, the author examine's certain mechanisms through which these alpha particles can be used to facilitate current-drive.

Observation and analysis of a two-parallel-temperature electron tail during lower-hybrid current-drive on a tokamak

A ‘‘two-parallel-temperature’’ feature of the fast-electron distribution function has been observed during lower-hybrid current-drive experiments on the Versator-II tokamak (B. Richards, Ph.D. thesis, Massachusetts Institute of Technology, 1981). Computational modeling predicts this feature and indicates that the colder tail is initiated by low-power, high-N∥ waves from the sidelobes of the antenna N∥ spectrum. The modeling indicates these waves bridge the ‘‘spectral gap,’’ enabling current drive even though the wave parallel phase velocity greatly exceeds the electron thermal velocity for the majority of the wave power.

Space-dependent quasi-linear theory of lower-hybrid waves

The quasi-linear diffusion coefficients for the inherently space-dependent wave amplitudes in lower-hybrid heating and current-drive experiments are derived and compared with the usual space-independent approximation. The distribution function is derived and the dependence on the radio-frequency field amplitude gradient of the ‘plateau’, which is an essential quantity in thetheoretical description of LH experiments, is stressed.

Neutral-Beam Current-Driven High-Poloidal-Beta Operation of the DIII-D Tokamak

Neutral-beam current-drive experiments in the DIII-D tokamak with a single null poloidal divertor are described. A plasma current of 0.34 MA has been sustained by neutral beams alone, and the energy confinement is of $H$-mode quality. Poloidal $\ensuremath{\beta}$ values reach 3.5 without disruption or coherent magnetic activity suggesting that these plasmas may be entering the second stability regime.

Particle confinement improvement during 2.45 GHz lower-hybrid current-drive experiments

Particle confinement behavior during 2.45 GHz lower-hybrid current drive has been investigated on the Versator II tokamak [Phys. Fluids 29, 1985 (1986)]. It is found that during combined Ohmic and rf current drive the global particle confinement time τp increases by up to a factor of 2 compared to purely Ohmically driven discharges, as observed in earlier 800 MHz experiments at lower densities n̄e ≤6×1012 cm−3. In the present experiments, τp increases have been observed at densities up to n̄e =2×1013 cm−3.

Particle confinement and the anomalous Doppler instability during combined inductive and lower-hybrid current drive

Increases in average density have been observed during lower-hybrid current-drive experiments (LHCD) on the Versator II tokamak [Phys. Rev. Lett. 48, 152 (1982)], and on other LHCD experiments where inductive-Ohmic and lower-hybrid current drive are combined. In the present experiment it is found that this density increase is the result of an improvement in global particle confinement time τp in comparison to purely Ohmic discharges with similar plasma parameters. Furthermore, it is found that the improved confinement deteriorates when the anomalous Doppler relaxation instability of the anisotropic high-energy electron tail becomes unstable during LHCD, and the loss rate of tail electrons increases. However, energetic electron losses alone are at least an order of magnitude too small to explain this deterioration in particle confinement.

Current rampup by lower-hybrid waves in the PLT tokamak.

Recent lower-hybrid current-drive experiments have clearly demonstrated that the current in a tokamak discharge can be maintained by rf drive alone. We have extended the operating regime of such plasmas to include ramping up of the current. We find that at densities of \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ approximately 20% of the launched rf power is converted to magnetic field energy.

Stimulated mode conversion at lower-hybrid frequencies.

The prevalence of plasma density fluctuations in tokamaks restricts the applicability of mode-conversion theories based on quiescent plasmas. The fluctuations stimulate mode conversion between otherwise distinct fast and slow cold plasma waves near the lower-hybrid frequency. A pair of coupled differential equations representing the two waves is derived and solved under the assumption of random turbulence. This conversion may explain the observed ${N}_{\ensuremath{\parallel}}$ spectrum and density cutoff in tokamak lower-hybrid current-drive experiments.

The PLT rotating pumped limiter

A limiter with a specially contoured front face and the ability to rotate during tokamak discharges has been installed in a PLT pump duct. These features have been selected to handle the unique particle removal and heat load requirements of ICRF heating and lower-hybrid current-drive experiments. The limiter has been conditioned and commissioned in an ion-beam test stand by irradiation with 1 MW power, 200 ms duration beams of 40 keV hydrogen ions. Operation in PLT during ohmic discharges has proven the ability of the limiter to reduce localized heating caused by energetic electron bombardment and to remove about 2% of the ions lost to the PLT walls and limiters.

On the generation of superthermal electrons in lower-hybrid current-drive experiments

It is shown that a small, low-phase-velocity part of a power source spectrum can strongly enhance the amount of the current generated in lower-hybrid current-drive experiments. A good agreement between the calculated and observed current is found.

Study of the slow-wave structure as an ICRF launcher

The feasibility of the slow-wave structure as an ICRF launcher is studied. The well-defined travelling-wave spectrum of this structure makes it a plausible candidate for current-drive experiments. Using a model which is realistic in ICRF experiments, the impedance function, the propagation constant, and the damping length along the structure are calculated as a function of plasma/antenna separation. We find that with reasonably accurate control of plasma position and sufficient spacing between the plasma and antennas, which can be expected from reactors, the use of the slow-wave structure for ICRF launching is feasible.

Synchrotron emission from runaway electron distributions

Synchrotron emission from a relativistic anti-loss-cone (runaway) distribution is investigated numerically and compared with various analytical approximations. The results are applied to recent measurements of enhanced emission during current-drive experiments on the Princeton Large Torus (PLT) as well as to impulsive solar microwave bursts.