Biasing Effect on Modifying of the Tokamak Plasma Horizontal Displacement

In this contribution we presented the first biasing experiments performed on the IR-T1 tokamak. For this purpose, a movable biasing system was designed, constructed, and installed, and then the positive voltage applied to an electrode inserted inside the tokamak limiter and the plasma current and also the plasma horizontal displacement in the absence and presence of the biased electrode based on the multipole moments and magnetic probes techniques were measured. Measurements result has shown a modification of plasma confinement by decreasing the plasma displacement.     

Biasing Effect on the Edge Plasma Electrical Fluctuations in IR-T1 Tokamak

The effect of biasing on the edge plasma electrical fluctuations and their control was investigated. Biasing was applied to both positive and negative polarity in the range of +380 to ?380 V. As soon as biasing was applied, the electrical fluctuations, fluctuations frequency and H_α emission reduced significantly and all of them showed the positive effect of biasing in the edge plasma electrical fluctuations reduction. Experiments also showed that positive biasing is more effective than negative biasing.     

Plasma Horizontal Displacement Measurement Using Flux Loops in IR-T1 Tokamak

In this paper we presented poloidal flux loops technique for measurement of plasma horizontal displacement in the IR-T1 tokamak. In this technique, two poloidal flux loops were designed and installed on outer surface of the IR-T1 tokamak chamber, and then the plasma displacement was obtained from them. To compare the result obtained using this method, analytical solution is also experimented on the IR-T1. Results of the two methods are in good agreement with each other.     

Determination of the Plasma Internal Inductance and Evaluation of its Effects on Plasma Horizontal Displacement in IR-T1 Tokamak

We analyzed dynamic equilibrium properties of a large aspect ratio and low Beta tokamaks, in particular deriving a modified relation for the Shafranov shift in the presence of poloidal flow and external vertical field, and demonstrate it experimentally on the IR-T1 tokamak. Poloidal flow can produce modifications in the equilibrium properties. By increasing Alfvenic Mach number from zero, flow produce outward force, and plasma shifted in outward direction. If the poloidal Alfvenic Mach number equal to one, singularity will observe in the solution of generalized Grad–Shafranov equation. Also inversion of Shafranov shift in the transition of flow speed between sub-Alfvenic to super-Alfvenic speeds can be observed due to inward force produced by flow.     

Radial and Vertical Displacement Events of Plasma in the Damavand Tokamak

The plasma current in tokamak is under the influence of forces in such a way it tents toward the radial expansion. The forces resulting from external self-induced, internal inductance, thermal energy, and magnetic field fluctuations on the plasma column, cause radial expansion. To keep plasma in its position, the Lorentz force should be applied by vertical magnetic field to balance these forces. Control of the plasma position in the radial direction is very complicated. Poloidal beta, β_θ, and the internal inductance parameter depend on plasma current where plasma current parameters themselves are not steady in tokamak. The experimental data of Damavand tokamak is used to compare radial displacement with theoretical prediction. Temporal variation of plasma current along with time variation of R and Z positions of the plasma column is studied. The vertical displacement event takes place because of the elongated cross section of plasma column. Theoretical and experimental results show reasonable agreement.     

Effects of Hot Limiter Biasing on Tokamak Runaway Discharges

In this research hot limiter biasing effects on the Runaway discharges were investigated. First wall of the tokamak reactors can affects serious damage due to the high energy runaway electrons during a major disruption and therefore its life time can be reduced. Therefore, it is important to find methods to decrease runaway electron generation and their energy. Tokamak limiter biasing is one of the methods for controlling the radial electric field and can induce a transition to an improved confinement state. In this article generation of runaway electrons and the energy they can obtain will be investigated theoretically. Moreover, in order to apply radial biasing an emissive limiter biasing is utilized. The biased limiter can apply +380 V in the status of cold and hot to the plasma and result in the increase of negative bias current in hot status. In fact, in this experiment we try to decrease the generation of runaway electrons and their energy by using emissive limiter biasing inserted on the IR-T1 tokamak. The mean energy of these electrons was obtained by spectroscopy of hard X-ray. Also, the plasma current center shift was measured from the vertical field coil characteristics in presence of limiter biasing. The calculation is made focusing on the vertical field coil current and voltage changes due to a horizontal displacement of plasma column.     

