Fast ion generation with novel three-ion radiofrequency heating scenarios:

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1 1 Fast ion generation with novel three-ion radiofrequency heating scenarios: from JET, W7-X and ITER applications to aneutronic fusion studies Yevgen Kazakov 1, D. Van Eester 1, J. Ongena 1, R. Bilato 2, R. Dumont 3, E. Lerche 1, M. Mantsinen 4 and A. Messiaen 1 1 Laboratory for Plasma Physics, LPP-ERM/KMS, Brussels, Belgium 2 Max-Planck-Institut für Plasmaphysik, Garching, Germany 3 CEA, IRFM, F Saint-Paul-lez-Durance, France 4 Catalan Institution for Research and Advanced Studies, Barcelona, Spain For more details: [1] Ye.O. Kazakov et al., Nucl. Fusion 55 (2015) [2] Ye.O. Kazakov et al., Phys. Plasmas 22 (2015)

2 ICRF and fast-ion generation 2 ICRF minority heating used since 70 s: (H)-D, ( 3 He)-D, ( 3 He)-H, ( 3 He)-DT, etc. Typical concentrations of minority species X mino = n mino /n e ~ 5% Good absorption & fast-ion generation (in present-day machines) Theory: T.H. Stix, Nucl. Fusion (1975) Energies of fast ions generated due to minority ICRF heating: Fast-ion tail energy depends on: 1) ICRH power density, <P RF > 2) Minority concentration, X mino 3) Plasma density, n e Present-day experiments: tail energies of a few hundred kev observed Effective temperature, T eff [kev] JOI restriction 650 kev 150 kev T e = 4 kev, P ICRF = 5 MW n e0 = 4x10 19 m -3 n e0 = 8x10 19 m Hydrogen concentration, X[H] (%) [E. Lerche, JET Science Meeting Generating, measuring and utilising fast ions, Dec. 2014]

3 Three-ion ICRF heating: efficient wave absorption at very low minority concentrations 3 Three-ion ICRF: very similar to minority heating, but much lower X mino is needed Case 1: ( 3 He)-D-H plasma Case 2: ( 3 He)-D-T plasma ( 3 He)-D: X[ 3 He] ~ 5-6% ( 3 He)-H: X[ 3 He] ~ 3% ( 3 He)-DT: X[ 3 He] ~ 3-4% ( 3 He)-T: X[ 3 He] ~ 2% bulk plasma heating fast ion generation Fast 3 He generation (X mino < 0.5%) is possible in H:D 70:30 plasmas Helium-3 minority heating in D-H and D-T plasmas is very different

4 Outline 4 Short overview of presently used ICRF scenarios Three-ion (Z)-Y-X ICRF absorption: basics Applications ( 3 He)-D-H: fast-ion generation in JET, W7-X, ITER (Be)-D-T: bulk ion (T i ) heating in ITER Summary and conclusions

5 Recall theory: wave dispersion and polarization 5 Fast wave (excited by ICRF antenna) is elliptically polarized E + / E : left/right-hand polarized component (ions / electrons) Left-hand polarized component E + is responsible for (thermal) ion heating [Stix, NF-1975] Fundamental ICRF heating (N=1) requires: 1) Resonant ions, ω = ω ci 2) Left-hand polarized component E + Plasma (mainly) imposes wave polarization

6 Heating of single-ion species plasmas 6 Fundamental ICRF heating (e.g., ω =ω cd ) is inefficient because of E + 0 [E. Lerche et al., PPCF-2009] The scheme can be somewhat improved if using NBI to Doppler-shift the absorption region Possible option: harmonic ICRF heating (N 2) finite Larmor radius (FLR) effect (plasma preheating required; normally ICRH+NBI > MeV ions see, M. Mantsinen et al., PRL-2002; M. Schneider et al., this meeting, I-5) Efficient heating at ω = 2ω cd was observed inconsistent with theory [J. Adam, PPCF-1987]

7 Two-ion species plasmas: Minority heating 7 Residual hydrogen ions always present Two ion species: (majority ions polarization, minority ions absorption) Scenario (H)-D (D)-T Minority ion H D E + / E 1/3 [D. Start et al., NF-1999] Damping Strong Two-ion minority heating: Minority concentrations of ~5% are typically used in present-day experiments Limited capability for ion absorption at very low X mino («1%) ( 3 He)-H 3 He 1/5 Medium Three-ion ICRF heating: ( 3 He)-D (Z)-Y-X 3 He Z 1/7»1 Weak How? See soon X 3 = n 3 /n e ~ 0.1 1% (impurity ions) Much larger energy per resonant ion. Then, O(100 kev) O(1 MeV)

