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In this article, the design idea of a 57-pin Langmuir probe array is presented, together with some experimental results. The manufacture technique of the probe array is discussed. Two-dimensional distributions of the plasma density, the floating potential and their fluctuations in the poloidal cross section are obtained. The blobs, which move radially and poloidally, cause great particle and energy transport. The charge neutralization by the poloidal movement of particles greatly decreases the vertical electric field in the helimak plasma. This leads to a confinement time which is two orders of magnitude larger than the theoretical prediction.

A program of electron cyclotron heating (ECH) with 4 MW at 140 GHz has been launched for developing scenarios of stable high performance through control of the pressure and current density profiles...

An upgraded electron cyclotron emission imaging (ECEI) system consisting of new optics lenses with necessary electronics for receiving and processing signals for two dimension (2D) ECEI diagnostics was installed on EAST. Hyperboloid lens were adopted in the new system to optimize the spatial resolutions. The mixers array of sixteen elements measured the plasma electron cyclotron emission at eight frequencies simultaneously, and the profiles of the electron temperature and its fluctuation in an area of 20 cm (vertical) × 6 cm (horizontal) could then be analyzed. Evolution of sawtooth precursor and crash in EAST was observed.

The statistical analyses of fluctuation data measured by electrostatic-probe arrays clearly show that the self-organized criticality (SOC) avalanches are not the dominant behaviors in a toroidal ECR plasma in the SMT (Simple Magnetic Torus) mode of KT-5D device. The f −1 index region in the auto-correlation spectra of the floating potential V f and the ion saturation current I s , which is a fingerprint of a SOC system, ranges only in a narrow frequency band. By investigating the Hurst exponents at increasingly coarse grained time series, we find that at a time scale of τ > 100 μs , there exists no or a very weak long-range correlation over two decades in τ. The difference between the PDFs of I s and V f clearly shows a more global nature of the latter. The transport flux induced by the turbulence suggests that the natural intermittency of turbulent transport maybe independent of the avalanche induced by near criticality. The drift instability is dominant in a SMT plasma generated by means of ECR discharges.

A newly developed moveable magnetic probe array was installed on KT-5C tokamak. The profiles of radial and poloidal magnetic fluctuations of the plasma have been measured for ( 0.5 r / a 1.1 ). The experimental results indicate that there is a radial gradient which is greater than relative electrostatic fluctuations and the magnetic fluctuations contribute a little to losses. A strong coherence between fluctuations of 4 mm nearby two points suggests that the magnetic fluctuations have quite a long correlation length.

Electrode biasing experiments are carried out on the KT-5C tokamak to investigate the effects of the radial electric field E/sub r/ on turbulence. A modestly enhanced E/sub r/ gradient layer is formed at the plasma edge by the electrode biasing. In the E/sub r/ gradient layer, reductions in the fluctuation amplitude, radial correlation length and turbulent particle flux are observed. These results support the theoretical models on the turbulence suppression by decorrelation due to E /spl times/ B flow shear.

Tokamak experimental research in China has made important progress. The main efforts were related to quasi-steady-state operation, LHCD, plasma heating with ICRF, IBW, NBI and ECRH, fuelling with pellets and supersonic molecular beams, and first wall conditioning techniques. Plasma parameters in the experiments were much improved, for example ne = 8 × 1019m-3 and a plasma pulse length of >10 s were achieved. ICRF boronization and conditioning resulted in Zeff close to unity. Steady state full LH wave current drive has been achieved for more than 3 s. LHCD ramp-up and recharge have also been demonstrated. The best ηCDexp ≈ 0.5(1+0.085exp(4.8(BT-1.45)))neICDRp/PLH = 1019 m-2 A W-1. Quasi-steady-state H-mode-like plasmas with a density close to the Greenwald limit were obtained by LHCD, where the energy confinement time was nearly five times longer than in the ohmic case. The synergy between IBW, pellet and LHCD was tested. Research on the mechanism of macroturbulence has been extensively carried out experimentally. AC tokamak operation has been successfully demonstrated.

The radial wavenumber spectra of electrostatic fluctuations measured by Langmuir probe arrays on the KT-5C tokamak show a quasimode-like structure which results in a net radial outward propagation of the turbulent fluctuations. The power-averaged radial wavenumber is approximately constant in the core region and increases near the limiter. The radial wavenumber spectral width stays nearly constant from core to edge and is about two times greater than the poloidal wavenumber spectral width. The measured fluctuation levels and wave action fluxes are in good agreement with model predictions (Mattor N and Diamond P H 1994 Phys. Plasmas 1 4002, Mattor N 1995 Phys. Plasmas 2 766) which suggest that drift wave propagation could be a source of edge turbulence.

A sharp variation at some radial positions superimposed on a slow change in the profiles of the fluctuation levels, fluctuation-driven particle and energy fluxes, which is referred as spatial intermittency, is observed in the core plasma of the Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190.]. The peaks in the profiles are located in the vicinity of low-q rational surfaces, and fluctuation spectra perpendicular to the magnetic field become more anisotropy there. The intermittency may be related to the radial variations in the nonlinear mode couplings near the low-q resonant surfaces.

A new fluctuation phenomenon is observed through Langmuir probe measurements at the edge plasma in the KT-5C tokamak by applying a -90 degrees phase shift feedback. Using a two point correlation technique, it is found that this fluctuation mode has a longer poloidal wavelength and a definite frequency when compared with the usual edge turbulence. It is also found through bispectral analysis that this mode is a spontaneously excited quasi-coherent mode, which has almost no contribution to the cross-field particle flux

A CO2 laser phase contrast imaging system has been developed for tokamak KT-5C. The system properties have been explored using a step phase modulator. It has been found that by setting the focus of an undiffracted beam outside the groove of the phase plate, a single-point detection system can still be used to measure fluctuations with resolution of wavenumber.

