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  1. Behaviors of Impurity and Hydrogen Recycling on the HT-7 TokamakJ. Huang*, B.N. Wan, X.Z. Gong, Z.W. Wu and the HT-7 TeamInstitute of Plasma Physics, Chinese Academy of Sciences P.O.Box 1126, Hefei, Anhui 230031, P.R.China* E-mail: janejoy@ipp.ac.cnMay 22, 2006 17th PSI, Hefei, Anhui, China

  2. Outline • Introduction • Experimental Setup • Data Analytical Methods • Results and Discussion • Summary

  3. Introduction • The achievement of steady state plasma operation is one of the requirements for future fusion reactors. The problems involved are non-inductive current drive, plasma control, heat exhaust, particle control, etc. • HT-7 is a medium sized superconducting tokamak with limiter configuration. Its main purpose is to explore high-performance plasma operation under steady-state conditions. Recycling and impurity control are key issues in obtaining reproducible non-inductive current drive discharges. • ICRF wall conditioning techniques for impurity reduction and recycling control have been routinely used in HT-7 tokamak with the aim of supporting future large superconducting devices such as EAST, ITER • During long pulse discharges in HT-7, impurity and hydrogen recycling show a complicated behaviors and time evolution, investigated by passive spectroscopy measurement and particle balance equation.

  4. (a) (b) (c) (d) Experimental Setup • Two interference filter scopes with • photodiode arrays • -- (the total emission of Ha/Da) • Visible spectrography • -- (the emission of CIII) • Two monochromators • -- (the emission of OII/OV) • (d)Two Opticals Multi-channel Analysis (OMA) system • SP-300i: (focal length: 300cm; 1200 gr/mm: 68×84mm, 0.1nm) • LN/CCD: (1340×400 pixels, 20×20μm) -- impurity spectral lines • SP-750:(focal length: 750cm; 1200gr/mm: 68×84mm, 0.023nm) • ICCD: (EEV 384×578 pixels, 23×23μm) -- Ha/Da line shapes

  5. Data Analytic Methods -- Particle Balance Model Ne : the total number of electrons  : fueling efficiency Sg : gas feed rate by the piezo-electric valve p : a particle confinement time effective particle confinement time • It can be estimated from the decay time of the plasma density after the external fueling is stopped.

  6. Data Analytic Methods -- Isotopic Ratio H/(H+D) • H/(H+D) can show the properties of wall recycling because hydrogen comes from the wall and limiter surfaces with working gas deuterium • The ratio can be achieved from the area of the D(H) line shape according to the spectroscopic measurement. • The optical signal is proportional to the local flux of D(H) entering the plasma from the wall. • The D and H neutral particle influxes derived from the D(H) emission are normally underestimated in the presence of hydrogen or deuterium molecules involved in the recycling, which depends on the surface temperature and surface material • The uncertainty of the ratio due to molecular processes can be partially compensated in the H/(H+D) ratio because H2, D2 and HD molecules fuel the plasma in a similar way. • the asymmetric D(H) line shape is studied and simulated by Degas2 code shown in [P3-89 B. Xiao “Balmer-α Spectroscopic Study on Hydrogen Recycling and Molecular Effects in HT-7” ]

  7. Impurity Behaviors in Long Pulse Discharges The relation between the product of Ip*ne and Tne • An uncontrolled density increase is often observed in long pulse discharges in HT-7, due to impurities radiation and global recycling from the internal elements. • Impurity and hydrogen emission increase with power of LHCD • The high impurity concentration can reduce the LHCD efficiency and limit the duration Impurity and hydrogen behaviors as a function of LHCD power

  8. Impurity Reduction by RF Boronization Comparison of Two Typical OH Discharges Evolution of Impurity Emission after Boronization • ICRF wall boronization using carborane (C2B10H12) was applied to reduce impurity influx • Significant decrease impurity radiation was observed • The plasma performance improvements are characterized by reduced impurity levels Comparison of Impurity Spectral Lines

  9. Evolution of H/H+D after Boronization (a) phase for uncontrollable density (b) transition phase (c) phase for controllable density • The hydrogen recycling exhibits different features, divided into three phases according to H/(H+D): • (a) period for uncontrollable density • The H/(H+D) ratio is high, up to 90%, due to a large amount of hydrogen absorbed in the film • high hydrogen recycling leading to an uncontrollable density rise • RF conditioning after boronization was to remove the huge hydrogen content in the film • (b) transition period • (c) period for controllable density • More than 150 shots later the ratio reaches a steady-state value of about 10-25%.

  10. Longest Pulse Discharge shot 83026 Ip~50KA, ne~0.75, Bt~1.8T, PLHCD~100KW, f=2.45GHZ, Td~306s

  11. Properties of Wall Recycling -- Effective Particle Confinement Time Ip~50KA, ne~0.5, Bt~1.8T, PLHCD~120KW, f=2.45GHZ, Td~60s • Time evolution of the effective particle confinement time in a 60S LHCD discharge • The effective particle confinement time increased with the time from 1 to 2.25s • Wall inventory gradually increased due to accumulation of particle flux out of plasma, and corresponding to the increase in edge recycling

  12. Summary • An uncontrolled density increase is often observed in long pulse discharges in HT-7, correlated with hydrogen and impurity influx from the internal elements. The high impurity concentration can reduce the LHCD efficiency and limit the plasma duration. • To reduce impurity emission RF-boronization has been routinely used in the HT-7 tokamak. It can improve plasma performance and enhance the current drive efficiency. • After ICRF boronization wall recycling exhibits different features during long pulse discharges. When the H/(H+D) ratio was reduced to less than 25%, the electron density was easily controlled. The longest discharge with Te(0) ~ 1.0 keV and central electron density 0.75x1019/m3 was up to 306 s. • Wall properties was investigated by effective particle confinement time. It shows that wall inventory gradually increased due to accumulation of particle flux out of plasma, and corresponding to the increase of the recycling coefficent R

  13. Thank you for your attention

  14. Discharge of Transition to Controllable Density • The H/(H+D) ratio is 65%, and the level of impurities carbon and oxygen is low • After 2.5s, the density has a rapid increase because of increasing hydrogen from the film and R is up to and above 1 indicating the density is out of control • The LHCD efficiency was not high enough to sustain the plasma at a certain plasma current and electron density for a long duration. Larger LHCD power caused more hydrogen flux from the layer • In this phase the high hydrogen recycling terminates the discharge

  15. Discharge of Controllable Density Ip~50KA, ne~0.5, Bt~1.8T, PLHCD~120KW, f = 2.45GHZ • The ratio H/(H+D) was reduced to 15%, because hydrogen was continually removed from the fresh film by discharges and partly replaced by deuterium. • R~1, the electron density was easy to control • the main characteristics of this period are that: 1) both the level of impurities and the edge recycling are very low 2) the target plasma has good performance 3) the LHCD efficiency is high

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