Efficient generation of energetic ions in multi-ion plasmas by radio-frequency heating

ARTICLES PUBLISHED ONLINE: 19 JUNE 2017 | DOI: 10.1038/NPHYS4167 E icien gene a ion o ene ge ic ions in mul i-ion plasmas by adio- equency hea ing Ye. O. Kazako 1*, J. Ongena1, J. C. W igh 2, S. J. Wuki ch2, E. Le che1,3, M. J. Man sinen4,5, D. Van Ees e 1, T. C aciunescu6, V. G. Kip ily3, Y. Lin2, M. Nocen e7,8, F. Nabais9, M. F. F. Na e9, Y. Ba ano 3, J. Bielecki10, R. Bila o11, V. Bobko 11, K. C ombé1,12, A. Cza necka13, J. M. Faus in14, R. Fel on3, M. Fi zge ald3, D. Galla 4, L. Giacomelli8, T. Gol inopoulos2, A. E. Hubba d2, Ph. Jacque 3, T. Johnson15, M. Lennholm16,17, T. Loa e 18, M. Po kolab2, S. E. Sha apo 3, D. Valca cel3, M. Van Schoo 1, H. Weisen14, JET Con ibu o s†and he Alca o C-Mod Team† We desc ibe a new echnique o he e icien gene a ion o high-ene gy ions wi h elec omagne ic ion cyclo on wa es in mul i-ion plasmas. The discussed ‘ h ee-ion’ scena ios a e especially sui ed o s ong wa e abso p ion by a e y low numbe o esonan ions. To obse e his e ec , he plasma composi ion has o be p ope ly adjus ed, as p esc ibed by heo y. We demons a e he po en ial o he me hod on he wo ld-la ges plasma magne ic con inemen de ice, JET (Join Eu opean To us, Culham, UK), and he high-magne ic- ield okamak Alca o C-Mod (Camb idge, USA). The ob ained esul s demons a e e icien accele a ion o 3He ions o high ene gies in dedica ed hyd ogen–deu e ium mix u es. Simul aneously, e ec i e plasma hea ing is obse ed, as a esul o he slowing-down o he as 3He ions. The de eloped echnique is no only limi ed o labo a o y plasmas, bu can also be applied o explain obse a ions o ene ge ic ions in space-plasma en i onmen s, in pa icula , 3He- ich sola la es. In magne ized plasmas, cha ged pa icles gy a e a ound he magne ic ield lines wi h hei cha ac e is ic cyclo on equencies ωcs =qsB/ms, whe e qsis he pa icle’s cha ge, msis he pa icle’s mass, and Bis he local magni ude o he magne ic ield. A a ie y o s ong wa e–pa icle in e ac ions is possible when he wa e equency is close o he pa icle’s cyclo on equency o i s ha monics1–3. Ion cyclo on esonance hea ing (ICRH) is a powe ul ool used in o oidal magne ic usion esea ch. In ecen decades, se e al e icien ICRH scena ios we e iden i ied heo e ically and e i ied expe imen ally2–4. In b ie , his echnique elies on ex e nal exci a ion o as magne osonic wa es in he plasma, using specially designed ICRH an ennas loca ed a he edge o he de ice (see Fig. 1a). An ennas consis o a se ies o me allic s aps ha ca y adio- equency (RF) cu en s a a gi en equency deli e ed by an ex e nal gene a o . The adially a ying o oidal magne ic ield hen de e mines he loca ion o he ion cyclo on laye s ω=pωci (p=1,2,. . .), in he icini y o which he RF powe can be e icien ly abso bed by ions. The elec ic ield o he exci ed as wa es can be decomposed as a sum o he le -hand pola ized componen E+, o a ing in he sense o ions, and he opposi ely o a ing igh -hand componen E−. Wa e abso p ion by non-ene ge ic ions is e iden ly acili a ed by he p esence o a su icien ly la ge E+nea he ion cyclo on esonance. To illus a e his, we no e ha undamen al cyclo on hea ing in single-ion plasmas is ine ec i e since E+almos anishes a ω≈ωci. The choice o plasma composi ion, namely he numbe o ion species and hei ela i e concen a ions, allows one o con ol he adial dependence o he a io E+/E−. In wo-ion plasmas composed o one main ion species and a ew pe cen o mino i y ions wi h qi/midi e en om ha o he main ions, RF powe abso p ion a he mino i y ion cyclo on equency is s ongly enhanced5,6. These mino i y hea ing scena ios bene i om he enhanced E+in he icini y o he ion–ion hyb id (IIH) cu o - esonance pai , loca ed close o he mino i y cyclo on esonance2. I he IIH laye is no p esen in he plasma, as is he case a e y low mino i y concen a ions in wo-ion plasmas, he RF powe abso p ion by mino i ies is e y limi ed. On he o he hand, a mino i y concen a ions signi ican ly abo e he op imal alue o a ew pe cen , he IIH pai is loca ed oo a away om he mino i y cyclo on laye , hus u he educing hei abso p ion e iciency. Ins ead, such plasmas a e ypically used o localized elec on hea ing h ough mode con e sion (see e . 7 o mo e de ails). 1Labo a o y o Plasma Physics, LPP-ERM/KMS, TEC Pa ne , 1000 B ussels, Belgium. 2Plasma Science and Fusion Cen e , Massachuse s Ins i u e o Technology, Camb idge, Massachuse s 02139, USA. 3Culham Cen e o Fusion Ene gy (CCFE), Culham Science Cen e, Abingdon OX14 3DB, UK. 4Ba celona Supe compu ing Cen e (BSC), 08034 Ba celona, Spain. 