Ohmic Heating Plasma Research Articles

Penetration of n = 2 resonant magnetic field perturbations in EAST

This paper presents the penetration of n = 2 magnetic field perturbations, where n is the toroidal mode number. The n = 2 intrinsic error field (IEF) is measured in an ohmic heating plasma using the compass scan method, i.e. the toroidal asymmetry in the threshold current for the penetration of n = 2 resonant magnetic perturbations (RMPs). Its amplitude is 55.5 A in equivalent coil current or B r,3/2 = 0.1 G and the toroidal phase of the IEF is around 170.6° (129°). Phasing scans (scans of the phase difference between the upper and lower coil currents) of the n = 2 RMPs are carried out to obtain the effects of the n = 2 spectrum on field penetration. The observed dependence of the field penetration on the spectrum is consistent with those of simulations using the MARS-F code. One of the interesting phenomena is that the n = 2 mode often stimulates an n = 1 mode. The dominant poloidal harmonic of the n = 1 mode is m = 2, and the dominant poloidal harmonic of the n = 2 mode is m = 3. The evolution of the n = 1 mode has two stages, i.e., an initial small island growth stage, and a later saturation stage. In the initial stage, the amplitude of the n = 1 magnetic island grows, while the phase remains fixed. When the amplitude of the magnetic island exceeds a certain threshold, it enters the second stage, in which the magnetic island is locked into another phase and its amplitude starts to saturate. The phase in the initial small island stage depends linearly on the phase of the applied n = 2 RMP, which suggests that the n = 2 mode is directly driven by the coupling between the n = 1 and n = 2 modes. The phase in the second stage is either locked to the phase close to the previously measured n = 1 IEF, or is locked to the phase close to the n = 2 response field. This suggests that the final phase of the n = 1 mode depends on competition between the locking effect induced by the n = 1 IEF and the nonlinear coupling effect between the two modes. This might be an issue in the MHD control application using high-n RMPs in the future ITER device.

Preliminary observation of medium-Z impurities by means of the soft x-ray pulse height analysis diagnostic on EAST

A soft x-ray pulse height analysis (PHA) system, based on a silicon drift detector (SDD), has been successfully implemented on EAST to measure spectra of soft x-ray emission. Utilizing the soft x-ray PHA diagnostic, the behavior of intrinsic and injected medium-Z impurities is first investigated on EAST and the preliminary observations are presented. There are hardly any Kα emission peaks of metallic impurities including Ti (Z = 22), Cr (Z = 24), Fe (Z = 26), Ni (Z = 28) and Cu (Z = 29) in the soft x-ray spectra for an ohmic heating plasma due to lower electron temperatures, while remarkable peaks can be seen in a lower hybrid wave (LHW) heated plasma. The profiles of Kα intensities of these metallic elements have been obtained with about 7 cm spatial resolution. It is observed that the Kα emissions come mainly from the core plasma column of ρ ⩽ 0.3 and found that their intensities are roughly proportional to the power of the LHW launched into the plasma. The Ar Kα peaks can also be monitored when argon gas is puffed into the plasma. It is qualitatively confirmed that the Kα lines of argon emitted from the lower ionic states are predominantly located in the outer plasma region, whereas the Kα lines with higher energies are mainly from high ionization stages in the central region.

Construction and Plasma Initiation of the Tokamak-Helical Hybrid Device TOKASTAR-2

TOKASTAR, a hybrid magnetic configuration between tokamak and stellarator, has been proposed for the study of magnetic confinement optimization. The construction of the small device named TOKASTAR-2 was started in April 2008, and completed in March 2009. As a preliminary experiment, spatial distribution of electron temperature (Te) and electron density (ne) are measured in a simple toroidal magnetic field configuration. Initial ECH plasmas with Te ∼ 10 eV, ne ∼ 1016 m−3 are confirmed. In the preliminary experiment of ohmic heating plasma production, plasma current (Ip) of 25 A is observed. The plasma equilibrium is calculated by the TOSCA code, and the magnetic flux surface and optimal vertical field coil currents are clarified for the future experiment.