New Results on Plasma Displacement in IR-T1 Tokamak by Bias Modification

In this work we presented recent results on plasma displacement in IR-T1 Tokamak using modification in bias system. In this regard, a movable biasing system was designed and constructed. The positive voltage applied to an electrode inserted inside the tokamak limiter and the plasma current and also the plasma horizontal displacement in the absence and presence of the biased electrode based on the multipole moments and magnetic probes techniques were measured. Measurements result has shown a modification of plasma confinement by decreasing the plasma displacement.     

A Simplified Technique for Determination of Plasma Displacement in the IR-T1 Tokamak

In this paper we presented a simplified technique for the determination of plasma displacement based on poloidal flux measurement in IR-T1 tokamak. This instrument consists of a two semicircle loops which installed toroidally on inner and outer sides of tokamak chamber and connected with each other. Really, this instrument detects the difference of poloidal flux on High Field Side (HFS) and Low Field Side (LFS), which we needed in calculating of the Shafranov shift. Main benefit of our proposed instrument is its simplicity. Based on this technique we measured the plasma position, and to compare the result obtained using this technique, array of four magnetic probes are also designed, constructed and installed on outer surface of the IR-T1 tokamak and plasma position obtained from them. Results are in good agreement with each other.     

A Novel Optical Technique Based on Image Processing for Determination of Tokamak Plasma Displacement

Plasma displacement is one of the main problems of tokamak plasma equilibrium. Control of plasma displacement has important role in plasma confinement and to achieve optimized tokamak plasma operation. In this contribution we presented a navel and simple optical technique for determination of tokamak plasma column displacement. For this purpose, an image processing technique used for the output signal of CCD camera and then plasma emission intensity profile and plasma position obtained.     

Feedback Control of Plasma Current and Horizontal Position in HT-7 Tokamak

There is a strong magnetic coupling between poloidal field coils of superconductive tokamak HT-7, These coils are connected to individual power     

Plasma Flows within the Context of Biasing Experiments

The understanding and reduction of turbulent transport in magnetic confinement devices is not only an academic task, but also the matter of practical interest, since high confinement is chosen as the regime for ITER and possible future reactors it reduces both the size and the cost. Since the pioneering work on CCT a lot of work has been devoted to the effect of electric field biasing carried out on many tokamaks, which in general leads to a strongly varying radial electric fields as a function of radius and a resulting sheared E×B flow, giving rise to improved confinement properties.The issue of plasma flows is utterly fundamental for understanding of tokamaks aimed at the achievement of fusion energy. This appears in the well known neoclassical theory as the most accomplished and self-consistent basis for understanding of fusion plasmas. It pertains to the novel concept of "zonal flows" emerging from the recent development of gyro-kinetic transport codes. The poloidal and toroidal flows are also cruc     

Design,Construction, and Installation of Movable Electrode Biasing System on Tokamak

The first results of the movable electrode biasing experiments performed on the IR-T1 tokamak are presented. For this purpose, a movable electrode biasing system was designed, constructed, and installed on the IR-T1 tokamak, and then the positive voltage applied to an electrode inserted inside the tokamak limiter and the plasma current, poloidal and radial components of the magnetic fields, loop voltage, and diamagnetic flux in the absence and presence of the biased electrode were measured. Results compared and discussed.     

Relations Between the Plasma Diamagnetic Effect and Plasma Basic Parameters in IR-T1 Tokamak

Precise determination of the poloidal Beta, internal inductance, plasma energy, plasma pressure, plasma temperature, plasma resistance, plasma effective atomic number, and plasma energy confinement time are essential for tokamak experiments. In this paper an experimental method especially based on the plasma diamagnetic effect for measurements of these parameters in IR-T1 tokamak are presented. For these purposes a diamagnetic loop with its compensation coil, and also an array of magnetic probes designed, constructed, and installed on outer surface of the IR-T1. Also in this work we measured the Shafranov parameter, plasma current, plasma voltage, and the plasma density by the magnetic probes, Rogowski coil, poloidal flux loop, and the Langmuir probe measurements, respectively.     

