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Proposal for the observation Proposal for the observation of ITB in synergetic of LHCD of ITB in synergetic of LHCD

& IBW discharges in HT-7 & IBW discharges in HT-7 EXP2005EXP2005

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

ASIPP

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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

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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 & LHCD synergetic discharges.

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Introduction(cont.)

Major issues regarding 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.

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ITB structure

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 and reduced gradients for the other regions (except for the edge region).

Optimized ITB profiles lie at a large radius ρITB and possess moderate gradients (large ITB halfwidth).

ρSYM is the symmetry radius of the ITB

ITB foot

ITB shoulders

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ITB Study results in JT-60U Obtained the long pulse(tpulse = 9s) discharges with double transport barriers (ITB & ETB ).

A high beta with βN = 2.7 was maintained for 7.4 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%).

/N t t PaB I

89 89/H

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ITB formation conditions

Two types of ITBs were 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 Bp at the ITB) to change the state from a weak to a strong ITB.

eff max min(dEr/dr) = ((dEr/dr) -(dEr/dr) )/2

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Threshold power of ITB formation• The threshold power Pth can be affected by the power deposition profile relative to the q profile.

• The q profile 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.

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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.

Existent period of ITB

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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.

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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.)

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Obtained results of electron-ITB study

with the synergy of LHCD & IBW

in the HT-7 tokamak

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Synergetic effect The operation mode utilizing synergy effect of IBW and LHCD provid

e 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.

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Background HT-7 main parameters: R=1.22m, a=0.27m, Ip = 100 ~ 250 kA, Bt=1.6 ~ 2 T,

= 1~ 6.51019/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 investiga

tion. High performance discharges: H89*N >3 (4) , H93>1 (1.5) for ~50E

)0(ne

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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

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Ip=120kA , ne0=1.35×1019m-3 , LHW~400kW , IBW~200kW

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.

H89*N > 2 for ~ 200E and fLHCD+fBS = 80% ,

(fBS = 38% , fLHCD = 42%)

High performance long-pulse discharge on HT-7

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Te(0) > 4keV by LHW&IBW synergy

The electron distribution be broadened

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HX radial profile

• Electron ITB of Te was 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.

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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 , χe except for the prevenient profile of Te, Ne and intensity of HXR.)

Commence study of ion ITB next if the relevant parameters (e.g. ion temperature Ti ) is available.

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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.

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Summary & discussion

Showed study 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.

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