8 Two-ion species plasmas: Mode conversion 8 At higher minority concentration, a transition from minority ion to mode conversion heating occurs Typical FW dispersion for MC heating IIH resonance L-cutoff R-cutoffs MC layer: x S x x L, IBW/ICW conversion x R x S x L (FW reflection) FW x R Radial position of the MC layer depends on the minority/majority density ratio ω = ω cd ω ch

9 MC layer moves with minority concentration 9 JET-like example: D-H plasma, f = 32.5MHz, B 0 = 3.2T, T 0 = 4keV, n e0 = 4x10 19 m -3, n tor = 27 (dipole) ω cd X[D] = 10% 40% ω ch Double-pass absorption (HFS) (LFS) By varying the deuterium concentration from 0% to 100%, one moves the MC layer from the D cyclotron resonance to the H resonance

10 Three-ion ICRF: (Z)-Y-X scenario 10 D-H plasma R Wave polarization: IIH resonance : linear polarization E + = E L-cutoff : E vanishes! E + carries almost 100% of the FW power An enhanced E + near the L-cutoff in two-ion species plasmas. None of the minority/majority species is able to profit from that. Fundamental ICRF heating requires: 1) Resonant ions, ω = ω ci 2) Left-hand polarized component E +

11 Three-ion ICRF: (Z)-Y-X scenario 11 ( 3 He)-D-H plasma R Wave polarization: IIH resonance : linear polarization E + = E L-cutoff : E vanishes! E + carries almost 100% of the FW power An enhanced E + near the L-cutoff in two-ion species plasmas. None of the minority/majority species is able to profit from that. Add the third ion species ( 3 He) at a small concentration to absorb the power.

12 Feel the difference 12 Inefficient electron MC heating No 3 He Very efficient absorption by helium-3 ions X[ 3 He] = 0.1% Low efficiency! Adding a very small concentration of helium-3 ions (X[ 3 He] ~ 0.1%) boosts ICRF absorption in D:H 1:2 plasmas Mode conversion without mode conversion

13 ( 3 He)-D-H scenario: optimal D:H ratio 13 First-order approximation: 1 hydrogen (1/1); 2 deuterium (1/2); 3 helium-3 (2/3) (H:D ~ 2:1) X[H] = 70.5% ± 4% Numerical result (TOMCAT): Efficient DPA absorption, p 3He > 50% (X[ 3 He]=0.15%) at X[H]=70.5 ± 4% 0.15%

14 Three-ion ICRF scenarios: various options 14 bulk ions #1 resonant ions bulk ions #2 (Z/A) i 1/1 Hydrogen 2/3 1/2 ~ /3 ( 3 He)-D-H Helium-3 Deuterium, Helium-4 (Be)-D-T 9 Be 4+, 7 Li 3+, 11 B 5+, 22 Ne 10+ (impurities!) Tritium Ion concentration Even studying aneutronic fusion is possible, (B)-T-H p + 11 B 3α + 8.7MeV

15 Acceleration of Boron ions in T-H plasmas 15 Aneutronic fusion: Hydrogen, (Z/A)=1 Boron, (Z/A)=5/11 Tritium, (Z/A)=1/3 (B)-T-H ICRF heating: X[B] < 0.1%, T:H ~ 15%:85% Double-pass ICRF absorption by B ions

16 16 Short overview of presently used ICRF scenarios Three-ion (Z)-Y-X ICRF absorption: basics Applications ( 3 He)-D-H: fast-ion generation in JET, W7-X, ITER (Be)-D-T: bulk ion (T i ) heating in ITER Summary and conclusions Hydrogen, (Z/A)=1 Helium-3, (Z/A)=2/3 Deuterium, (Z/A)=1/2

17 Three-ion ICRF scenarios: fast-ion generation 17 Power deposition from TORIC X[ 3 He]=0.1%, X[H]=70% (X[D]=22%, X[Be]=2%) Flat p abs dependence in the range X[ 3 He]= 0.05% 1% (very precise 3 He control not required) [MW/m 3 /MW inj ] IIH resonance MC layer (JET) Potential in JET: X[ 3 He] = %, E 3He ~ 1 MeV/MW inj.

