Hefei Tokamak-7 Research Articles

Theoretical calculation of electron density and temperature in the edge of tokamak

In this work, we use a method based on the concept of particle confinement time [Formula: see text] uniqueness to calculate the electron density and temperature in ohmically heated, edge plasma of the Hefei tokamak-7. Here, with the help of the data taken from Johnson and Hinnov’s table, we have done an extensive work to find electron densities and temperatures that satisfy the [Formula: see text] uniqueness to evaluate the temporal evolution of electron density [Formula: see text] and temperature [Formula: see text]. The results are in good agreement as measured from the Langmuir probe array in previous works.

Effects of internal inductance on the Shafranov parameter by using the solution of equilibrium problem

In this work, the dependence of Shafranov parameter on plasma internal inductance has been studied by using the solution of Grad–Shafranov equation (GSE) for Hefei Tokamak-7. The Shafranov parameter was obtained from the solution of GSE, using the expansion of free functions, which is quadratic in flux function. Then, we can find the dependence of Shafranov parameter on plasma internal inductance.

Development of high-speed and wide-angle visible observation diagnostics on Experimental Advanced Superconducting Tokamak using catadioptric optics

A new wide-angle endoscope for visible light observation on the Experimental Advanced Superconducting Tokamak (EAST) has been recently developed. The head section of the optical system is based on a mirror reflection design that is similar to the International Thermonuclear Experimental Reactor-like wide-angle observation diagnostic on the Joint European Torus. However, the optical system design has been simplified and improved. As a result, the global transmittance of the system is as high as 79.6% in the wavelength range from 380 to 780 nm, and the spatial resolution is <5 mm for the full depth of field (4000 mm). The optical system also has a large relative aperture (1:2.4) and can be applied in high-speed camera diagnostics. As an important diagnostic tool, the optical system has been installed on the HT-7 (Hefei Tokamak-7) for its final experimental campaign, and the experiments confirmed that it can be applied to the investigation of transient processes in plasma, such as ELMy eruptions in H-mode, on EAST.

Data processing and analysis of the imaging Thomson scattering diagnostic system on HT-7 tokamak

A high spatial resolution imaging Thomson scattering diagnostic system was developed in ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences). After about one month trial running on the superconducting HT-7 (Hefei Tokamak-7) tokamak, the system was proved to be capable of measuring plasma electron temperature. The system setup and data calibration are described in this paper and then the instrument function is studied in detail, as well as the measurement capability, an electron temperature of 50 eV to 2 keV and density beyond 1 × 10(19) m(-3). Finally, the data processing method and experimental results are presented.

Investigation of anomalous Doppler instability in ohmic and LHCD discharges in the HT-7 tokamak

We present the results of our experimental studies on a discharge in the Hefei Tokamak-7 (HT-7) device with a specific instability whose emergence coincides with a sudden jump in electron cyclotron emission (ECE) that shows the transverse energy of the plasma. It is shown that, in this slide-away discharge regime, the current is carried by a relatively small group of runaway electrons and that the period of high-frequency small-amplitude oscillations varies from 1 to 2 ms, which depends on the value of the loop voltage in ohmic discharges. In low hybrid current drive (LHCD) plasmas, the period of high-frequency small-amplitude oscillations in the ECE signal during the whole period of anomalous Doppler instability (ADI) is about 2 ms. It is also found that the ADI in ohmic and LHCD discharges can restrain the magnetic oscillations.

Electron temperature fluctuation in the HT-7 tokamak plasma observed by electron cyclotron emission imaging

The fluctuation of the electron temperature has been measured by using the electron cyclotron emission imaging in the Hefei Tokamak-7 (HT-7) plasma. The electron temperature fluctuation with a broadband spectrum shows that it propagates in the electron diamagnetic drift direction, and the mean poloidal wave-number k̄θ is calculated to be about 1.58 cm−1, or k̄θρs ≈ 0.34. It indicates that the fluctuation should come from the electron drift wave turbulence. The linear global scaling of the electron temperature fluctuation with the gradient of electron temperature is consistent with the mixing length scale qualitatively. Evolution of spectrum of the fluctuation during the sawtooth oscillation phases is investigated, and the fluctuation is found to increase with the gradient of electron temperature increasing during most phases of the sawtooth oscillation. The results indicate that the electron temperature gradient is probably the driver of the fluctuation enhancement. The steady heat flux driven by electron temperature fluctuation is estimated and compared with the results from power balance estimation.