5ICREA, Pg. Lluis Companys 23, 08010 Ba celona, Spain. 6Na ional Ins i u e o Lase , Plasma and Radia ion Physics, 077126 Bucha es , Romania. 7Dipa imen o di Fisica, Uni e si à di Milano-Bicocca, 20126 Milan, I aly. 8Is i u o di Fisica del Plasma, CNR, 20125 Milan, I aly. 9Ins i u o de Plasmas e Fusão Nuclea , IST, Uni e sdade de Lisboa, 1049-001 Lisboa, Po ugal. 10Ins i u e o Nuclea Physics, Polish Academy o Sciences, 31-342 K akow, Poland. 11Max-Planck-Ins i u ü Plasmaphysik, 85748 Ga ching, Ge many. 12Depa men o Applied Physics, Ghen Uni e si y, 9000 Gen , Belgium. 13Ins i u e o Plasma Physics and Lase Mic o usion, 01-497 Wa saw, Poland. 14EPFL, Swiss Plasma Cen e (SPC), 1015 Lausanne, Swi ze land. 15KTH Royal Ins i u e o Technology, 114 28 S ockholm, Sweden. 16Eu opean Commission, 1049 B ussels, Belgium. 17JET Exploi a ion Uni , Culham Science Cen e, Abingdon OX14 3DB, UK. 18CEA, IRFM, 13108 Sain -Paul-Lez-Du ance, F ance. †A ull lis o au ho s and a ilia ions appea a he end o he pape . *e-mail: ye gen.kazako[email p o ec ed] NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics 973 © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. ARTICLES NATURE PHYSICS DOI: 10.1038/NPHYS4167 10 10 20 20 30 30 40 40 50 50 60 60 60 70 70 80 80 80 90 90 90 ω = ωci Hyd ogen concen a ion, X[H] (%) Helium−3 concen a ion, X[3He] (%) F ac ion o RF powe abso bed by 3He ions 50 55 60 65 70 75 80 85 90 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 CPS17.32–1c ba ICRH ILA an enna 1 2 3 4 ICRH A2 an enna Figu e 1 | A new echnique o as -ion gene a ion in magne ized mul i-ion plasmas. The goal o ou s udy is o alida e ha , in p ope ly chosen mul i-ion plasmas, elec omagne ic ion cyclo on wa es can be e ec i ely abso bed by a e y low numbe o esonan ions a ω≈ωci. This echnique opens he possibili y o high-e iciency gene a ion o ene ge ic ions in magne ized plasmas. a, Inside iew o he wo ld-la ges magne ic con inemen usion de ice, Join Eu opean To us, showing di e en ion cyclo on esonance hea ing (ICRH) an ennas a he edge. The inse shows an example o he compu ed RF elec ic ield pa e n in a c oss-sec ion o he JET plasma. b, ‘Th ee-ion’ scena ios equi e esonan ions wi h a (Z/A) a io in be ween ha o he wo main ions, essen ially ollowing he ‘sandwich’ p inciple (Z/A)2<(Z/A)3<(Z/A)1. The igu e shows he ac ion o RF powe abso bed by 3He mino i y ions o he D–(3He)–H h ee-ion scena io as a unc ion o H and 3He concen a ions. The compu a ions we e made by he TOMCAT code o he pa ame e s o he JET expe imen s discussed in his pape (B0=3.2 T, =32.5 MHz, ne0 =4×1019 m−3,T0=4keV, k(an ) k=3.4m−1). The zones wi hin he dashed lines co espond o a single-pass abso p ion la ge han 50%. The code p edic s wa e abso p ion by a iny amoun o 3He ions (∼0.1–0.2%) in H–D plasmas wi h H concen a ions in he ange 70–80%, in ag eemen wi h equa ion (2). The e is, howe e , an elegan way o use mix u e plasmas o channel RF powe o ions: simply add a hi d ion species wi h a cyclo on esonance laye close o he IIH cu o - esonance pai . Unde hese condi ions, a new IIH pai appea s in close p oximi y o he cyclo on esonance o he hi d ion species, e en i hei concen a ion is ex emely low! Fo his hea ing scheme o wo k, he Z/A alue o he esonan ions should be ‘sandwiched’ be ween ha o he wo main plasma ions (Z/A)2<(Z/A)3<(Z/A)1(1) whe e Ziand Aia e he cha ge s a e and he a omic mass o ion species i. We use indices ‘1’ and ‘2’ o he main ions wi h he la ges and lowes cyclo on equencies, espec i ely, and index ‘3’ o he abso bing mino i y. Deposi ing nea ly all RF powe o a e y small numbe o mino i y ions is maximized in plasmas wi h main ion concen a ions8,9 X∗ 1≈1 Z1 (Z/A)1−(Z/A)3 (Z/A)1−(Z/A)2 ,X∗ 2≈1 Z2 (Z/A)3−(Z/A)2 (Z/A)1−(Z/A)2 (2) whe e Xi=ni/ne. Hea ing mino i y ions a highe concen a ions is equally possible; plasma mix u es wi h X1&X∗ 1a e mo e op imal in his case10. The me hod can also be ex ended o plasmas con aining mo e han h ee ion species by sligh ly adap ing he plasma composi ion. Fo p oo -o -p inciple demons a ion, we selec a plasma mix u e composed o wo hyd ogen iso opes, H ions wi h (Z/A)=1 and he hea ie D ions wi h (Z/A)=1/2, and 3He ions wi h hei unique (Z/A)=2/3 as a esonan abso be . Equa ion (2) p edic s ha 3He ions should e icien ly abso b RF powe in H–D (o H–4He) plasmas i he hyd ogen concen a ion is ∼67%. This is suppo ed by modelling wi h he TOMCAT code11, using plasma pa ame e s ele an o he JET expe