It is found on KT-5C tokamak that an edge fluctuation mode can be excited by using a pair of floating Langmuir probes. This mode is propagating along the magnetic field with its amplitude decreasing drastically when it is off the magnetic field line. Preliminary feedback experiments show that the broadband edge turbulence can be reduced or enhanced depending upon the phase shift of the feedback network.

Experiments using a biased multiblock limiter in the KT-5C tokamak show that positive biasing is more effective than negative biasing in modifying the edge electric field, suppressing fluctuations and improving plasma confinement. The biasing effect varies with the limiter area, the toroidal magnetic field and the biasing voltage. By positive biasing, the edge profiles of the plasma potential, the electron temperature and the density become steeper, resulting in a reduced edge particle flux, an increased global particle confinement time and lower fluctuation levels of the edge plasma.

In this work, numerous antiferromagnetic (AFM) and weak ferromagnetic (WFM) iron oxides were fabricated and used as contrast agents in magnetic resonance imaging application. These non-superparamagnetic iron oxides exhibit r2/r1 ratios in a range of 20–110, and hence making them potential for T2 contrast enhancers. Most strikingly, a Langevin-function relationship was found between r2/r1 ratios and the specific surface area (S) values, r2/r1 ∼ L−2(S−1), which is quite unique for AFM and WFM iron oxides. This finding enables us to precisely tailor their r2/r1 ratios simply by manipulating their specific surface area via controlling morphology and structure in preparation.

Features of a low frequency ion mode were observed together with the electron drift wave type fluctuations by a Langmuir probe array in the bulk plasma inside the velocity shear layer in the KT-5C tokamak. The measured ion mode wavenumber, scaling of the ion mode with chord-averaged density, and the estimated ηi value suggest that the observed ion mode may be the theoretically predicted ηi mode driven by ion temperature gradient.

Core and edge turbulences were measured by Langmuir probe arrays in the KT-5C tokamak plasma. The radial wavenumber spectra show a quasimode like structure which results in a net radial outward propagation of the turbulent fluctuations. The measured fluctuation levels and wave action fluxes are in good agreement with model predictions by Mattor et al., suggesting that drift wave propagation could be a source of edge turbulence.

Investigations of a possible radiation drive of edge turbulence in tokamaks require sensitive radiated power measurements having high temporal and spatial resolution. A commercially available 16‐element X–UV‐enhanced photodiode array having essentially 100% quantum efficiency from ∼25 to ≥6 keV is used. The frequency response is presently limited by the amplifiers to ∼50 kHz, although the ultimate response is expected to be much higher (≳200 kHz). Each element’s view is ∼0.4 cm wide along the major radius at the plasma midplane while the view of the entire array is ∼6.3 cm wide and is scannable over the entire plasma cross section. The details of the system as well as observations of fine spatial structures and fluctuations undetectable by conventional bolometers are presented.

The electrode biasing experiments were carried out on the KT‐5C tokamak to investigate the effects of the radial electric field Er on turbulence in a toroidal plasma. It is observed the radial electric field is mainly contributed by the poloidal flow both in Ohmic and in electrode biasing discharges. The changes of Er is led by the changes of poloidal flow, and it is the Reynolds stress driving the poloidal flow in Ohmic discharges in the device, but the Reynolds stress is not so important in the electrode biasing discharges. A modestly enhanced E×B shear layer is formed at the plasma edge by the electrode biasing. In the sheared E×B layer, reductions in the fluctuation amplitude, and the radial correlation length as well as the turbulent particle flux, are observed, Indicating that the turbulence suppression by de‐correlation is due to the modestly enhanced E×B shear layer.

Sawtooth oscillations were investigated using an electron cyclotron emission imaging diagnostic technique on the HT-7 tokamak. High-order harmonic modes are observed in sawtooth precursors. They cause sharp pressure points, leading to the occurrence of reconnection events at more than one place, and which are not preferential on the low field side of the q ~ 1 radius at low density. The reconnection proceeds in two stages. In the first stage, a weak reconnection happens which is characterized by a slow emergence of a small amount of heat and particles through the finite openings. Subsequently, in the second stage, most of the remaining heat and particles in the core escape outward. The observations at low density indicate that high-order harmonic modes are responsible for the secondary reconnection stage, which has seldom been mentioned in reconnection model. The characteristics of the modes are then described by investigating a large number of sawteeth oscillations with different densities and qa values. It is found that when the density increases above a threshold at qa = 3.3, the m/n = 1/1 mode amplitude significantly exceeds its harmonics. The threshold decreases as qa increases.

Compatible operation of steady state mode and pulse mode is realized in the KT-5D device. New power supplies with the operation control systems for the steady state toroidal magnetic field as well as for the vertical field are added, and the rf wave injection systems for sustaining steady state plasmas are upgraded. After the modification, the device now can work not only as a tokomak with pulsed plasma currents as it was but also as a simple magnetized torus with steady state plasma discharges. It allows more flexible and efficient experimental researches on the magnetically confined plasmas to be carried on in the same device.

A direct imaging system, viewing visible light emission from plasmas tangentially or perpendicularly, has been set up on the KT5D toroidal device to monitor the real two-dimensional profiles of purely ECR generated plasmas. This system has a typical spatial resolution of 0.2mm (1280×1024pixels) when imaging the whole cross section. Interesting features of ECR plasmas have been found. Different from what classical theories have expected, a resonance layer with two or three bright spots, rather than an even vertical band, has been observed. In addition, images also indicate an intermittent splitting and drifting character of the plasmas.