Performance and stability of near-unity aspect ratio plasmas in the Pegasus Toroidal Experiment

The Pegasus Toroidal Experiment [R. Fonck et al., Bull. Am. Phys. Soc. 41, 1400 (1996)] is a spherical torus designed to study the limits of plasma behavior as the aspect ratio A approaches unity. Access to near-unity A is achieved through the use of a novel high-stress reinforced solenoid magnet. High toroidal beta βt is obtained in ohmically-heated plasmas by operation at low field with densities up to the Greenwald limit. Values of βt up to 20% and normalized beta up to 5 have been obtained. The ratio of plasma current to toroidal field rod current, known as the toroidal field utilization, reaches values as large as 1 but appears to approach a “soft” boundary at that level related to both ohmic flux limitations and the onset of resistive magnetohydrodynamic (MHD) activity. The m/n=2/1 and 3/2 modes are most frequently observed, in agreement with the inferred safety factor profiles. Experiments are beginning to access the external kink stability boundary at edge safety factor q95=5, which is significantly higher than that observed in conventional tokamaks. Calculations using the DCON code [A. H. Glasser and M. S. Chance, Bull. Am. Phys. Soc. 42, 1848 (1997)] confirm instability to the ideal kink.

Density Profile and Diffusion Coefficient During IBW Heating in the HT-7 Superconducting Tokamak

The electron density coefficient was studied during IBW heating in the HT-7 tokamak. The frequency of IBW was 30 MHz, and the injected power was 130 kW. An improved confinement mode was induced by IBW heating in the HT-7. The peaked density profile was formed gradually, and the Ha radiation intensity was decreased abruptly. A five-channel far-infared (FIR) hydrogen cyanide (HCN) laser interferometer was used to measure the electron density profile. The evolution of the density profile and the soft X ray intensity profile were studied during the improved confinement phase. The electron density diffusion coefficient was studied carefully for IBW heating discharges. The results were compared with Ohmic heating plasma and with different toroidal field Bt (2.0 T and 1.8 T). For 30MHz frequency of IBW heating, the higher Bt leads the lower electron density diffusion coefficient at the region of r/a > 0.6 in the HT-7.

Investigation of coupling of magnetohydrodynamic modes by soft x-ray computer tomography on the WT-3 tokamak

The internal structure of the stationary m=1 and m=2 modes in an ohmic heating plasma and the double m=1 mode structure in a lower hybrid current drive plasma are investigated on the WT-3 tokamak [Maehara et al., Nucl. Fusion 38, 39 (1998)] using computer tomography after the application of the singular value decomposition to the soft x-ray signals. The results show that, in both cases, two coexisting modes have the same frequency and have a fixed mutual phase relation, indicating that two modes are coupled and rotate as one body in the toroidal direction. It is found that the mutual inductance of two loops of helical current filaments for producing magnetic islands always takes the maximum at the experimentally observed positions of two-mode structures. This result means not only that the electromagnetic coupling of two current loops is at the maximum, but also that the two loops are in the dynamically stable position.

Soft X-ray computer tomography of locked mode with five detector arrays on the WT-3 tokamak

In the WT-3 tokamak, mode locking of the m=2 mode is artificially induced by the application of lower hybrid current drive in an ohmic heating plasma. The time evolution of the plasma internal structure during the mode locking is investigated by soft X-ray computer tomography with five detector arrays. It is found that the m=1 mode does not rotate as well as the m=2 mode and that the m=1 and m=2 modes grow continuously during the mode locking.

Suppression of a pressure driven m=1 mode in a lower hybrid current drive plasma by electron cyclotron heating in the WT-3 tokamak

A pressure driven m=1/n=1 mode is excited by lower hybrid current drive in the WT-3 tokamak [T. Maehara et al., Nucl. Fusion 38, 39 (1998)]. The excitation of the mode is accompanied with the decrease of the magnetic shear and with the peaking of the soft x-ray emissivity profile inside the q=1 surface. The crescent-shaped mode structure appeared on the contour map of the soft x-ray emissivity is consistent with that of the quasi-interchange mode. The m=1 mode can be suppressed by electron cyclotron heating near the q=1 surface. The range of the location of the electron cyclotron resonance layer effective for the complete suppression is much wider and the time scale for the suppression is much faster than those in the case of the suppression of the tearing mode in the ohmic heating plasma.