Observation of instabilities during density limit experiments in the Hefei Tokamak-7

During a recent campaign at the Hefei Tokamak-7 (HT-7) [M. Asif et al., Phys. Plasmas 12, 082502 (2005)], experiments were performed with a moveable graphite limiter to investigate the influence of the horizontal plasma position on the density limit. It has been found that in HT-7 density limits differ by the radiative instability in the plasma boundary and various magnetohydrodynamic (MHD) instabilities, which precedes the disruption. Control of the impurity content, edge, and recycling properties prevents the growth of the multifaceted asymmetric radiation from the edge (MARFE) and makes it possible to exceed the Greenwald limit in HT-7. It has been observed that the m=2, MHD component dominates during almost the whole discharge duration, when the density limit is approached at values of the edge safety factor 3&amp;lt;q(a)&amp;lt;4. The observed sawtooth oscillation shows that oscillation with m=1 and n=1, where m and n are the poloidal and toroidal mode numbers, respectively, triggers m=2 and n=1 in sawtooth collapse, which finally causes disruption in the HT-7 tokamak.

The phenomenon of multifaceted asymmetric radiation from the edge with new graphite limiters in Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters have been developed in Hefei Tokamak-7 (HT-7) for long-pulse operation. In this paper, the phenomenon of multifaceted asymmetric radiation from the edge (MARFE) has been studied in the ohmic discharges in HT-7. The best correlation has been found between the total input ohmic power and the product of the edge line-averaged density and Z eff. The critical density of the MARFE onset is observed in the region of , where f GW=n¯e/n GW (here n¯e is the maximum line-averaged electron density and n GW is the Greenwald density).

Study of the operational region with new graphite limiters in the Hefei tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei tokamak-7 for long pulse operation. The extension of the operational region and the density behaviour with graphite limiters have been studied in this paper. An extended high-density region at high plasma currents and low q a was obtained. The density profile with graphite limiters was studied in order to compare the results with those of the previous molybdenum limiters.

Improved confinement mode induced by a MARFE during lower hybrid current drive in the HT-7 tokamak

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the Hefei tokamak-7 (HT-7) superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena and an improved confinement mode induced by a MARFE are observed in LHCD plasmas when PLH>160 kW and the edge safety factor q(a) is slightly less than 6.5. It is found that an improved confinement mode induced by a MARFE, characterized by Hα line emissions drops and the line-averaged density increase is triggered in the MARFE discharges. The MARFE event occurs at t=1120 ms following the L–H transition, and the improved confinement phase exists for about 65 ms from t=1140 ms after the L–H transition. The LHCD plasma results basically are typical with MARFEs and it is an improved confinement mode; they are unique in that they appear at very low values in the HT -7 tokamak.

Topology of toroidal helical fields in non-circular cross-sectional tokamaks

The ordinary differential magnetic field line equations are solved numerically; the tokamak magnetic structure is studied on Hefei Tokamak-7 Upgrade (HT-7U) when the equilibrium field with a monotonic q-profile is perturbed by a helical magnetic field. We find that a single mode (m, n) helical perturbation can cause the formation of islands on rational surfaces with q = m/n and q = (m +/- 1, +/- 2, +/- 3,...)/n due to the toroidicity and plasma shape (i.e. elongation and triangularity), while there are many undestroyed magnetic surfaces called Kolmogorov-Arnold-Moser (KAM) barriers on irrational surfaces. The islands on the same rational surface do not have the same size. When the ratio between the perturbing magnetic field B-r(r) and the toroidal magnetic field amplitude B(phi)0 is large enough, the magnetic island chains on different rational surfaces will overlap and chaotic orbits appear in the overlapping area, and the magnetic field becomes stochastic. It is remarkable that the stochastic layer appears first in the plasma edge region.

Plasma density behavior with new graphite limiters in the Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff1∕2fGW=0.9∼1.2, where fGW=n¯e∕nGW. (Here n¯e is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter.

Recent progress on steady-state high-performance plasma research in the Hefei Tokamak-7

High performance discharges under steady-state condition in the Hefei Tokamak-7 (HT-7) have been investigated. Lower hybrid current drive (LHCD) was used both for sustaining plasma current and current density profile control. The experiments demonstrated that features of ion Bernstein wave heating in controlling the electron pressure profile can be integrated into LHCD plasmas and the assist localization of LHCD. This local synergy effect was used to tailor the current density profile in a negative shear configuration and to control the internal transport barrier (ITB) under steady-state conditions. The high performance with a stationary ITB at the footprint of the minimum q and βNH89&amp;gt;2 was sustained for &amp;gt;220 τE or &amp;gt;20 τCR. The fraction of noninductive plasma current was larger than 80% in such discharges with considerable bootstrap current. The duration at H89&amp;gt;1.2 and βN∼1 has been extended to nearly 8 s, longer than 400 τE. More than 90% of the plasma current was sustained by LHCD and bootstrap current. High-plasma performance and sustaining time was limited by the magnetohydrodynamic instabilities and the recycling, which caused an uncontrollable rise of the electron density.