imen s desc ibed below. Figu e 1b shows dominan RF powe abso p ion by a small amoun o 3He ions, down o concen a ions X[3He]≈0.1–0.2%. Plasma hea ing wi h he h ee-ion D–(3He)–H scena io a highe X[3He] ≈ 0.5–1% is equally possible. We no e ha he ecipe o he plasma composi ion gi en by equa ion (2) is alid o as magne osonic wa es, exci ed a he low magne ic ield side and p opaga ing owa ds egions wi h inc easing B, as in mos o p esen -day usion machines. E icien plasma hea ing wi h h ee-ion ICRH scena ios A se ies o dedica ed expe imen s we e pe o med on he Alca o C-Mod okamak12 (MIT, Camb idge, USA; majo adius R0≈0.67 m, mino adius apl ≈0.23 m) and on he wo ld-la ges magne ic usion de ice JET (Join Eu opean To us, Culham, UK; R0≈3 m, apl ≈1 m). The goal o hese s udies was o demons a e ha indeed a small amoun o 3He ions can e icien ly abso b RF powe in H–D mix u es. The Alca o C-Mod expe imen s we e un a high cen al elec on densi ies ne0 ≈(2–3)×1020 m−3 and e y high o oidal magne ic ield B0=7.8 T a a plasma cu en Ip=1.2 MA. In he JET expe imen s, ne0 ≈4×1019 m−3 and B0=3.2 T, Ip=2.0 MA we e used. Acco dingly, ICRH equencies =ω/2π=78.0–80.0 MHz (Alca o C-Mod) and =32.2–33.0 MHz (JET) we e chosen o loca e he 3He cyclo on esonance in he plasma cen e in bo h de ices. The Alca o C-Mod plasmas we e hea ed wi h 4–5 MW o ICRH powe only. In JET plasmas, 3.2 MW o neu al beam injec ion (NBI) was added p io o applying ∼4 MW o ICRH. Figu e 2 shows he ime e olu ion o he cen al elec on empe a u e Te0 and plasma s o ed ene gy Wpin esponse o he applied ICRH on Alca o C-Mod and on JET. These esul s con i m ou ea lie p edic ions (Fig. 1b) o he e iciency o 3He abso p ion a concen a ions o a ew pe mille (h) in H–D plasmas. The op imal 3He concen a ion o his scena io in C-Mod plasmas was app oxima ely X[3He] ≈0.5%. In JET, e en lowe 3He concen a ions ∼0.2% we e success ully applied. In JET expe imen s, he edge iso opic a io H/(H+D)was a - ied be ween 0.73 and 0.92 and he 3He concen a ion be ween 0.1% and 1.5% o assess he sensi i i y o ICRH on he de ailed plasma composi ion. The co e hyd ogen concen a ion was es ima ed om he measu ed edge H/(H+D) a io as X[H]≈0.9×H/(H+D), accoun ing o he p esence o impu i ies in he plasma and addi ional D co e uelling om he D-NBI sys em. We ind e icien 974 © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics NATURE PHYSICS DOI: 10.1038/NPHYS4167 ARTICLES PICRH (MW) X[3He] ≈ 5−7% X[3He] ≈ 0.5% Alca o C-Mod: (3He)−D and h ee-ion D-(3He)−H scena iosa 0 1 2 3 4 Th ee-ion hea ing pulses on JET: #90753 and #90758 b 0 1 2 3 4 5 PD-NBI = 3.2 MW Te0 (keV) 2 3 4 5 6 #90753: X[3He] ≈ 0.2−0.4%, X[H] ≈ 68−74% #90758: X[3He] ≈ 0.1−0.3%, X[H] ≈ 80−82% 1 2 3 4 5 Wp (kJ) PICRH (MW)Te0 (keV) Wp (MJ) Time, (s) Time, (s) Two-ion scena io, (3He)−D Th ee-ion scena io, D-(3He)−H 0 40 80 120 0.6 0.7 0.8 0.9 1.0 #90753 #90758 0.0 0.5 1.0 1.5 2.0 6 8 10 12 14 16 18 CPS17.32-2c Figu e 2 | Illus a ion o he pe o mance o he D–(3He)–H h ee-ion ICRH scena io on Alca o C-Mod and JET okamaks. a, Alca o C-Mod h ee-ion hea ing pulse (#1160901009, X[3He]≈0.5%, ed) and (3He)–D pulse (#1160823003, X[3He]≈5–7%, black). b, JET h ee-ion hea ing pulses #90753 (X[H]≈68–74%, X[3He]≈0.2–0.4%, blue) and #90758 (X[H]≈80–82%, X[3He]≈0.1–0.3%, ed). Whe eas a ew % o 3He is needed o mino i y hea ing in H o D majo i y plasmas, s ong wa e abso p ion in H–D plasmas is achie ed wi h abou en imes less 3He. plasma hea ing o a ai ly b oad ange o he iso opic a io (see also Supplemen a y Figs 5 and 6). In pa icula , cen al plasma hea ing wi h 1Te0/1PICRH >0.5 keV MW−1was obse ed o H/(H+D)≈ 0.78–0.91 mix u es a 3He concen a ions below 0.5%. Figu e 2a also includes he e olu ion o Te0 and Wp o 3He mino i y hea ing in he Alca o C-Mod D plasma wi h X[3He]≈5–7% (pulse 1160823003). Compa ed o his (3He)–D scena io, he h ee-ion hea ing scena io in C-Mod showed a la ge inc ease in he plasma s o ed ene gy (1Wp/1PICRH =22 kJ MW−1 e sus 14 kJ MW−1). A di ec compa ison o he hea ing pe o mance o he h ee- ion discha ges was no possible o he JET discha ges discussed he e. Howe e , i can be assessed compa ing he measu ed he mal plasma ene gy o ha de i ed om a so-called scaling law. These scaling laws p edic he ene gy con inemen alue o a gi en plasma expe imen as a unc ion o speci ic enginee ing pa ame e s (Ip,B0,ne,...; e . 13) and esul om a s a is ical analysis o da a collec ed om mul iple okamaks wo ldwide. He e, we use he well-es ablished ITERL96-P and IPB98(y,2) scalings o he ene gy con inemen ime τE(equa ions (24) and (20) in e . 