Saskatchewan Torus-Modified Tokamak Research Articles

Development of a broad band AC power amplifier for real time turbulence feedback control experiment in the Saskatchewan Torus-modified (STOR-M) tokamak.

A gated oscillating power amplifier has been developed for high frequency biasing and real time turbulent feedback experiments in the Saskatchewan Torus-modified tokamak. This oscillator is capable of providing a peak to peak oscillating output voltage of around ±60 V with a current around 30 A within the frequency band 1 kHz-50 kHz without any distortions. The overall output power is amplified by a two-stage metal oxide semiconductor field-effect transistor power op-amp as well as nine identical push-pull amplifiers in the final stages. The power amplifier input signal, collected from the plasma floating potential during the tokamak discharge, is optically isolated from the tokamak vessel ground. The filtered floating potential fluctuations with a band width of 5 kHz-40 kHz were amplified and fed to an electrode inserted into the plasma edge to study the response of the plasma turbulence. We observe that magnetic fluctuations are suppressed due to real time feedback of the floating potential.

Effects of edge safety factor on the toroidal flow velocities during the resonant magnetic perturbations and compact torus injection experiments on the STOR-M tokamak

ABSTRACTThe effects of the edge safety factor on the toroidal flow velocity modification due to resonant magnetic perturbation (RMP) and tangential compact torus injection (CTI) have been investigated on the Saskatchewan Torus-Modified (STOR-M) tokamak. The (l = 2, n = 1) RMP coils are wound against the outer surface of the vacuum vessel. The safety factor has been varied by changing the plasma current at a fixed toroidal magnetic field. As the edge safety factor is reduced, the magnetic perturbations on the q = 2 surface are expected to shift radially outwards, moving closer towards the RMP coils. The evolution of the plasma flow velocity was measured through ion Doppler spectroscopy (IDS). It has been found that RMP changes the flow velocity towards the direction of the plasma current direction and this effect is enhanced when the edge safety factor is reduced. When CT was injected tangentially along the plasma current direction, similar changes in the flow velocity have been found. However, the change is not sensitive to the edge safety factor in the CT injection case.

Effect of resonant magnetic perturbations on edge plasma parameters in the STOR-M tokamak

ABSTRACTEdge plasma properties have been modified in the Saskatchewan Torus-Modified tokamak by means of resonant magnetic perturbations (RMP). It has been found that the radial profiles of ion saturation current and floating potential in the edge region can be modified by an externally applied static (m = 2, n = 1) RMP field. An increase in the pedestal plasma density (n) and more negative electric field (Er) have been observed in the plasma edge region. It is believed that the RMP field altered the plasma transport in the edge and scrape-off layer regions, leading to a higher density pedestal and a potential drop in some cases. During the enhanced confinement phase, it is possible to identify a region where intermittent transport events, the so-called blobs, are created and the holes of lower density left behind.

Modification of toroidal flow velocity through momentum Injection by compact torus injection into the STOR-M tokamak

In the Saskatchewan torus-modified (STOR-M) tokamak, tangential compact torus injection (CTI) experiments have been performed with normal (counter-clockwise, CCW, top view) and reversed (clockwise, CW, top view) plasma current directions while the compact torus (CT) injection direction remains in the CCW direction. The intrinsic toroidal flow direction reverses when the discharge current is reversed. However, the change in the toroidal flow direction is always toward the CTI direction (CCW). It has been determined that the momentum in high density and high velocity CT is more than ten times larger than the intrinsic toroidal rotation momentum in the typical STOR-M plasma. Therefore, the modification of the plasma toroidal rotation velocity is attributed to momentum transfer from CT to the tokamak discharge.

Modification of plasma rotation with resonant magnetic perturbations in the STOR-M tokamak

The toroidal plasma flow velocity of impurity ions has been significantly modified in the Saskatchewan Torus-Modified (STOR-M) tokamak by means of resonant magnetic perturbations (RMP). It has been found that the toroidal flow velocities of OV and CVI impurity ions change towards the co-current direction after the application of a current through a set of (l = 2, n = 1) RMP field coils. It has been observed that the reduction of the toroidal flow velocity is closely correlated to the reduction of the magnetohydrodynamic (MHD) fluctuation frequency measured by Mirnov coils. Modulation of the flow velocity has been achieved by switching the RMP current pulses. Non-resonant magnetic perturbations have also induced a much smaller change in the toroidal plasma flow. A theoretical model has been adopted to assess the contributions of different drift mechanisms to magnetic islands rotation in STOR-M.

Plasma current start-up by the outer ohmic heating coils in the Saskatchewan TORus Modified (STOR-M) iron core tokamak.

