STOR-M Tokamak

The STOR-M tokamak is a research tokamak designed and built in the Laboratory for studies on plasma heating, anomalous transport, and developing novel tokamak operation modes and advanced diagnostics. After the closure of Tokamak de Varennes in 1997, STOR-M is the only device in Canada devoted to magnetic fusion. Efforts are being made to revitalize fusion research in Canada which is the only G8 country not participating in the ITER program.

STOR-M is equipped with a sophisticated feedback control system for horizontal and vertical plasma positions, a driver for fast rising Ohmic current, a circuit system for alternating current (ac) operation, Compact Torus Injector, and various diagnostic instruments. In the following, major experiments carried out with STOR-M are their impacts on tokamak research are described.

Alternating Current (ac) Tokamak Operation

Stable alternating current operation of a tokamak was first demonstrated in STOR-1M (1987) and subsequently reproduced in STOR-M and the Joint European Torus (JET) at 2 MA currents. Genuine steady state tokamak fusion reactors require non-Ohmic current drive. For example, injection of microwaves at the lower-hybrid resonance frequency is a well developed technology for non-inductive current drive. However, the power requirement for driving multi mega ampere tokamak currents is a significant fraction of reactor output even if a large fraction of the toroidal current is self-generated as the bootstrap current. Inductive (Ohmic) current drive is highly efficient and not subject to plasma density limitation as rf wave current drive. The principal objective of the ac operation experiments carried out on STOR-1M and STOR-M tokamaks is to demonstrate the feasibility of quasi steady state (rather than genuinely steady state) tokamak reactors. Recently, 1.5-cycle ac operation has been achieved in STOR-M without the feared accumulation of impurities during the current reversal phases.

Compact Torus Injection

Compact torus (CT) injection is an emerging new technology to fuel the core of tokamak fusion reactors in the future. Fueling technologies currently available, such as cryogenic deuterium-tritium pellet injection, may not be adequate to fuel directly the core of reactors where most fusion reactions take place. The compact torus is a fully ionized, self-confined high density plasmoid and if accelerated to a velocity larger than the Alfven velocity, it will penetrate deep into a tokamak discharge. The first non-disruptive CT injection has been demonstrated on Tokamak de Varennes using the Compact Toroid Fueller designed and fabricated in the Laboratory under a contract with the Canadian Fusion Fuels Technology Project. Subsequently, a smaller Compact Torus Injector was built with the funds provided by NSERC through the Strategic Program. It was installed on STOR-M in 1995. In addition to expected plasma fueling (increase in the plasma density), CT injection has been found to induce a phenomenon similar to the Ohmic H-mode (high confinement mode) discovered earlier on STOR-1M and STOR-M tokamaks. Similar improved confinement after CT injection has been observed on TdeV as well.

Ohmic H-Modes

H-mode is an improved confinement phase in tokamaks over nominal (L-mode) confinement. In large tokamaks with powerful supplementary heating, transition to H-mode occurs when the heating power exceeds a threshold. In STOR-1M and STOR-M tokamaks, a unique heating method, turbulent heating, developed earlier in the Laboratory for non-tokamak toroidal devices, has been applied. After the current pulse, the plasma confinement time is tripled. This observation of Ohmic H-mode was made first on STOR-1M and later on STOR-M and other tokamaks with various means including fast gas puffing and electrode biasing. The significance of the observation is in demonstrating that there is room for improved confinement even in Ohmic discharges. Concurrent with improved confinement, significant reduction of the fluctuations of plasma density and of magnetic fluctuations at the plasma edge have been observed. The plasma potential is lowered and an edge transport barrier develops. At present, the causality problem remains unresolved, that is, it is not clear plasma auto-biasing is a cause or result of improved confinement

Advanced Plasma Diagnostics

Microwave reflectometry has been developed not only for measuring the amplitude of plasma density fluctuations but also for correlation length measurements which is an important quantity in estimating the anomalous plasma diffusivity. In the core region of STOR-M, plasma density fluctuation measurement based on scattering of 2 mm microwave is being conducted with emphasis on short wavelength modes driven by the electron temperature gradient (ETG mode). X-ray tomography for fast magnetic reconnection phenomena during CT injection is planned. It is also planned to develop novel diagnostics based on far infrared lasers for the ITER through international collaboration.