13) o L-mode and H-mode okamak plasmas. τEis he cha ac e is ic ime du ing which he plasma main ains i s ene gy i he hea ing powe is suddenly swi ched o 1. Unde s a iona y condi ions i is gi en by he a io o he s o ed plasma ene gy di ided by he o al hea ing powe . Supplemen a y Figs 1–4 show he esul s ob ained o L-mode JET discha ges hea ed wi h di e en ICRH mino i y scena ios, including he a ios τE/τE,scaling. F om he de ini ion o τEgi en abo e, i ollows immedia ely ha τE/τE,scaling is equal o he a io o he co esponding s o ed ene gies. Fo he h ee-ion hea ing pulse #90758 (Fig. 2b), we ob ain τE/τIPB98(y,2)≈0.85–0.88 and τE/τITERL96−P≈1.43–1.48. This compa es e y well o τE/τE,scaling alues o he excellen (H)–D mino i y hea ing scena io in JET plasmas (Supplemen a y Fig. 1). E icien gene a ion o high-ene gy ions Ene ge ic ions play a c ucial ole in usion plasmas14. Indeed, he success o magne ic usion elies upon good con inemen o as alpha pa icles (4He ions wi h bi h ene gies 3.5 MeV). This is equi ed o sus ain high plasma empe a u es and o economical ope a ion o a usion eac o 1. Howe e , hese ene ge ic 4He ions can also igge ins abili ies ha deg ade he plasma pe o mance. To mimic he beha iou o usion-bo n alphas, bu wi hou ac ually using D–T plasmas, ICRH has been ex ensi ely used in he pas . Fo undamen al ion cyclo on abso p ion he acqui ed ion ene gies scale wi h he abso bed RF powe pe pa icle15. Since h ee-ion scena ios allow minimizing he numbe o esonan pa icles down o hle els, ions wi h a he high ene gies can be gene a ed. Fo plasma densi ies and ICRH powe le els a ailable in he JET and C-Mod expe imen s, sel -consis en powe deposi ion compu a ions wi h he codes AORSA16, PION17 and SCENIC18 p edic ed accele a ion o 3He ions o ene gies o a ew MeV. Figu e 2b shows as epe i i e d ops in Te0 (so-called ‘saw oo h’ oscilla ions) wi h a pe iod o ∼0.2 s du ing he NBI-only phase o JET pulses #90753 and #90758 ( =7–8 s). Ex ended saw oo h pe iods up o ∼1.0 s a e seen when ICRH is applied on op o NBI. Simila ly, in he h ee-ion Alca o C-Mod discha ge in Fig. 2a, he saw oo h pe iod inc eases om ∼0.13 s du ing he 2 MW ICRH phase o ∼0.23 s du ing he 4 MW phase. The obse a ion o long- pe iod saw ee h is a i s indica ion o he c ea ion o ene ge ic ions by ICRH, as he p esence o as ions in a plasma is well known o ha e a s abilizing e ec on saw ee h19,20. An independen con i ma ion o accele a ing 3He ions o high ene gies is p o ided by gamma- ay emission spec oscopy on JET21,22. Figu e 3a shows he gamma- ay spec um o pulse #90753 du ing =8–14 s (PICRH =4.4 MW), eco ded wi h he LaB 3 spec ome e 23. The obse ed lines o igina e om 9Be(3He, pγ)11B and 9Be(3He, nγ)11C nuclea eac ions be ween as 3He ions and be yllium (9Be) impu i ies. These impu i ies a e in insically p esen in JET plasmas wi h he ITER-like wall. The epo ed plasmas we e con amina ed wi h ∼0.5% 9Be, as es ima ed by cha ge exchange measu emen s. The obse a ion o he Eγ≈4.44 MeV line implies immedia ely he p esence o con ined as 3He ions wi h ene gies >0.9 MeV ( e . 21). Alpha pa icles, bo n in concu en 3He–D usion eac ions, also con ibu e o he gamma-emission a his ene gy NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. 975 ARTICLES NATURE PHYSICS DOI: 10.1038/NPHYS4167 R (m) Z (m) #90752 1.5 2.0 2.5 3.0 3.5 4.0 R (m) 1.5 2.0 2.5 3.0 3.5 4.0 −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0 1.5 2.0 #90753 0 20 40 60 80 100 120 140 Coun s Gamma ene gy, Eγ (MeV) #90753, Th ee-ion scena io ( = 8.0−14.0 s) #91323, (3He)−H scena io ( = 8.0−15.0 s) 0 20 40 60 80 100 120 140 3456789 6.91 MeV 6.48 MeV 5.85 MeV 5.5 MeV 7.28 MeV 4.44 MeV 7.98 MeV 8.56 MeV 8.92 MeV 5.02 MeV CPS17.32-3c abc ω = ωci (3He) ω = ωci (3He) Figu e 3 | Gamma- ay emission om 3He +9Be nuclea eac ions, p o ing he p esence o ene ge ic ICRH-accele a ed 3He ions. a, Gamma- ay spec a measu ed in JET pulse #90753 ( h ee-ion scena io, X[3He]≈0.2–0.4%, ed) and in pulse #91323 ((3He)–H scena io, X[3He]≈1–2%, blue). The e o ba s ep esen he squa e oo o he numbe o coun s in each channel o he spec um and a ise om he unde lying Poisson s a is ics o he gamma- ay de ec ion p ocess. b,c, The JET plasma c oss-sec ion and 19 lines-o -sigh o he neu on/gamma came a. The econs uc ed high-ene gy gamma- ay emission (Eγ=4.5–9.0MeV) isualizes he popula ion o he con ined ene ge ic 3He ions (E[3He]>1–2 MeV). Pulses #90752 (b) and #90753 (c) had a nea ly iden ical plasma composi ion (X[H]≈70–75%, X[3He]≈0.2–0.4%) and RF hea ing powe (PICRH =4.3–4.4MW), excep o he ICRH an enna phasing. A ac o -o - wo inc ease in he γ- ay emissi i y was obse ed in pulse #90753, in which 2MW o RF powe was coupled o he plasma wi h +π/2 phasing (see ex o mo e de ails). h ough 4He +9Be eac ions. Figu e 3a also shows a numbe o cha ac e is ic gamma lines a Eγ>4.44 MeV, o igina ing om ansi ions be ween highe exci ed s a es o 11B and 11C nuclei (p oduc s o 3He +9Be eac ions). The exci a ion e iciency o such high-ene gy le els inc eases by a ac o o en when he ene gy o he p ojec ile 3He ions inc eases om 1 MeV o 2 MeV ( e . 