Edge turbulent transport with lower hybrid current drive in the Hefei Tokamak-7

The edge plasma transport around the last closed flux surface was investigated using Langmuir probes in a lower hybrid current drive (LHCD) experiment in the Hefei Tokamak-7 [J. K. Xie et al., in Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. Fluctuations of plasma density, plasma temperature, and poloidal electric field in the boundary plasma are reduced, hence the electrostastic turbulence-induced particle flux and heat flux both significantly decrease after the application of LHCD. The suppression of density fluctuation is more sensitive to the lower hybrid wave (LHW) than that of temperature fluctuation, which is in agreement with the model proposed by Ware in the presence of flow shear [A. S. Ware et al., Phys. Plasmas 5, 173 (1998)]. The difference between turbulent convective and conductive fluxes depends on such a discrepancy between normalized density fluctuation and temperature fluctuation. It is speculated that the reduced turbulence-induced transport is possibly attributable to a sheared flow resulting from the varying radial electric field, which is formed possibly due to the loss of the energetic electron generated by LHW. In addition, studies show that the effect of magnetohydrodynamic activity, possibly modified by LHCD, on plasma transport could be another possible candidate for the reduced electrostatic turbulent flux.

Control of profiles and transport by ion Bernstein waves in the Hefei Tokamak-7

Electron pressure profile and transport behaviors in ion Bernstein waves (IBW) heated plasmas were investigated at three frequencies (24, 27, and 30 MHz) in the Hefei Tokamak-7. Both on-axis and off-axis electron heating with global peaked and locally steepened electron pressure profiles were realized, if the ion cyclotron resonant layer was properly selected. Improvement of the IBW-heated plasma confinement was observed except in the 24 MHz case. Reduction of the local electron heat transport was evident from sawtooth heat pulse propagation. High performance plasma volume was extended in the IBW off-axis heating mode, which resulted in improvement of the global plasma confinement. These studies demonstrate that IBW is a potentially powerful tool for active control of plasma profiles and transport.

Improved confinement through internal transport barrier formation with lower hybrid current drive in the Hefei Tokamak-7

An improved confinement through internal transport barrier formation with lower hybrid current drive (LHCD) is obtained on the Hefei Tokamak-7 [J. K. Xie et al., Proceedings of the 16th International Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 685]. The experimental energy confinement time increases from 14.6 ms (Ohmic discharge phase) to 24.5 ms (LHCD phase), which is consistent with the value predicted by the high confinement mode scaling law of ITER93 (edge localized mode free). The confinement factor H (H=τE/τEITER89P) up to 1.42 is obtained during the improved confinement phase. The analysis shows that the modification of the plasma current profile and E×B are two possible reasons for the confinement improvement.

In search of zonal flows using cross-bispectrum analysis in the boundary plasma of the Hefei Tokamak-7

Langmuir probes have been used to measure the electrostatic Reynolds stress and the floating potential fluctuation in the boundary plasma of the Hefei Tokamak-7 (HT-7) [J. Li, B. N. Wan, and J. S. Mao, Plasma Phys. Controlled Fusion 42, 135 (2000)]. The cross bispectrum of 〈ṼrṼθφ̃f〉 indicates the existence of difference-frequency nonlinear phase coupling and the generation of fluctuations near the geodesic acoustic mode frequency. The inverse cascade process might be linked to the generation of zonal flows by small-scale electrostatic drift-wave turbulence.

Plasma density behavior in the Hefei tokamak-7

The density profiles were measured in the Hefei tokamak-7 (HT-7) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1997), p. 61] ohmic discharges by means of a new multichannel far-infrared (FIR) laser interferometer. The progress on the extension of the HT-7 ohmic discharge operation region was introduced. The experiment results at the density limit, the multifaceted asymmetric radiation from the edge (MARFE) phenomena, the rf (radio frequency) boronization experiments, and the fueling efficiency studies were reported. The plasma physics in the molecular beam injection (MBI), the pellet injection (PI), and the gas puffing (GP) fueling experiments was studied and discussed.