Calibration of plasma heat flux measurement

Transcription

1 ICPP2008 Satellite Wrokshop in Aso September 14, 2008, Aso, Japan Calibration of plasma heat flux measurement Hiroto Matsuura Osaka Pref. Univ.(Japan) Contents of My presentation * What is "sheath" and "thermal probe" * Heat flux measurement for DC glow plasma and estimate energy reflection coefficent * direct heat flux measurement for Divertor Simulator * Heat flux estimate for pulse plasma(preliminary)

2 What is Sheath Plasma is a compound fluid of * Positive Ions(heavy) * Electrons(light) At boundary between plasma and solid, negative charge is accumulated and, with established electrostatic potential gradiant, * Ions are accelalated * Electrons are repelled Transport through this plasma boundary layer is dependant on sheath structure.

3 Transport through sheath and Probes Probe Ref parameter Charge (Ze) Langmuir probe (Current) Langmuir... Te,Ni,Vs,... Momentum (mv) Combined force-mach-probe (Pressure) Lunt Ti,... Energy (mv^2) Thermal probe (Heat flux) Stamate Matsuura Kado Nagaoka Nni,Ti,Te,Ni,..

4 Sheath heat flux Divertor heat flux reduction with detached plasma is an important task in fusion reactor design. However, direct measurement data of heat flux itself is almost not available yet. Calculation of heat flux is not so accurate. Sheath heat transfer factor is often assigned artificially, but it depends on sheath potential drop, ion specis, surface condition, and so on. Ti>Te in divertor, but ion temperature measurement is more difficult than electron.

5 Probe bias effect Langmuir probe I-V characteristic Thermal probe Q-V characteristic

6 Glow discharge device

7 Thermal probe 2(TP2)

8 Heat balance model during discharge after discharge

9 Perturbation method When discharge is terminated suddenly, the jump of surface temperature differential is propotional to heat flux. c ρ V T t T before after = Q before t 3 QS = ΔT S T[K] QS[W] 3 QS = ΔT time[arb.] dt[k]

10 Probe bias modulation From Vf, probe potential is set to be negative, and heat flux increase due to ion accellration through sheath. 120

11 Estimate of energy reflection coefficient

12 Importance of energy reflection coefficient of surface RE=0.0 RE=0.6 Q-V scan data can not be explained without RE

13 Importance of bias potential and energy reflection effect In the estimate plasma heat flux, relation ( Q ~ Te * Iis ) is assumed, but its proportional coefficient ( heat transfer factor) is often given artificially. Effect of bias potential and energy reflection (RE) has been paid less attention. As shown in this work, experimental data for ion heat flux ( V < Vf ) and electron flux ( V > Vf ) can not be explained with single theoretical curve without considering proper RE. Direct measurement of divertor plasma heat flux and detail study on sheath potential will be indispensible.

14 PIG arc discharge Divertor simulator MAP-II

15 Gradient type thermal probe(gtp)

16 TC raw data of GTP Discharge condition is kept constant. (Te=3[eV]) Probe vias is negative compared with Vf(=-14.3[V]). Thermal probe raw data(#25853) 70 TC temp.[deg.] -89.0, -70.2, -51.1, -79.6, TC1 TC2 TC3 TC t[min.]

17 Temperature gradiant method Two or more thermocupples with high sampling time are used to determine temperature gradient ( and time evolution) in the senser experimentally. Heat Flux(Q) Heat flux[w/m 2 ] 0 MAP-II Thermal probe data(#25853) -89.0, -70.2, -51.1, -79.6, Q plasma GRAD T DOT T t[min.] M.Osakabe et al.; Rev. Sci. Instrum., 72(2001)586

18 Results of GTP About 10[min.] after bias change, conductive heat flux ( indicated with "GRAD T" ) shows exact plasma heat flux. Transit change of plasma heat flux with biasing is also fairly well compensated withthe transition term ( indicated with "DOT T" ). By comparison GTP data with the theoritical Q-V curve, ion temperature can be deduced. Though obtained value (Ti ~ 0.5[eV]) are reasonable compared with the experimental database obtained with spectroscopy method, more precise calibration of heat flux measurement and estimation of RE or effective recombination energy are still necessary.

19 Heat flux measurement for H-J divertor plasma(preliminary) Pulse length is shorter than thermal diffusion time. From time delay and peak temperature, heating energy can be determined. (to be reported at next JSPF meeting)

20 Summary of This work Plasma heat flux of low pressure DC glow plasma was estimated from probe surface temperature with 0D heat balance model. Comparison probe current and heat flux for pertabation of bias voltage enable us to estimate energy reflection coefficient of probe surface, without which experimental data for ion heat flux and electron flux can not be explained with single theoretical curve. Plasma heat flux of high density divertor simulator was directly measured with 1D temperature gradient method. Transit response of heat flux is also measured.

21 Acknowledgements Osaka Pref. University K.Michimoto,T.Jida, Y.Matsumura, K.Nakanoi The Univ. of Tokyo S.Kado,K.kurihara NIFS K.Nagaoka,M.Osakabe MAP-II experiment is performed with the support and under the auspiese of the NIFS Colaborative Program (NIFS04KOAB09).

22 Probe circuit