24). Fo compa ison, we also display he γ-spec um eco ded in JET pulse #91323, in which 3He ions (≈1–2%) we e hea ed as a mino i y wi h up o 7.6 MW o ICRH in an almos pu e H plasma (see Supplemen a y Fig. 3). Figu e 3a clea ly shows highe gamma-coun a es o he h ee-ion pulse #90753 (X[3He]≈0.2–0.4%), al hough a ac o o wo less ICRH powe was injec ed in o he plasma. In JET, we u he enhanced he e iciency o as -ion gene a- ion by changing he con igu a ion o ICRH an ennas om dipole o +π/2 phasing. The phasing de ines he dominan kkand he spec- um o emi ed wa es, whe e kkis he wa enumbe pa allel o B. The +π/2 phasing launches wa es p edominan ly in he di ec ion o he plasma cu en wi h ypical alues |k(an ) k|≈3.4m−1, which is wo imes smalle han o dipole phasing (|k(an ) k|≈6.7m−1). Since he wid h o he abso p ion zone scales wi h |kk|, educing i has he ad an age o inc easing he abso bed RF powe pe ion. Fu - he mo e, he +π/2 phasing allows one o exploi he RF-induced pinch e ec , bene icial o localize he ene ge ic ions owa ds he plasma co e25. The esul is clea ly isible in Fig. 3b,c, showing he wo- dimensional omog aphic econs uc ion o he Eγ=4.5–9.0 MeV gamma- ay emission21 o wo compa able h ee-ion hea ing pulses #90752 and #90753. Bo h had a simila edge H/(H+D) a io, a ying om ∼0.84 a he beginning o he pulse o ∼0.75 a he end (X[H] ≈ 68–76%), and X[3He] ≈ 0.2–0.4%. In pulse #90752 (Fig. 3b), all ICRH powe was applied using dipole phasing, while in pulse #90753 (Fig. 3c) abou hal o he ICRH powe (2.1 MW) was launched wi h +π/2 phasing. Ene ge ic 3He ions a e mo e cen ally localized and he numbe o gamma- ay coun s inc eases by a ac o o wo in pulse #90753. The pe iod o he saw oo h oscilla ions also inc eases om ∼0.54 s o ∼0.78 s. We also obse ed exci a ion o Al én eigenmodes (AE) in JET plasmas wi h equencies ≈320–340 kHz in pulses, whe e PICRH ≥2 MW was deli e ed wi h +π/2 phasing. These ins abili ies a e exci ed i a su icien ly la ge numbe o ene ge ic ions wi h eloci ies compa able o he Al én eloci y is p esen in he plasma. Figu e 4a shows he AE dynamics o JET pulse #90758 (p e iously shown in Fig. 2b), wi h a sequen ial exci a ion o modes wi h mode numbe s om n=8 o n=5 du ing a long-pe iod saw oo h. The MHD code MISHKA26 yields eigen equencies (0) AE ≈285–295 kHz o n=5–7 modes in he plasma ame. E en close co espondence o he obse a ions is ob ained when plasma o a ion due o NBI ( o ≈5 kHz measu ed a R≈3.25 m) is aken in o accoun ( (lab) AE = (0) AE +n o ≈320kHz). Fu he analysis o he condi ions o ene ge ic ions o in e ac wi h he n=5 AE mode yields 3He ions wi h ene gies ≈1.5–2.5 MeV. A simila AE ac i i y was also de ec ed in he Alca o C-Mod expe imen s du ing a saw oo h cycle wi h a pe iod ex ended up o ∼40 ms (PICRH = 5 MW). As shown in Fig. 4b, AEs a equencies AE ≈1,270–1,300 kHz (n≈12) we e obse ed 30 ms a e he saw ee h c ash. In e es ingly, he no malized equency a io AE/ A(0)≈0.56–0.61 is simila o he AE modes obse ed on bo h de ices. He e, A(0)= A(0)/2πR0, wi h A(0) he on-axis Al én eloci y. This u he highligh s he simila i y o he h ee- ion hea ing expe imen s on he wo de ices. How many ‘ h ee-ion’ scena ios exis ? These no el scena ios allow g ea lexibili y in he choice o he h ee ion componen s. Table 1 summa izes he (Z/A) alues o usion- ele an ion species. The iso opes o hyd ogen ha e Z/A=1 (p o ons), 1/2 (D ions) and 1/3 (T ions). Fusion plasmas can also con ain 4He and ligh impu i y species, eleased in plasma– wall in e ac ions. In he co e o high- empe a u e plasmas, hose ions (4He, 12C, 16O, and so on) a e ypically ully ionized wi h Z/A=1/2, jus as he D ions. We also no e he iso ope 3He, which has a unique Z/A=2/3. O he ion species such as 9Be4+, 7Li3+,22Ne10+, and so on ha e a Z/A a io in he ange 0.43 and 0.45, and b ing ex a possibili ies. Among hese, be yllium is o pa icula impo ance. Plasmas in JET and he u u e okamak ITER na