The Influence of Neutral Particles on Edge Turbulence and Confinement in the HT-7 Tokamak

1. APFA 2003 The Influence of Neutral Particles on Edge Turbulence and Confinement in the HT-7 Tokamak Mei Song, B. N. Wan, G. S. Xu, B. L. Ling, C. F. Li Institute of Plasma Physics, Chinese Academy of Sciences, P.O.Box 1126, Hefei, China 230031 E-mail: sm@ipp.ac.cn 4th General Scientific Assembly of Asia Plasma & Fusion Association On New Development of Plasma Physics and Fusion Technology, Hangzhou China, Oct 13-16, 2003

2. Motivation and Abstract • Motivation: The dominant free energy source responsible for the turbulence in the plasma boundary region has not yet been fully identified. It is recognized that more than one single mechanism would be needed to explain all the experimental observations of turbulence in the plasma boundary region. Particular attention has been paid to the understanding of the impact of atomic physics processes on turbulence in recent years. Atomic processes such as impurity radiation and ionization can be drivers of instabilities that might cause anomalous transport in the plasma edge region. Profit from the effective RF wall conditioning developed on the HT-7 tokamak, we investigated the role of atomic processes in modifying the edge turbulence and flows using a reciprocating Langmuir probe. • Abstract:Accompanying the effective reduction of impurity radiation, strong shears of Er and Vph came into being and the turbulence suppression and de-correlation were observed in the edge region of coated wall plasma. All these led to the reduction of the edge transport and the improvement of plasma confinement. In density scanning experiments, an enhanced shear of the radial electric field was observed in the edge plasma with the increase of the density, which may account for the enhancement of the transport barrier and the improvement of particle confinement. The results suggest a close link between wall conditions and boundary plasma. In addition the features of the ionization and radiation driven turbulence, and , had been observed in the plasma edge region, which indicates the important role of the ionization and radiation in turbulence driving on the HT -7 tokamak.

3. A Successful RF Lithium/Silicon Coating Ip ~ 140 kA, ~ 2.0 × 1019 m-3, Bφ ~ 2 T, ohmically heated After RF coating, the Ha decreased, the recycling and the density became easily controlled, and a wider stable operation region was obtained. The Zeff was decreased by a factor of 2 ~ 3 after the coating, which implies that not only the influx of intrinsic impurity was suppressed but also the coated elements had no significant penetration into the main region.

4. Measured Profiles Before and After RF Coating Electron temperature Electron density Before RF coating After RF coating After RF coating, the profile of electron density was broadened and the temperature was increased.

5. Steepened Boundary Profiles and Enhanced Er Shear The four tips of the array were made of tungsten wire, 0.5 mm in diameter and 2 mm in length, and arranged carefully at the corners of a 2 mm × 2 mm square. Two toroidally separated tips were configured as a floating double probe system. The other two poloidally separated tips measured the floating potentials. The detailed arrangement and discussion of uncertainty can be found in the Ref. [PPCF45, 2003, P1805]. Main diagnostics: Langmuir probe The pneumatically driven probe can radially scan over 8 cm in 80 ms. The data were digitized at 1 MHz with 12 bit resolution using a multi-channel digitizer. After the RF coating, the gradients of electron density and temperature in the edge region were increased, which is consistent with the broadening of the profiles given above. There was only small gradient on the profile of Vp in the bare metal wall case. In the RF coated wall case, however, the two potential profiles had large gradients near the radial location of the limiter and dropped more rapidly inside it, which indicates that there was an enhanced shear of the radial electric field Er in the edge region.

6. Suppression of Fluctuations and Modification of Turbulence The edge fluctuations were suppressed after the RF Lithium/Silicon coating. The non-Boltzmann relation: exists in the HT-7 edge plasma whatever before or after RF coating. In the coated wall case, we found a clear change in the propagation direction of the fluctuations from the ion diamagnetic direction in the SOL to the electron diamagnetic direction in the edge region. The cross point is radially close to the peak of the plasma potential profile, which implies that the Vph is dominated by the E×B rotation and the velocity shear is mainly due to the shear in VE×B.

7. Depression of Fluctuation-Induced Particle Fluxes Due to the suppression of the fluctuations (as shown in the above paper) and the decrease of correlations between fluctuations in the shear region, the turbulent particle flux was reduced in the edge plasma after the RF coating Considering the dominant role of the turbulent particle flux in the total particle loss for the boundary plasma, the suppression of turbulent particle flux can at least partly account for the improvement of the particle confinement.

8. Scanning Experiments of the Central Line Averaged Density Before RF coating After RF coating Ip ~ 140 kA, Bφ ~ 2 T, Ohmically heated With the rise of the central line averaged density, the ne in the boundary plasma increased with a steeper profile, while the Te decreased. The enhanced radiation with the gas puffing is expected responsible for the decrease of the Te. The pressure gradient became higher with the rise of the density, which indicates the better confinement in the higher density.

9. Improved Particle Confinement With the Rise of ne and the Dominant Role of Turbulent Particle Flux in the Total Particle Loss The particle flux deduced from fluctuations near the limiter location shows a much slighter increase than that of the central line averaged density, resulting in the calculated particle confinement time increases with the density. The particle confinement time estimated from emission and that for probe signals have a similar relationship with density, which shows the evident correlation of the turbulent particle flux and the global particle loss in the HT-7 tokamak. The good agreement between them indicates that fluctuation driven transport plays an important role in particle confinement in the HT-7.

10. Phase Angles Between Fluctuations of Te and ne, and That of Te and Is Edge SOL In the edge region, the phase angle between electron temperature and density fluctuations was close to pi in the whole frequency range, while that of electron temperature and ion saturation current clearly deviated from pi. In the SOL, both of them clearly deviated from pi. The clear deviation of them in both the edge region and the SOL means a negligible contribution from the temperature fluctuation in phase of the density fluctuation.

11. Two Main Features of the Atomic Physics Driven Instabilities in the Edge Plasma With the rise of the central line averaged density the averaged phase angles at r=25.5 cm approached to pi and the relative fluctuation level of electron temperature got to similar with that of density . Two main features of the atomic physics driven instabilities Though the measurements of the fluctuations of temperature, density and electron field could cause some uncertainties, the results presented here indicate at least the most possible important role of atomic physics in the edge turbulence driving on the HT-7 tokamak.

12. Summary and Acknowledgement Summary:The boundary plasma behaviors under different wall conditions on the HT-7 tokamak were investigated using a reciprocating Langmuir probe system. After RF lithium/silicon coating, impurity influxes and edge recycling were significantly reduced. Strong Er and Vph shear accompanying with the turbulence suppression and de-correlation were found, which led to the reduction of the edge transport and improvement of plasma confinement. With the increase of the density an enhanced shear of Er was observed in the edge plasma. It could at least partly account for the reduction of the transport and the improvement of the particle confinement. The close link between wall conditions and boundary plasma suggests the importance of the interplay between atomic processes and the formation of the radial electric field shear in the boundary plasma. The impurity radiation and neutral particle are possible to influence the plasma turbulence characteristics and therefore confinement. The observations of and at higher densities in the plasma edge region consistent with the main predictions of the ionization and radiation driven theory. These observations also indicate the possible importance of the ionization and radiation of neutral particles in turbulence driving, although the measurements of the temperature and electron field fluctuations could cause some uncertainties. Acknowledgement:This work was supported by the National Natural Science Foundation of China under Grant No. 10175069.