Effect of Various Parameters on Edge Plasma Stability in IR-T1 Tokamak

Edge plasma turbulence was investigated over a wide range of plasma and field parameters in the IR-T1 tokamak for the first time. Fluctuation levels and spectra were measured using two arrays of Langmuir probe in the region r/a = 0.75–1.2. Under almost all conditions the edge plasma was turbulently unstable, with a broadband fluctuation spectrum in the drift wave range of frequencies f = 10–1000 kHz. A stable state was observed only in the very cold, low-current discharge formed at unusually high neutral filling pressure. Otherwise, the relative fluctuation level as monitored by the ion saturation current was very high, in the range $ {{\tilde{\rm J}^{ + } } \mathord{\left/ {\vphantom {{\tilde{\rm J}^{ + } } {\bar{\rm J}^{ + } }}} \right. \kern-0pt} {\bar{\rm J}^{ + } }} \cong 0.2{-}0.8 $ , while the fluctuation power spectra were roughly invariant in shape. The relative fluctuation level was always highest near the wall and decreased monotonically toward the plasma centre.     

Using Neural Networks to Forecast Tokamak Plasma Disruptions

A feed-forward neural network with two hidden layers was used in this work to forecast major and minor disruptive instabilities in tokamak discharges. Using alternatively experimental magnetic data and soft X-ray signals, the neural net was trained with one or two disruptive plasma discharge, and a different disruptive discharge was used for validating the training process. After being properly trained     

Comparison Between Discrete Magnetic Coils and Multipole Coils for Measurement of Plasma Displacement in IR-T1 Tokamak

We have expressed two different methods to determine the plasma positions in the IR-T1 tokamak. An array of magnetic probes are placed polidally outside the chamber surface and their signals are plugged in the displacement equation and horizontal displacement of plasma column is obtained. The results of this measurement are compared with the experimental data obtained from sensors of multipole moment. It was detected a fluctuation in plasma displacement obtained with discrete magnetic coils, so the method of multiple moments is better than the discrete magnetic coils for the determination plasma displacement that used for feed back control.     

Observation of Pedestal Plasma Turbulence on EAST Tokamak

Pedestal plasma turbulence was experimentally studied by microwave reflectometry on EAST tokamak. The characteristics of edge pedestal turbulence during dithering L-H transition, ELM-free H-mode phase and inter-ELM phase have recently been studied on EAST. An edge spatial structure of density fluctuation and its dithering temporal evolution is observed for the first time on the EAST tokamak during the L-H transition phase. A coherent mode usually appears during the ELM-free phase prior to the first ELM on EAST tokamak. The mode frequency gradually decreases as the pedestal evolves. Analysis shows that the coherent mode is in the pedestal region inside the separatrix. In plasma with type-III ELMs, a precursor mode before ELM is usually observed. The frequency of the precursor was initially about 150 kHz and gradually decreased till the next ELM. The mode amplitude increases or shows saturation before ELM. In the plasma with compound ELMs composed of high and low frequency ELMs, the precursor was also observed before the high frequency ELM while the harmonic oscillations with frequencies of 20 kHz, 40 kHz and 60 kHz appear before the low frequency ELM.     

Initial Plasma Startup Test on SUNIST Spherical Tokamak

The goal of the Sino-United Spherical Tokamak (SUNIST) at Tsinghua University is to extend the understanding of toroidal plasma physics at a low aspect ratio (R/a ≈ 1.3) and to demonstrate a maintainable target plasma by non-inductive startup. The SUNIST device isdesigned to operate with up to 13 kA of ohmic heating field current, and to 0.15 T of toroidal field at 10 kA of discharge current. All of the poloidal fields can provide 30 mVs of Volt-seconds transformer. Experimental results of plasma startup show that SUNIST has remarkable characteristics of high ramp rate (dIp/dt ≈ 50 MA/s ), high normalized current IN of about 2.8 (IN = Ip/αBT),and high-efficiency (Ip/IROD ≈ 0.4) production of plasma current while operating at a low toroidal field. Major disruption phenomena have not been observed from magnetic diagnostics of all testing shots. Initial discharges with 52 kA of plasma current (exceeding the designed value of 50 kA),2 ms of pulse length and 50 MA/s of ramp rate have been achieved easily with pre-ionized filament.