u ally con ain a small amoun o 9Be impu i ies. Since (Z/A)T< (Z/A)9Be < (Z/A)D,9Be ions can e icien ly abso b RF powe and ans e mos o hei ene gy o D and T ions du ing hei collisional slowing-down, a ea u e pa icula ly a ac i e o 976 © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics NATURE PHYSICS DOI: 10.1038/NPHYS4167 ARTICLES AE equency, AE (kHZ) 360 350 340 330 320 310 n = 6 n = 5 n = 5 n = 6 n = 7 n = 8 n = 7 n = 8 a Te0 (keV) Time, (s) 2 3 4 5 15.2 15.4 15.6 15.8 16.0 16.2 16.4 CPS17.32-4c b AE equency, AE (kHZ) 10−6 10−5 10−4 10−3 10−2 10−1 100 In ensi y ((× 1016 m−2)2 kHz−1) Time, (s) Te0 (keV) 1,250 1,270 1,280 1,300 1,260 1,290 3 4 5 6 7 0.90 0.92 0.94 0.96 0.98 1.00 Figu e 4 | Exci a ion o Al énic eigenmodes in magne ic luc ua ion spec og ams, ano he p oo o he p esence o ICRH-accele a ed as ions. a, JET pulse #90758. b, Alca o C-Mod pulse #1160901023. The e olu ion o he cen al elec on empe a u e Te0 is also plo ed in he bo om pa o he igu es. 3He/4He 4He/H 10 −2 10−1 100 101 10−3 10−2 10−1 100 a 3He/H Es ima ed hyd ogen concen a ion, X[H] (%) 10−4 10−3 10−2 10−1 100 60 65 70 75 80 85 90 95 100 b X[H] ≈ 1/(1 + 2n4He/nH) JET: h ee-ion expe imen s Figu e 5 | Th ee-ion ICRH scena ios also explain some o he obse a ions o ene ge ic ions in space en i onmen s, in pa icula , 3He- ich sola la es. a,4He/H and 3He/4He a ios o 3He- ich sola la es. Da a aken om Table 1 and Fig. 2 o e . 32, including he o iginal e o ba s. The da a cloud wi hin he ed line co esponds o a n4He/nH a io e y simila o ou heo e ical p edic ions o a hypo he ical h ee-ion 4He–(3He)–H scena io a wo k in space plasmas (see ex o mo e de ails). b, The a io n3He/nH=(n3He/n4He)×(n4He/nH), measu ed in he MeV-ene gy ange, e sus H concen a ion es ima ed om X[H]≈1/(1+2n4He/nH) o he same da ase ( e . 32). A la ge 3He enhancemen o he e en s a X[H]≈70–75% is seen. The e o ba s o n3He/nHa e di ec ly aken om Table 1 o e . 32. The e o ba s o he es ima ed H concen a ion a e compu ed using he ela ion be ween X[H] and n4He/nH, and aking he maximum and minimum alues o n4He/nH o a pa icula 3He- ich e en . Table 1 | (Z/A) a io o di e en ion species in usion plasmas. Ion species T 9Be, 7Li, 22Ne D, 4He, 12C, ... 3He H (Z/A)i1/3≈0.43–0.45 1/2 2/31 a usion eac o 10. As ano he example o he h ee-ion echnique, we men ion he obse ed pa asi ic o -axis abso p ion o ICRH powe by 7Li impu i ies in D–T plasmas o he Tokamak Fusion Tes Reac o 27. Low- empe a u e plasmas o e an e en la ge a ie y o scena ios since ligh ion species a e no necessa ily ully ionized. Rele ance o space plasmas As discussed abo e, ion species wi h Z/A=1/2 a e nea ly iden ical o D ions om he wa e p opaga ion poin o iew. The e o e, helium ions (Z=2, A=4) can eplace D. Acco ding o equa ion (2) and Fig. 2b, hyd ogen plasmas addi ionally including 10–17% o 4He ions a e op imal o e ec i e RF powe abso p ion by a small amoun o 3He ions. The p esen ed expe imen al esul s p o ide also an addi ional insigh in o he unde s anding o he 3He- ich sola la es28–30, known o he pas ou decades. These e en s a e cha ac e ized by an anomalously la ge abundance a io 3He/4He ∼1 in he ene gy ange ∼1 MeV/nucleon, compa ed wi h a ypical alue o 3He/4He ∼5×10−4in he sola co ona. The p oposed heo e ical models o explain anomalous 3He-en ichmen gene ally ely on selec i e ene gy abso p ion by hese ions ia wa e in e ac ion mechanisms making use o he unique cha ge- o-mass a io o 3He. Fisk sugges ed p e-hea ing o 3He ions ia elec os a ic ion cyclo on wa es in H–4He plasmas, ollowed by a second-s age accele a ion p ocess28. C ucial in his model o he wa e abso p ion NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. 977 ARTICLES NATURE PHYSICS DOI: 10.1038/NPHYS4167 by 3He ions is also ha ing a plasma mix u e, consis ing o H and 4He ions. On he o he hand, Reames highligh s in his e iew ( e . 