1. ASIPP Long pulse and high power LHCD plasmas on HT-7 Xu Qiang

2. Abstract Long pulse plasma (up to 400 s) has been achieved by lower hybrid current drive (LHCD) on HT-7 tokamak. High power current drive of lower hybrid wave (up to 800 kW) was also performed recently. The lower hybrid wave (LHW) power scan was performed in the plasma parameter ranges of I P = 130 and 160 kA,  n e = 1.5×10 19 m -3, P LHW = 70 – 800 kW. The lower hybrid current drive (LHCD) efficiency is studied for different injected powers and for different densities. Improved particle confinement is observed by application of LHCD as characterized by an increase of central line averaged electron density and decrease of D  emission. The dependence of energy confinement time (  E ) on plasma density and LHW power is experimentally studied in detail.

3. Introduction of the HT-7 tokamak The HT-7 tokamak is superconducting device, which was rebuilt from the original Russian T-7 tokamak in 1994. It has amajor radius of R = 1.22 m, minor radius of a = 0.27 m in the circular cross- section. There are two layers of thick copper shells, and between them 24 superconducting coils are located, which can create and maintain extremely a toroidal magnetic field B t of 2.5 T. In the first experimental campaign of the 2007, we made the high power experiments, the power of the LHCD system is up to 800kW.

4. Overview of HT-7 Experimental setup I p = 100~250 kA (250)  n e = 1~8x10 19 m -3 (6.5) T e = 1~5 keV (4.5) T i = 0.2~1.5 k eV (1.5) ICRF: f = 15~30MHz, P = 0.3MW(0.35) f = 30~110MHz, P = 1.5MW LHCD: f = 2.45GHz, P = 1.2MW(0.8)

5. A typical long-pulse LHCD discharge (toroidal field B t = 1.8 T, injected LHW power P LHW = 100 kW) on HT-7

6. A typical high power current drive of lower hybrid wave on HT-7

7. LHCD efficiency The current drive efficiency is an important physical quantity in evaluating the lower hybrid current drive experiment. The experimental current drive efficiency is defined by  exp =  n e RI rf /P rf (10 19 Am − 2 W −1 ) where I rf is the current driven by LH wave  n e is line averaged density ， R is the major radius of the plasma ， P rf is the injected LH wave power. When the loop voltage reaches zero, the plasma current is driven fully non-inductivity. The LH wave driven non-inductive can be obtained from the relative drop in loop voltage, I rf ＝ (  V/V OH )I P,  V=V OH － V LHW

8. LHCD efficiency versus different injected LHW power on HT-7 tokamak

9. The I rf (current driven by LH wave) versus different injected LHW power on HT-7 tokamak

10. The LHW driven current (normalized by injected power as RI rf /P rf ) versus different electron density on HT-7 tokamak

11. The waveforms of a typical high power LHW (>300KW) discharge (shot 78673). (a) D a radiation; (b) line average electron density; (c) plasma current; (d) plasma loop voltage; (e) central electron temperature; (f) LHW power.

12. The dependence of energy confinement time on different line averaged density, where plasma current is 120 – 150 kA, the injected LHW power is 100 – 600 kW for all shots of solid square data, and data of ohmic discharge is plotted by closed circle.

13. The dependence of energy confinement time on injected LHW power, where plasma current is 130 and 160 kA, and average density is 1.5×10 19 m -3

14. Intensities of C III, O II and radiation power (measured by XUV bolometer) is linearly proportional to the injected LHW power on HT-7 tokamak

15. Summary The current drive (CD) efficiency experiment with lower hybrid wave (LHW) was performed on HT-7 tokamak at high power ~ 800 kW. It is observed that the LHCD efficiency is decreased with increasing LHW power. It is observed that the LHW driven current (defined by RI rf /P rf ) is decreased with increasing density. It is observed that the particle confinement time is increased by LHCD.

16. The impurity radiation power is proportional to the injected LHW power. The energy confinement time is not improved during high power lower hybrid wave experiments. Dynamic impurity control may be a key issue in the high power LHCD experiments on HT-7 tokamak in the future.