Proposal for the observation of ITB in synergetic of LHCD & IBW discharges in HT-7 EXP2005

1. ASIPP Proposal for the observation of ITB in synergetic of LHCD & IBW discharges in HT-7 EXP2005 J.Liu , X.Gao Institute of Plasma Physics, Chinese Academy of Sciences P.O.Box 1126, Hefei,Anhui, 230031 P.R.China Email: jinlj@ipp.ac.cn

2. Outline • Introduction • Study of ITB for high-integrated performance in JT-60U . • Obtained results of ITB study in discharges with synergy of LHCD & IBW in HT-7 . • Experimental analysis proposal • Summary and discussion

3. Introduction • Plasmas with ITBs can reach high normalized beta (N) and high poloidal beta (p), due to the pressure profile is broadened enough to avoid MHD instabilities . • Electron ITBs & ion ITBs are important in improving energy and particle confinement in ITER . • The formation of electron ITBs was studied using electron cyclotron (EC) heating with scan of neutral beam (NB)power in JT-60U positive shear (PS) and reversed shear(RS) plasmas. • In the HT-7 tokamak , Control of the current density profile and high performance via formation of an ITB-like profile in Te was achieved in the IBW & LHCDsynergetic discharges.

4. Introduction(cont.) Major issuesregarding application of the ITB to ITER or demonstration reactors are: • ITB formation condition and ITB control. • formation of electron ITBs to improve energy confinement. • sustainment of ITBs, especially ion ITBs. under reactor-relevant conditions, with dominant electron heating and small central particle fuelling. • Impurity accumulation in ITBs. Impurities(He,C,Ar) were investigated in ITB plasma.

5. ITB structure ITB shoulders Internal transport barrier (ITB) ------ a profile structure with large values in the Ti, Te and ne inside the radial location of the ITB, with large gradients in the Ti ,Te , ne profile in the region of the ITB andreduced gradients for the other regions (except for the edge region). Optimized ITBprofileslie at a large radius ρITBand possess moderate gradients (large ITB halfwidth). ITB foot ρSYMis the symmetry radius of the ITB

6. ITB Study results in JT-60U • Obtained the long pulse(tpulse = 9s) discharges with double transport barriers (ITB & ETB ). • A high beta withβN= 2.7was maintained for7.4 s(~60 E). • A electron ITB with high temperature plasma (Te ~ 25KeV) in a wide region (~30% of a) was obtained with ECH in a RS plasma sustained by LHCD. • The operation of RS plasmas has been extended to the low-q (qmin ~2), high-Ip (Ip= 0.8MA) region . HHy2= 2.1–2.3 , H89 = 3.3–3.8 , n/nGW= 0.65 under full CD condition, where (fBS ~ 80%).

7. ITB formation conditions Two types of ITBswere observed at JT-60U: • weak ITB (with a wide zone of low heat diffusivities χ inside ITB‘foot’) . • strong ITB ---“box-type”ITB (with high gradients and very low heat diffusivities χ). • Weak ITB is controlled continuously easier than strong ITB. • The effective Er shear (dEr/dr)eff characterizes the depth of the ‘notch’ structure of Er. • Exists a critical value of (dEr/dr)eff (depends on the Bpat the ITB) to change the state from a weak to a strong ITB.

8. Threshold power of ITB formation • The threshold power Pth can be affected by the power deposition profile relative to the qprofile. • Theqprofile at the start of EC was inverted. • The electron ITB was formed around 1MW when χe starts to decrease. • With 2.7MW of EC heating power, it shows a clear ITB structure. • The profiles of EC power deposition are shown for the highest EC power cases. Radial profiles of Te (top), q and EC power deposition (bottom) for different EC powers in and RS plasmas.

9. Existent period of ITB The result on RS plasma. Dominant ECH(P = 2.6MW) was applied from t = 4.3 s, NB power was increased stepwise from t = 4.7 s.

10. Analysis of the upper results • A strong electron ITB was established by EC heating as shown by the Te and χe profiles at t = 4.75 s. • The ITB for ne and Ti started to establish at t = 5.15 s and clear box-type ITBs were formed before t = 5.6 s. • The ITB foot locations were similar for both Te and Ti during the evolution of the profiles .(indicating a strong linkage between electron and ion ITBs.) • The ion thermal diffusivity χi decreased with increase in NB power and development of Ti ITB, the electron thermal diffusivity χe remained almost constant.