30) ha he 3He- ich e en s a e associa ed wi h s eaming 10–100 keV elec ons. He sugges s ha such elec on beams migh be a sou ce o elec omagne ic ion cyclo on wa es. The ad an age o his explana ion is ha elec omagne ic wa es can di ec ly accele a e ions o MeV ene gies, wi hou he need o a seconda y p ocess, which is a se ious simpli ica ion compa ed o he heo y by Fisk. Ro h and Teme in de eloped a single-s age model o he esonan accele a ion o 3He ions o high ene gies, u ilizing elec omagne ic ion cyclo on wa es in H plasmas31. Thei s udy esembles closely he (3He)–H mino i y hea ing in okamaks. Figu e 3a, showing he γ- ay spec um o JET pulse #91323, con i ms gene a ion o MeV- ange 3He ions wi h his scena io in a usion hyd ogen plasma. Figu e 3a also illus a es ha a signi ican ly la ge numbe o high-ene gy 3He ions was gene a ed using he D–(3He)–H h ee- ion scena io unde simila condi ions. Thus, we hypo hesize ha esonan abso p ion o elec omagne ic wa es by a small amoun o 3He ions in H–4He plasmas ( ha is, e ec i ely he h ee-ion 4He–(3He)–H scena io) can be ano he e ec i e mechanism o 3He accele a ion in space plasmas. This p oposal hen combines in one scena io he ad an ages o he heo ies o Fisk and Teme in–Ro h. We ecall ha in JET expe imen s e icien RF powe abso p ion by 3He ions was obse ed in H–D plasmas wi h X[H]≈ 68%–82% (see Fig. 2b and Supplemen a y Figs 5 and 6). Equi alen H–4He mix u es wi h he same H concen a ions should ha e a n4He/nH a io in he ange be ween 0.11 and 0.24. Figu e 5a summa izes he 4He/H and 3He/4He a ios o a num- be o obse ed 3He- ich sola la es, aken om Table 1 and Fig. 2 o e . 32. Rema kably, ou es ima es a e consis en wi h he da a poin s a n4He/nH≈0.1–0.3. This becomes e en clea e i he same da ase is plo ed as a unc ion o he es ima ed hyd ogen concen a- ion X[H] ≈ 1/(1+2n4He/nH) and using he measu ed numbe o ene ge ic 3He ions no malized o he numbe o p o ons, n3He/nH as an indica o o he e iciency o 3He accele a ion. Figu e 5b shows a la ge 3He enhancemen o e en s wi h X[H]≈70–75%, hus p o iding addi ional suppo o ou hypo hesis. Da a a ailabili y. The da a ha suppo he plo s wi hin his pape and o he indings o his s udy a e a ailable om he co esponding au ho upon easonable eques . Recei ed 19 Decembe 2016; accep ed 9 May 2017; published online 19 June 2017 Re e ences 1. Ongena, J. e al. Magne ic-con inemen usion. Na . Phys. 12, 398–410 (2016). 2. Adam, J. Re iew o okamak plasma hea ing by wa e damping in he ion cyclo on ange o equency. 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Ramadu ai, S. e al.3He- ich sola la es. P amana – J. Phys. 23, 305–311 (1984). 33. Li audon, X. e al. O e iew o he JET esul s in suppo o ITER. Nucl. Fusion (in he p ess). Acknowledgemen s This pape is dedica ed o he la e P. E. M. Vandenplas, ounde and i s di ec o o LPP-ERM/KMS, in ecogni ion o his li elong ou s anding commi men o usion esea ch, in pa icula o ICRH. The suppo om he JET and Alca o C-Mod Teams is wa mly acknowledged. We a e g a e ul o A. Ca dinali, C. Cas aldo, R. Dumon , J. E iksson, T. Fülöp, C. Gi oud, C. Hellesen, S. Menmui and M. Schneide o ui ul discussions. This wo k has been ca ied ou wi hin he amewo k o he EURO usion Conso ium and has ecei ed unding om he Eu a om esea ch and aining p og amme 2014–2018 unde g an ag eemen no. 633053. The iews and opinions exp essed he ein do no necessa ily e lec hose o he Eu opean Commission. This wo k was also suppo ed by he US DoE, O ice o Science, O ice o Fusion Ene gy Sciences, SciDAC Cen e o Simula ion o Wa e Plasma In e ac ions unde DE-FC02-01ER54648 and he Use Facili y Alca o C-Mod unde DE-FC02-99ER54512. The Alca o C-Mod Team au ho lis is ep oduced om e . 12. The JET Con ibu o s au ho lis is ep oduced om e . 33. Au ho con ibu ions All au ho s ha e con ibu ed o he publica ion, being a iously in ol ed in he design o he expe imen s, in unning he diagnos ics, acqui ing da a and inally analysing he p ocessed da a. Addi ional in o ma ion Supplemen a y in o ma ion is a ailable in he online e sion o he pape . Rep in s and pe missions in o ma ion is a ailable online a www.na u e.com/ ep in s. Publishe ’s no e: Sp inge Na u e emains neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a ilia ions. Co espondence and eques s o ma e ials should be add essed o Ye.O.K. Compe ing inancial in e es s The au ho s decla e no compe ing inancial in e es s. 978 © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. All igh s ese ed. NATURE PHYSICS | VOL 13 | OCTOBER 2017 | www.na u e.com/na u ephysics NATURE PHYSICS DOI: 10.1038/NPHYS4167 ARTICLES Alca o C-Mod Team E. S. Ma ma 1, S. G. Baek1, H. Ba na d1, P. Bonoli1, D. B unne 1, J. Candy2, J. Canik3, R. M. Chu chill4, I. Cziegle 5, G. Dekow1, L. Delgado-Apa icio4, A. Diallo4, E. Edlund4, P. Enne e 1, I. Faus 1, C. Fio e1, Chi Gao1, T. Gol inopoulos1, M. G eenwald1, Z. S. Ha wig1, C. Holland5, A. E. Hubba d1, J. W. Hughes1, I. H. Hu chinson1, J. I by1, B. LaBomba d1, Yijun Lin1, B. Lipschul z6, A. Loa e7, R. Mumgaa d1, R. R. Pa ke 1, M. Po kolab1, M. L. Reinke6, J. E. Rice1, S. Sco 4, S. Shi aiwa1, P. Snyde 2, B. So bom1, D. Te y1, J. L. Te y1, C. Theile 8, R. Viei