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.     

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.     

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.     

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.     

Tokamak edge plasma rotation in the presence of the biased electrode

Electrode biasing system was designed, constructed, and installed on the IR-T1 tokamak, and then biasing experiments were carried out. Also, using a Mach probes the effects of radial electric field (produced by biased electrode) on the poloidal and toroidal components of the edge plasma velocity were investigated. The results showed an increase in both toroidal and poloidal components of the edge plasma velocity during biasing regime. Results compared and discussed. During positive biasing, increased E_r tends to slow the poloidal rotation in the electron diamagnetic drift direction, i.e., to speed up rotation in the ion diamagnetic drift direction. An increased toroidal rotation velocity has the opposite effect on the poloidal rotation.     

Plasma Internal Inductance in the Presence of External Resonant Fields in IR-T1 Tokamak

In this paper we presented experimental investigation of effects of local limiter biasing (V_biasing = +200 v, V_biasing = +320 v) on the plasma parameters as plasma current, loop voltage, poloidal beta, plasma pressure, plasma energy, plasma resistance, plasma temperature, plasma displacement, Shafranov parameter and plasma internal inductance in IR-T1 tokamak. For these purposes, array of magnetic probes and also a diamagnetic loop have been used. The results show that applied biased voltage V_biasing = +200 v causes to decrease of about 40 % in plasma internal inductance. The plasma resistance and the plasma displacement have been decreased by V_biasing = +200 v. The main result of the application of V_biasing = +200 v is flatting the plasma parameters profiles. In other words, the addition of biasing voltage V_biasing = +200 v to plasma could be effective for improving the quality of tokamak plasma discharge by creating the steady state plasma. The plasma current, plasma pressure, plasma energy, plasma temperature and shift parameter have increased after the application of limiter biasing with V_biasing = +320 v but they decrease rapidly.     

Toward modeling of the scrape-off layer currents induced by electrode biasing using 2D particle-in-cell simulations

The property of scrape-off layer (SOL) currents induced by a biased electrode is investigated by fully kinetic collisionless two-dimensional particle-in-cell (PIC) simulations. A reduced Vlasov–Darwin model is employed, which is capable of describing the low-frequency kinetic behavior without electromagnetic vacuum modes A linear decay distribution of electron currents parallel to the background magnetic field is exhibited. Simulation analyses indicate that the cross field ion current is a key factor in sheath formation and global current balance. The influences of electrode area, biasing voltage and plasma source on the SOL current profile are studied, respectively. Characteristic plasma parameters in the far SOL region of the EAST tokamak are used in simulations to assess the current driving ability of the electrode biasing method. Due to the limitations of computational power, the geometrical size of the simulation domain is significantly smaller than the realistic SOL, which may lead to an absence of the quasi-neutral region in the upstream plasma. At last, a heuristic method is proposed to calculate the upper bound of the total current strength.     

Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons (REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices. Experimental study on the suppression of runaway current by electrode biasing (EB) and limiter biasing (LB) has been performed on the J-TEXT tokamak, which could be an alternative way to suppress the runaway current. The experimental results show that the higher the voltage value, the smaller the runaway current in both EB and LB experiments. The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V. The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max (ERE) and radiation temperature THXR of hard x-rays (HXRs) are significantly reduced after the application of +200 V limiter biased voltage. The electric field generated by the biased voltage may be the key factor to suppress the runaway current, and the measured radial electric field increases obviously after the voltage is applied. This may result in an increase in the loss of REs to realize the suppression of runaway current.     

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.     

The application of limiter target electrostatic measurement system in J-TEXT tokamak

The limiter target electrostatic measurement system including limiter grounding current sensors and Langmuir probes have been newly developed for the measurement of the limiter target area on the Joint-Texas Experimental tokamak(J-TEXT).Current sensors fixed between graphite limiters and the vacuum vessel walls are used to measure the currents between limiters and vessel wall.Simultaneously,a rectangular poloidal array containing 54 Langmuir probes is embedded in the graphite tiles of limiters for a more localized measurement.Based on this system,the effect of both the plasma's inherent behavior,including plasma motion and the 2/1 tearing mode,and the electrode biasing on probe and sensor signals have been observed and analyzed in the experiments.     