18 W7-X experimental aim: prove good fast-ion confinement 18 Stellarator W7-X (IPP-Greifswald, first plasma 2015) M. Drevlak, NF-2014: protons, 60keV, r 0 /a = 0.06 as <β> Stellarator-reactor must be designed to provide confinement of fusion-born alphas HELIAS: α-particles (3.5 MeV) W7-X: protons (60 kev), 3 He (80 kev) Fast ion sources for W7-X: NBI (55 kev) ICRH (25 38 MHz, 2 MW) The main goal of ICRH in W7-X: source of fast ions (~ kev) at very high plasma densities n e m -3 (* also good in view of impurity control, HDH-mode)

19 ICRF minority heating in W7-X: H and 3 He resonant ions 19 LPP-ERM/KMS: ICRF antenna design B 0 2.5T (ECRH 140GHz), TEXTOR RF generators: f = MHz J. Ongena et al., PoP-2014 H: f 38MHz 3 He: f 25 MHz ICRF minority heating (ω = ω ci ): a) H ions at f 38MHz, b) 3 He ions at f 25MHz ICRF second harmonic (ω = 2ω ci ): D and 4 He ions at f 38MHz (ω ch = 2ω cd )

20 Challenge of ICRF in W7-X: produce keV ions in very high density plasmas 20 High-plasma density: improving wave absorption vs. complicating fast-ion generation Fast-ion confinement studies: n e0 ~ 2x10 20 m -3, T 0 ~ 3 kev (<β> ~ 4%) (goal: E mino ~ kev) V W7-X = 30m 3, V ~ 3m 3, X mino ~ 1% P RF > 7MW (!) (for full plasma density) H minority heating a good option for operation with reduced n e0 ~ 7x10 19 m -3 Solution: Decrease the concentration of resonant ions (1% 0.1%) to compensate for the high plasma density

21 ITER: fast particles in the non-activated phase 21 If neutron generation is prohibited, D majority ions can be replaced with 4 He ions (Z/A=1/2): ( 3 He)- 4 He-H heating with H: 4 He ~ 70%:15% f = 54 MHz (central) f = 52 MHz (r/a ~ 0.1) f = 50 MHz (r/a ~ ) P ICRF ~ 3 MW needed P ICRF ~ 5 MW needed P ICRF ~ MW needed A possibility to control the radial location of the fast-ion source by varying the ICRF frequency (present-day tokamaks: ICRF+NBI)

22 ( 3 He)- 4 He-H heating in ITER 22 Operation without any deuterium: H ~ 70 90%, 4 He ~ 5 15%, 3 He < 1% X[ 3 He] = 0.2% Also: efficient ICRF heating with the reduced 3 He consumption

23 New ITER-like wall in JET: W and Be impurities 23 Deuterium, (Z/A)=1/2 Beryllium, (Z/A)=4/9 Tritium, (Z/A)=1/3 [L. Horton, 2015]

24 (Be)-D-T heating in ITER 24 Required ICRF frequency: f = MHz Absorption by intrinsic Beryllium impurities dominates for a wide range of T and Be concentrations Larger fraction of bulk ion heating than for 3 He scenario (80% vs %) This heating scenario can be tested in JET DTE2

25 25 Short overview of presently used ICRF scenarios Three-ion (Z)-Y-X ICRF absorption: basics Applications ( 3 He)-D-H: fast-ion generation in JET, W7-X, ITER (Be)-D-T: bulk ion (T i ) heating in ITER Summary and conclusions

26 Mode conversion and three ion species 26 Three ion species: D (~90%), H (~10%) + 13 C, 21 Ne, 7 Li, 11 B, 40 Ar Fast Wave IBW absorption by impurities (ICRH antenna) (mode conversion) (ω = 2ω c3 ) Such an impurity RF heating was observed on TFR, T-10, T11-M, JET, HT-7, ω = 2ω ct heating in D-H plasma (H:D:T=35%:45%:20%; C. Castaldo et al., PoP-2010) Bulk ion heating was observed at ω = 3/2ω cd on AUG (F. Nguyen et al., EPS-2002) Instead of FLR (ω=2ω c3 ) and non-linear (ω=3/2ω ci ) damping mechanisms, we suggest N=1 heating of a third ion isotope. Note: FW heating, negligible MC! ω = 2ω c3 ω = ω c3 N = 2 (second harm.) N = 1 (fund.) for impurities fast ion generation (FLR effect) ( cold plasma effect) bulk plasma heating

27 Experimental evidence of three-ion ICRF scheme: ( 7 Li)-D-T heating in TFTR (1996) 27 J.R. Wilson et al., Phys. Plasmas 5, (1998) In TFTR, three-ion ICRF heating was eliminated by using isotopically enriched 6 Li pellets for conditioning and standard MC heating was recovered.

28 Conclusions 28 Three-ion ICRF scenarios: efficient RF power absorption at very small minority concentrations, < 1% (e.g., impurity ions) Three ion species is necessary, but not sufficient: a) b) a proper density ratio between the main two ion species ICRH MENU ( 3 He)-D-H heating Applications of the three-ion ICRF heating include: fast-ion generation (W7-X, non-activated phase of ITER, ) bulk ion heating during the ramp-up D-T phase (JET, ITER, DEMO) aneutronic fusion studies (Be)-D-T heating