A plasma current up to 15 kA has been driven with outer ohmic heating (OH) coils in the STOR-M iron core tokamak. Even when the inner OH coil is disconnected, the outer OH coils alone can induce the plasma current as primary windings and initial breakdown are even easier in this coil layout. This result suggests a possibility to use an iron core in a spherical tokamak to start up the plasma current without a central solenoid. The effect of the iron core saturation on the extension of the discharge pulse length has been estimated for further experiments in the STOR-M tokamak.

Control of magnetic islands in the STOR-M tokamak using resonant helical fields

The resonant interaction between magnetohydrodynamic (MHD) instability modes and the externally applied helical magnetic field is demonstrated in the Saskatchewan Torus-Modified (STOR-M) tokamak. The study is conducted both numerically and experimentally using a 2D MHD equilibrium code in the former and an (l = 2, n = 1) helical coil carrying a short current pulse in the latter. It is shown numerically that the resonant helical current can efficiently suppress the magnetic islands resonating on the (m = 2, n = 1) magnetic surface when the value of the safety factor at the plasma edge is relatively low (⩽4). It is also found numerically that (2, 1) islands are induced when the applied helical current exceeds a threshold. The experimental study is performed in STOR-M during low-q ohmic discharges with high MHD activities. The amplitude and frequency of (2, 1) Mirnov fluctuations are significantly reduced after the activation of the resonant field. Lesser suppression in sideband islands is also observed. Moreover, a phase of improved plasma confinement, characterized by a reduction in Hα emission level, a reduction in loop voltage and an increase in the soft x-ray emission, is induced after application of the resonant field.

Retarding field energy analyzer for the Saskatchewan Torus–Modified plasma boundary

The retarding field energy analyzer (RFA) is a simple and reliable diagnostic technique to measure the ion temperature in the scrape-off layer and edge of magnetic fusion devices. Design and operation features of a single-sided (facing the ion flow) RFA for ion temperature measurements in the Saskatchewan Torus-Modified (STOR-M) tokamak are described. Its compact size (21 x 15 x 20 mm3) allows RFA measurements without perturbing plasma significantly. Both ion and electron temperature have been measured by RFA in the STOR-M tokamak. A method is proposed to correct the effects of ion flow on the ion temperature using the simultaneously measured Mach number. The measured electron temperature is consistent with the previously reported Langmuir probe data. Abnormal behavior of the RFA has been observed in both ion and electron modes when RFA is inserted deep into the plasma.

Fuzzy-logic-based plasma-position controller for STOR-M

The successful operation of a tokamak fusion reactor rests on its ability to stably confine a very hot plasma. The quality of the discharge in the Saskatchewan TORus-Modified (STOR-M) tokamak is strongly related to the position of the plasma column within the discharge vessel. If the plasma column approaches too near the wall, then either minor or complete disruption occurs. Consequently it is necessary to be able to control dynamically the position of the plasma column throughout the entire discharge. A fuzzy-logic-based controller was designed and implemented for this purpose. This controller was evaluated on the STOR-M tokamak during actual operation, and was found to provide reliably a high quality of control. The actual performance of the fuzzy-logic-based controller at actually controlling the position of the plasma column within the STOR-M tokamak is evaluated against the actual performance of a proportional-integral-derivative (PID) controller.

Observation of radial propagation of electrostatic fluctuations in the Saskatchewan torus-modified tokamak

Radial propagation of electrostatic fluctuations in the edge region of the Saskatchewan torus-modified [W. Zhang et al., Phys. Fluids B 4, 3277 (1992)] tokamak has been measured using Langmuir probes. The propagation characteristics of the density fluctuations are analyzed by two point correlation techniques. A finite radial propagation speed is measured and is found to be a large fraction of the poloidal propagation speed. The speed of propagation is inversely proportional to the toroidal magnetic field, suggesting that the origin of these fluctuations could be drift wave type turbulence. The radial correlation length of turbulence is found to decrease with increasing magnetic field. The power averaged wave number shows an increase with increasing magnetic field, suggesting a decrease in the correlation lengths.

Control of the floating potential fluctuations via limiter biasing in the Saskatchewan Torus-Modified (STOR-M) tokamak

Limiter biasing experiments have been performed in the STOR-M tokamak (Saskatchewan Torus-Modified) [Phys. Fluids B 4, 3277 (1992)]. It is found that the floating potential fluctuation amplitudes in the scrape-off layer (SOL) depend mainly on the limiter potential and this potential determines the current flowing toward the limiter. The dependence of the fluctuation amplitude on the potential gradient in the SOL is rather weak. For positive limiter bias, a strong suppression of the floating potential fluctuation is observed, while for negative limiter biasing the floating potential fluctuations are significantly enhanced. The observed phenomena are consistent with recent theoretical studies [Phys. Fluids B 5, 3191 (1993)].