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.     

Comparative Measurements of Plasma Position Using Multipole Moments Method and Analytical Solution of Grad–Shafranov Equation in IR-T1 Tokamak

In this paper we present two methods for determination of plasma position in IR-T1 tokamak: a multipole moments method and analytical solution of equilibrium problem or Grad–Shafranov equation. In the multipole moments method a modified rogowski and saddle coils were designed, constructed, and installed on outer surface of IR-T1 tokamak chamber, then displacement of plasma column were measured from them. To compare the plasma position obtained using this method, analytical solution of Grad–Shafranov equation is also demonstrated on IR-T1 tokamak. Results are in good agreement with each other.     

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.     

Study on Three-Dimensional Displacement Measurement Method of EAST Cold Magnet Based on Computer Vision

The location of superconducting tokamak magnets decides the position and shape of plasma, it is significant of real-time acquiring the location of tokamak magnets to stably operate the tokamak. At present, magnet displacement measurement is taken in displacement sensor, but the factors affecting the measurement results have much because of tokamak structural features, this paper proposes an improved monocular laser triangle measuring method, it can effectively reduce the distractions, using the image processing software specially developed, we can quickly get magnet operating data, experimental results show that this method precision is high, this method can meet the requirements of the measure.     

The residual zonal flow in tokamak plasmas with a poloidal electric field

In a tokamak plasma with auxiliary heating by cyclotron waves, a poloidal electric field will be produced, and as a consequence influence the residual zonal flow(RZF) level. The poloidal electric field can also be induced through biasing electrodes at the edge region of tokamaks.Numerical evaluation for a large aspect ratio circular cross section tokamak for the electron cyclotron wave heating indicates that the RZF level decreases significantly when the poloidal electric field increases. Qualitatively, the ion cyclotron wave heating is able to increase the RZF level. It is difficult to apply the calculation to the real cyclotron wave heating experiments since we need to know factors such as the plasma profiles, the exact power deposition and the cross section geometry, etc. It is possible to use the cyclotron wave heating to control the zonal flow and then to control the turbulence level in tokamak experiments.     

Comparative study of plasma position measurements using multipole moments and discrete magnetic probes methods on IR-T1 tokamak

In this paper, we present a comparative study for estimating plasma column displacement using multipole moments and discrete magnetic probes methods. It is shown that multipole moments method gives a better performance in the real time determination of the plasma column displacement in IR-T1 tokamak.     

Plasma Confinement Modification in IR-T1 Tokamak by Velocity Shear Stabilization

E × B velocity shear effects on the plasma confinement were investigated in the IR-T1 tokamak. The investigations have been done at the presence of external applied electric and Resonant Helical magnetic Fields (RHF). In this work, experimental data have been measured by using two arrays of the Langmuir probes in both the radial and poloidal directions. A velocity shear stabilization mechanism has also been proposed to be responsible for an improvement in plasma confinement. The results show that E_r × B drift velocity (V_E×B) reduces about 90 % due to applied biasing and RHF at edge plasma. We have also observed that positive biasing and RHF lead to a significant decrease (>80 %) for radial turbulent transport (Γ_E×B) at edge plasma. In this paper, the electrostatic Reynolds stress (Rs) and the shearing rate γ_E×B have been calculated. We have also compared the Rs and γ_E×B at presence of the biasing and RHF and without biasing and RHF. A good correlation between confinement modifications and E_r × B velocity shear has been found suggesting that confinement enhancement originates at the edge plasma as a consequence of the formation of a particle transport barrier just inside the limiter.     

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.     

Effect of Centrifugal Force on Poloidal Distribution of Impurities in the Edge of Small Size Divertor Tokamak

Toroidal rotation of tokamak plasma causes an outward major radial displacement of impurities. Here the physics of this phenomenon is analysis and the distribution of impurities on a magnetic surface is determined for the cases of practical interest.     

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.