HT-7

1. ASIPP HT-7 The measurement of the light impurity radiation profiles for the impurity particle transport in the HT-7 tokamak Qian Zhou, B.N.Wan, Z.W.Wu, Juan Huang Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, P.R.China

2. Abstract HT-7 • The measurement of impurity(C, O, etc.) radiation profiles for the study of light impurity transport was performed in the HT-7 tokamak. • The volume emission rate of impurities can be obtained by Abel inversion from spectroscopy measurement. • An impurity transport code [2], on the assumption of a cylindrical symmetry, can obtain the diffusion coefficient Dk(r) and the convection velocity Wk(r) through simulating the experiment results of the volume emission rate of line emission from ionized light impurities and the effective ionic charge of the plasma Zeff(r). • In this way, the radial transport behaviors of the carbon impurities in different central line averaged densities of OH discharges were investigated in the HT-7 tokamak and the result shows clearly the empirical trend of improved confinement at higher electron densities.

3. Motivation HT-7 • Impurities play an important role on plasma behavior in tokamaks. They are responsible for large radiation losses, and the resistivity of plasma usually rises several times above that of pure plasma. • A full understanding of impurity transport behavior, in other words, finding a method of controlling the impurities in the plasma, plays an important role in the thermonuclear fusion research and the achievement of controlled fusion power. • The spectroscopy study of impurities is one important and effective approach of plasma impurity research.

4. The spectroscopy system and the volume emission rate HT-7 (a) The spatial evolution of the observed CV column brightness B(r) (o) and the smoothed column brightness curves ( blue line ). (b) The profile of CV volume emission rate k(r) obtained from the profile of B(r) in (a) by the special Abel inversion. (a) the observed CV (2271 Å) column brightness from the UV system.(b) the HN pulse signal of the center monitoring system.  the scanning period T plasma scanning time from down to up In this paper:  = 72.7ms ; T = 12.1ms Tmin in this system is 0.5ms with 3000 rev.min-1

5. Impurity transport model HT-7 A set of coupled differential equations: The neutral impurity density: The flux of the impurity ions: • Both neoclassical and anomalous transport have been considered • The assumption of a cylindrical symmetry • D and W are the anomalous factors The diffusion coefficient Dk The convection velocity Wk • Assumed that the neutral impurities flow into the plasma at thermal velocity V0, and the density n0( r) decreases rapidly through ionization as the impurities penetrate the plasma

6. Simulation HT-7 • Because the UV spectroscopy system has only been calibrated relatively, the absolute value of the column brightness of CV(227.1nm) is meaningless. • Since the locations of the shell peaks and the thickness of the shells of the volume emission rate are determined by the transport coefficient of impurities, we can simulate these by the code. • The results show that the shell peak location of the volume emission rate is at about 15.7cm.

7. Experiment in HT-7 tokamak HT-7 The scan of ne in OH discharges Ne scan (OH): Ip=100kA Vp=1.6V B=1.85T H/(H+D) ~ 20% The observed spatial range: 26.42cm The plateau range of discharges:380ms ~ 660ms Note:the radiation of Ha and the bremsstrahlung increased with the increase of ne.

8. Simulation results ( I ) HT-7 D=1.2 D=2.15 D=0.5 W=0.25 W=0.25 W=0.25 With the increase of ne, the diffusion coefficient Dk(r) and the convection velocity Wk(r) decreased. The empirical trend of improved confinement at higher electron densities is clearly shown in the graphs.

9. Simulation results ( II ) HT-7 • The change trends of Zeff with ne of experiment are similar with that of simulation Nzthe total number of impurity ions of plasma z(rp) the outward impurity flux at LCFS A|rpthe area of LCFS • The confinement time of impurities z has been calculated from the results of simulation • The particle confinement time p has been derived from the H/D measurement • z and p have the same trends with the increase of ne.

10. Simulation results ( III ) HT-7 (a) (b) (a) ne=1e19m-3(b) ne=1.5e19m-3(c) ne=2e19m-3 The impurity ion densities of various ionization states nk(r) /n0(rp) under different ne have been obtained by the simulation. nk(r) has been normalized to n0(rp). (c)

11. Summary HT-7 • The volume emission rate of CV (2271 Å) was obtained by a special Abel inversion from UV spectroscopy measurement system. • An impurity transport code, on the assumption of a cylindrical symmetry, was constructed. Both neoclassical and anomalous transports were taken into account in this code. The diffusion coefficient Dk(r) and the convection velocity Vk(r) were obtained through simulating the experiment results of the volume emission rate k(r)of line emission from ionized light impurities and Zeff(r). This code also provided the impurity ion densities of various ionization states nk(r)/n0(rp). • The results shows clearly that Dk(r) and Vk(r) decreased with the increase of ne, correspondingly, the confinement of particles was improved.

12. Contact Form HT-7

13. HT-7 ASIPP References [1] Ruan Huailin and Wan Baonian, International Journal of Infrared and Millimeter Waves 21 (12) p.1973-1987 December 2000 [2] Wei Xu, PhD Thesis, ASIPP, (1997) unpublished. [3] W. Lotz, IPP 1/62 (1967) [4] S. von Goeler, W. Stodiek, H. Eubank, H. Fishman, S. Grebenshchikov and E. Hinnov, Nucl. Fusion 15 (1975) 301. [5] EUR-CEA, MAKOKOT – Code d’evolution (August 1977).