H.M. Skarsgard

Stable AC tokamak discharges in the STOR-1M device

Stable and clean AC tokamak discharges have been achieved in the STOR-1M device. The plasma current is reversed from +4.1 kA to −4.0 kA within 1.9 ms. During the reversal, no disruptive behaviour is observed, the loop voltage changes smoothly from +1 V to −5 V without any spike, and impurities are not released. An electron density of (2–4) × 1012 cm−3 is maintained during current reversal. The possibility of continuous tokamak operation with a low frequency alternating plasma current is discussed.

Alternating current tokamak reactor with long pulses

Alternating current (ac) tokamak operation in the reactor parameter range is studied by considering the volt-second consumption. A simple condition for obtaining ac operation with nearly constant pulse length is given by 1/sub rho//R/sub rho/

Improved confinement and edge plasma fluctuations in the STOR‐M tokamak

An improved Ohmic confinement phase has been observed in the STOR‐M tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 323] after application of a turbulent heating (TH) pulse. This improved Ohmic confinement phase is characterized by (a) increased ne, (b) reduced Hα radiation from the edge, (c) reduced density and magnetic fluctuations at the edge, (d) a steeper density profile at the edge, and (e) a more negative radial electric field. Almost complete suppression of sawtooth oscillations during the improved confinement phase has also been observed. A linear dispersion relation describes the density and magnetic fluctuations in the frequency range up to 350 kHz. In the region r<a (r≳a) the propagation direction of the density fluctuations is in the electron (ion) diamagnetic direction. The reduction of rms density and magnetic fluctuations after the TH pulse is largely due to suppression of relatively high frequency (≤500 k...

An alternating current tokamak reactor with ohmic ignition and bootstrap current

In this paper an alternating current (ac) tokamak reactor with ohmic ignition and long pulses due to bootstrap current is proposed as a simple and quasi-continuous fusion power plant. An ohmic plasma current of 23 MA with a high toroidal field of {approximately} 10 T in the alternating Current Tokamak Reactor-Upgrade (ACTR-U) (10-m major radius and 2-m minor radius) provides the ohmic ignition. After entering the ignition regime, the plasma current is reduced by one-half to enhance the bootstrap current with a high-beta poloidal field ({beta}{sub p} {approximately} 2) to prolong the pulse length. When the ohmic transformer reaches the maximum flux, the plasma current is ramped down and reversed; ac operation follows. The authors thus demonstrate that an ohmic transformer alone is in principle sufficient for a quasi-continuous deuterium-tritium fusion reactor.

Plasma density at the current reversal in the STOR-1M tokamak with AC operation

The plasma density behaviour in the STOR-1M tokamak with alternating current (AC) operation is described using the Murakami-Hugill diagram (1/qa, nR/Bt). At the current reversal, Ip = 0 (1/qa = 0), the plasma density remains finite and the Murakami parameter is nR/Bt = (0.66 ± 0.22) × 1018m-2.T-1. Gas puffing before the current reversal does not noticeably increase the plasma density at the current reversal, but allows AC operation with larger currents and improves its reproducibility. A qualitative explanation for the finite plasma density at the current reversal is given on the basis of a short circuit effect by the limiter

Turbulent heating of a large toroidal plasma

Consideration is given to some of the scaling problems involved in the application of turbulent heating to much larger toroidal systems than those employed up to now. Single-pulse heating is studied in a model in which the time variation of the skin depth is allowed for and energy transport from the skin to the interior of the plasma maintains the poloidal beta factor, βp, at a constant value. With the total heating current limited to a value Im (for a safety factor q > 1), the ion acoustic instability can be generated only as long as the skin depth δ satisfies the condition δ < Im/(necs2πa) where n is the plasma density, cs the ion sound speed and a is the minor radius. Consequently, depending on the efficiency of the energy transport, it is not certain that single-pulse heating can raise the plasma temperature to that required for ignition of a fusion reactor, and some form of multiple-pulse-heating may be required.

Alternating current plasma operation in the STOR-M tokamak

One cycle alternating current (AC) plasma operation without a dwell time has been achieved in the STOR-M tokamak with good reproducibility using a newly developed ohmic heating circuit. The plasma current of +24 kA is smoothly ramped down in 10 ms with a rampdown rate of around 2.0 kA/ms and then ramped up to between -20 and -24 kA directly without a dwell time. The plasma density of up to (3.7+or-0.6)*1018 m-3 remains at the current reversal as observed in recent soft landing experiments. The key to a successful, reproducible and direct transition in AC tokamak operations on STOR-M is to control both the total vertical field by a feedback control system and the plasma density by careful gas puffing during the current reversal phase. This experiment has demonstrated that the initial loop voltage for the second negative current is minimized when the dwell time approaches zero, and the AC operation without dwelling is possible whenever the plasma current can be softly terminated with a finite residual plasma density

Generalized saddle point condition for ignition in a tokamak reactor with temperature and density profiles

In this paper, the concept of a generalized ignition contour map, is extended to the realistic case of a plasma with temperature and density profiles in order to study access to ignition in a tokamak reactor. The generalized saddle point is found to lie between the Lawson and ignition conditions. If the height of the operation path with Goldston L-mode scaling is higher than the generalized saddle point, a reactor can reach ignition with this scaling for the case with no confinement degradation effect due to alpha-particle heating. In this sense, the saddle point given in a general form is a new criterion for reaching ignition. Peaking the profiles for the plasma temperature and density can lower the height of the generalized saddle point and help a reactor to reach ignition. With this in mind, the authors can judge whether next-generation tokamaks, such as Compact Ignition Tokamak, Tokamak Ignition/Burn Experimental Reactor, Next European Torus, Fusion Experimental Reactor, International Tokamak Reactor, and AC Tokamak Reactor, can reach ignition with realistic profile parameters and an L-mode scaling law.

Microprocessor-based controller for plasma position in a tokamak

A high-speed microprocessor-based controller and its application to the plasma position in the STOR-M (Saskatchewan torus-medium) tokamak is described. An adaptive control procedure based on a high-speed digital signal processor has been used to improve the controller performance. The fundamental functions of this system are the plasma dynamics modeling, the controller adjustment based on parameter optimization, and real-time feedback control. The performance of this high-speed digital controller is described, and a simulation study is discussed. Plasma position measurement using magnetic probes is described. >

A controller for plasma motion in a Tokamak based on model estimation

A simple model and its application in designing the plasma position control system in the Saskatoon Torus Medium (STOR-M ) Tokamak are described. Estimating the model parameters, the design of a controller, plasma position measurement, the design of a power driver, and the implementation and testing of the complete system are included. The following assumptions are made to simplify the plasma position model: the plasma is treated as a moving axisymmetric, current-carrying filament; the iron-core of the transformer is approximated by an infinitely long cylinder; and the STOR-M vacuum vessel is constructed of type-304L stainless steel, is circular in cross-section, and is 4 mm in thickness. The model was very useful in the early stages of the design, but due to practical constraints the parameters of the model could not be determined very accurately. A least-squares-error algorithm was used offline to determine revised estimates of the models parameters. Based on these values, the PID (proportional-integral-derivative) controller was tuned. A 30 kA plasma has been successfully contained for a time of 50 ms, which is the maximum time required for this machine. >