Observation of Toroidicity-Induced Alfvén Eigenmodes in Ohmically Heated Plasmas by Drift Wave Excitation

At ASDEX Upgrade, in Ohmically heated discharges MHD modes scaling with f{approximately}B/{radical}(m{sub i}n{sub e}) have been observed, pointing towards Alfv{acute e}nic-type modes. Considering a local approximation for the frequency of toroidicity-induced Alfv{acute e}n eigenmodes (TAE-modes), a value for the localization of the modes of q{approx}2.6 results. More detailed calculations predict TAE-modes with the corresponding frequencies at this location. The excitation of TAE-modes without fast particles can be explained by the coupling of the electromagnetic part of the drift Alfv{acute e}n turbulence at the plasma edge to the ordinary Alfv{acute e}n wave spectrum. {copyright} {ital 1997} {ital The American Physical Society}

The L- to H-Mode Transition - an Experimental Viewpoint

The nature of the L- to H-mode transition is re-examined on the basis of the more recent experimental data, especially that obtained from Ohmically-heated plasmas where the transition is generally more gradual. This raises the question whether the transition is better explained as a gradual change in the characteristics of the turbulence at the plasma edge rather than by a sudden bifurcation in the transport process.

DD Fusion neutron measurements in the beam-heated stellarator deuterium plasmas on Heliotron E

This paper describes the measurement of D-D fusion neutrons in the toroidal stellarator plasmas on Heliotron E. We used three moderated 3He counters and a moderated BF 3 counter. Calibration of neutron detectors was previously done with a 252Cf spontaneous fission neutron source. We describe the measured fusion neutron emissions with several discharge conditions with hydrogen neutral beam injection heating to the deuterium plasmas. The Heliotron E plasmas are usually initiated by electron cyclotron resonance heating at a magnetic field of 1.9 T without an ohmic heating plasma current. We have tried to deduce the central deuterium ion temperature from measured neutron flux and line-averaged electron density. We have compared the central ion temperatures by neutron flux, with the passive charge exchanged particle analyses and diamagnetic stored energy. We discuss the possible error sources for the central neutron deduced deuterium temperature.

Plasma-surface interaction studies in the HL-1 tokamak

The plasma-surface interaction (PSI) studies in HL-1 were carried out under the condition of an all metal wall, typically at an ohmic heating plasma current of 100 kA, a pulse duration of 0.6 s and a toroidal magnetic field of 2 T. Two mass spectrometers, a Langmuir probe and surface probes were developed to study particle flux arriving at and leaving the surface. From the time evolution of the pumped rate for the major gaseous impurities of H 2O, CH 4 and CO after an ohmic heating discharge, it is shown that the formations of H 2O, CH 4 and CO are due to, respectively, different types of surface chemical reactions.

The preparation and characterization of transition metal nitride films: P T Dawson and S A J Stazky,J Vac Sci Technol, 20 (4), 1982, 966–967

This paper describes the measurement of D-D fusion neutrons in the toroidal stellarator plasmas on Heliotron E. We used three moderated 3He counters and a moderated BF3 counter. Calibration of neutron detectors was previously done with a 252Cf spontaneous fission neutron source. We describe the measured fusion neutron emissions with several discharge conditions with hydrogen neutral beam injection heating to the deuterium plasmas. The Heliotron E plasmas are usually initiated by electron cyclotron resonance heating at a magnetic field of 1.9 T without an ohmic heating plasma current. We have tried to deduce the central deuterium ion temperature from measured neutron flux and line-averaged electron density. We have compared the central ion temperatures by neutron flux, with the passive charge exchanged particle analyses and diamagnetic stored energy. We discuss the possible error sources for the central neutron deduced deuterium temperature.

Thomson Scattering Measurements on Ohmically Heated Plasmas in the JFT-2 Tokamak

Measurements were made on electron temperature and density profiles of ohmically heated hydrogen plasmas in the JFT-2 tokamak device by using Thomson scattering of light from a Q-switched ruby laser. The measured electton temperature ranged from 0.08 keV to 1.2 keV for electton densities of 2.0×1012 cm-3 to 4.4×1013 cm-3. The temperature profile was found to have a rather sharp distribution in the central region. The electron temperature was found to vary linearly with the plasma current and the ionic charge derived from the electron temperature and plasma conductivity decreased with the electron density. The experimental electron thermal conductivity KeEXP in the central region was compared with the theoretical prediction KeNC≪KeEXP\\lesssimKeDTE, where KeNC and KeDTE are the neoclassical and dissipative-trapped electron thermal conductivities respectively.

Kinematics of Ohmic Heated Plasmas in the B-1 Stellarator

A kinematic description of the B-1 stellarator plasma during various stages of Ohmic heating discharges is presented. It is shown that for specific ranges of plasma current, for a given confining field, the plasma assumes a shape which has an azimuthal distribution of the form exp im θ. It is further shown that the plasma column rotates at high velocity while the distortion is present. The possible relationship between these current ranges and the predictions of hydromagnetic instability theory is considered.