11. ITB control • The barrier can be sustained if RF power (e.g.LHRF) is applied. • ITB location can be controlled via the modification of j(r)using LHCD or NBCD. • The large ITB location enhances the confinement property. • The radius of qmin corresponded to the ITB foot location. For a narrow ITB width with steep gradient of Ti ,the location of the ion ITB foot is located inside the radius of qmin. (Contrarily, showed as the figure.)

12. Obtained results of electron-ITB study with the synergy of LHCD & IBW in the HT-7 tokamak

13. Synergetic effect • The operation mode utilizing synergy effect of IBW and LHCD provide a new way to obtain steady-state operation in advanced tokamak scenario. • LHCD was used to sustain plasma current and control current density profile. IBW has good features for heating electrons both locally and globally and for controlling the electron pressure profile. • IBWs can be used in conjunction with LHWs to aid the localization of the non-inductive current generated in the regime of LHCD. Features of off-axis IBWH integrated into LHCD plasma can extend high performance volume. • IBW and LHCD synergetic discharges was optimized through moving the IBW resonant layer (by changing Bt and selecting plasma parameters) to maximize the plasma performance and to avoid MHD activity.

14. Background • HT-7 main parameters: R=1.22m, a=0.27m, Ip = 100 ~ 250 kA, Bt=1.6 ~ 2 T, = 1~ 6.51019/m3, Te(0) = 0.5 ~ 1.5 keV . Feedback controlled: Ip (by the poloidal field system) Ne (by a pulsed gas injection system). • RF Power: IBW(50~350kW), LHW(up to 1.2MW) Frequency: IBW(24~30MHz), LHW(2.45GHz) N||peak : IBW(8 for 27MHz), LHW(1.25~3.45GHz) • Te was measured by a SX-PHA • A CdTe detector based HX array is used as main tool in present investigation. • High performance discharges:H89*N >3 (4) , H93>1 (1.5) for ~50E • .

15. ITB by synergy of IBW and LHCD • LHW: near on-axis current drive in • target plasma • IBW: 27MHz, off-axis heating • ~0.5a • Strong gradient in pressure ~0.5a, • showing an electron ITB . • Improved particle and energy • confinement ITB

16. Ip=120kA , ne0=1.35×1019m-3 , LHW~400kW , IBW~200kW High performance long-pulse dischargeon HT-7 H89*N > 2 for ~ 200E andfLHCD+fBS = 80% , (fBS = 38% , fLHCD = 42%) • Te profile at 2.75s by 250ms collecting time. • Much flat in central region • had a large gradient at 0.52a • showing an ITB-like structure.

17. Te(0) > 4keVby LHW&IBW synergy The electron distribution be broadened

18. HX radial profile • Electron ITB of Tewas strongly correlated with the location of the LHW driven fast electron current ( indicated by the HX radiation profile ). • The peak in the radial profile of I_HX in LHCD and IBW-heated plasma is correlated with the region around the resonant layer. • The peak of I_HX moved out forth with the increase of Bt. • High performance volume extended.

19. Experiment observation proposal Our study in future should focus on the points: • Analysis of relevant factors of formation of ITBs :The profiles of Te, q, ne, and pressure gradient, electron thermal diffusivityχe, magnetic shear s , electric field Er gradient , poloidal magnetic field Bp. • Ascertain the threshold power as the formation condition of ITB. (Analysis the cases of ITBs structure in different RF power of LHW & IBW.) • Determine the region of the electron ITBs and existent period of ITBs. ( Refering to the profile of q , χeexcept forthe prevenientprofile of Te, Ne and intensity of HXR.) • Commence study of ion ITB next if the relevant parameters (e.g. ion temperature Ti ) is available.

20. Relevant diagnostics &difficulties Needed diagnosis in HT-7: Far infrared laser interferometry(ne) Soft X-rays radial spectrum and intensity(Te &χe ) Hard X-rays radial spectrum(fast electron deposition) Neutral particle analysis(Ti) Shafranov shift (P) MSE---motional Stark effect(q profile) Some diagnostics (e.g. measurements for q profile ) is unavailable in HT-7.

21. Summary & discussion • Showedstudy results in JT-60U : the formation and characteristics of ITBs & the necessary conditions for ITBs to appear. • In HT-7 , electron ITBs with The properties of IBWs in controlling Te and Ne profiles integrated into the LHCD plasmas . • Active controlling strength of ITBs by modifications of j(r) profile is our work for the future.

22. Thanks!