a1, J. R. Walk1, G. M. Wallace1, A. Whi e1, D. Why e1, S. M. Wol e1, G. M. W igh 1, J. W igh 1, S. J. Wuki ch1and P. Xu1 1Plasma Science and Fusion Cen e , Massachuse s Ins i u e o Technology, Camb idge, Massachuse s, USA. 2Gene al A omics, San Diego, Cali o nia, USA. 3Oak Ridge Na ional Labo a o y, Oak Ridge, Tennessee, USA. 4P ince on Plasma Physics Labo a o y, P ince on, New Je sey, USA. 5Cen e o Ene gy Resea ch, Uni e si y o Cali o nia San Diego, San Diego, Cali o nia, USA. 6Depa men o Physics, Uni e si y o Yo k, Yo k, USA. 7Plasma Ope a ions Di ec o a e, ITER O ganiza ion, S . Paul lez Du ance, F ance. 8TCV Tokamak Physics, Cen e de Reche ches en Physique des Plasmas, Lausanne, Swi ze land. JET Con ibu o s S. Abdualle 39, M. Abhangi46, P. Ab eu53, M. A zal7, K. M. Agga wal29, T. Ahlg en101, J. H. Ahn8, L. Aho-Man ila112, N. Aiba69, M. Ai ila112, R. Albanese105, V. Ald ed7, D. Aleg e93, E. Alessi45, P. Aleyniko 55, A. Al ie 12, A. Alksee 72, M. Allinson7, B. Alpe 7, E. Al es53, G. Amb osino105, R. Amb osino106, L. Amicucci90, V. Amoso 88, E. Ande sson Sundén22, M. Angelone90, M. Anghel85, C. Angioni62, L. Appel7, C. Appelbee7, P. A ena30, M. A iola106, H. A nichand8, S. A shad41, A. Ash7, N. Ashikawa68, V. Aslanyan64, O. Asun a1, F. Au iemma12, Y. Aus in7, L. A o ina103, M. D. Ax on7, C. Ay es7, M. Bacha is24, A. Bacie o57, D. Baião53, S. Bailey7, A. Bake 7, I. Balboa7, M. Balden62, N. Balshaw7, R. Bamen 7, J. W. Banks7, Y. F. Ba ano 7, M. A. Ba na d7, D. Ba nes7, M. Ba nes27, R. Ba nsley55, A. Ba on Wiechec7, L. Ba e a O e34, M. Ba uzzo12, V. Basiuk8, M. Bassan55, R. Bas ow7, A. Ba is a53, P. Ba is oni90, R. Baughan7, B. Bau i 55, L. Baylo 73, B. Bazyle 56, J. Beal110, P. S. Beaumon 7, M. Becke s39, B. Becke 7, A. Becoule 8, N. Bek is35, M. Beldishe ski7, K. Bell7, F. Belli90, M. Bellinge 7, É. Belonohy62, N. Ben Ayed7, N. A. Ben e man7, H. Be gsåke 42, J. Be na do53, M. Be ne 62, M. Be y7, L. Be alo 55, C. Besliu7, M. Beu skens63, B. Bieg61, J. Bielecki47, T. Biewe 73, M. Bigi12, P. Bílko á50, F. Binda22, A. Biso i31, J. P. S. Biza o53, C. Bjö kas101, J. Blackbu n7, K. 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Zycho 65 1Aal o Uni e si y, PO Box 14100, FIN-00076 Aal o, Finland. 2Aix Ma seille Uni e si é, CNRS, Cen ale Ma seille, M2P2 UMR 7340, 13451 Ma seille, F ance. 3Aix-Ma seille Uni e si é, CNRS, IUSTI UMR 7343, 13013 Ma seille, F ance. 4Aix-Ma seille Uni e si é, CNRS, PIIM, UMR 7345, 13013 Ma seille, F ance. 5A izona S a e Uni e si y, Tempe, USA. 6Ba celona Supe compu ing Cen e , Ba celona, Spain. 7CCFE, Culham Science Cen e, Abingdon, Oxon OX14 3DB, UK. 8CEA, IRFM, F-13108 Sain Paul Lez Du ance, F ance. 9Cen e o Ene gy Resea ch, Uni e si y o Cali o nia a San Diego, La Jolla, Cali o nia 92093, USA. 10Cen o B asilei o de Pesquisas Fisicas, Rua Xa ie Sigaud, 160, Rio de Janei o CEP 22290- 180, B azil. 11Conso zio CREATE, Via Claudio 21, 80125 Napoli, I aly. 12Conso zio RFX, co so S a i Uni i 4, 35127 Pado a, I aly. 13Daegu Uni e si y, Jillyang, Gyeongsan, Gyeongbuk 712-174, Republic o Ko ea. 14Depa amen o de Física, Uni e sidad Ca los III de Mad id, 28911 Leganés, Mad id, Spain. 15Depa men o Applied Physics UG (Ghen Uni e si y), S -Pie e snieuws aa 41 B-9000 Ghen , Belgium. 16Depa men o Ea h and Space Sciences, Chalme s Uni e si y o Technology, SE-41296 Go henbu g, Sweden. 17Depa men o Elec ical and Elec onic Enginee ing, Uni e si y o Caglia i, Piazza d’A mi 09123 Caglia i, I aly. 18Depa men o Expe imen al Physics, Facul y o Ma hema ics, Physics and In o ma ics Comenius Uni e si y Mlynska dolina F2, 84248 B a isla a, Slo ak Republic. 19Depa men o Ma e ials Science, Wa saw Uni e si y o Technology, PL-01-152 Wa saw, Poland. 20Depa men o Nuclea and Quan um Enginee ing, KAIST, Daejeon 34141, Ko ea. 21Depa men o Physics and Applied Physics, Uni e si y o S a hclyde, Glasgow G4 ONG, UK. 22Depa men o Physics and As onomy, Uppsala Uni e si y, SE-75120 Uppsala, Sweden. 23Depa men o Physics, Chalme s Uni e si y o Technology, SE-41296 Go henbu g, Sweden. 24Depa men o Physics, Impe ial College London, SW7 2AZ, UK. 25Depa men o Physics, SCI, KTH, SE-10691 S ockholm, Sweden. 26Depa men o Physics, Uni e si y o Basel, Swi ze land. 27Depa men o Physics, Uni e si y o Ox o d, OX1 2JD, UK. 28Depa men o Physics, Uni e si y o Wa wick, Co en y, CV4 7AL, UK. 29Depa men o Pu e and Applied Physics, Queens Uni e si y, Bel as BT7 1NN, UK. © 2017 Macmillan Publishe s Limi ed, pa o Sp inge Na u e. 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