PAPER • OPEN ACCESS
Expe imen al cha ac e is ics o los as nega i e
ions on EAST okamak
To ci e his a icle: Z.X. Zhang
e al
2025
Nucl. Fusion
65 046001
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Nuclea Fusion
Nucl. Fusion 65 (2025) 046001 (12pp) h ps://doi.o g/10.1088/1741-4326/adb7ee
Expe imen al cha ac e is ics o los as
nega i e ions on EAST okamak
Z.X. Zhang1,2, J. Huang1,∗, J.F. Chang1,∗, C.R. Wu3, J. Galdon-Qui oga4,
A. Snicke 5,6, M. Isobe7,8, K. Ogawa7,8, K.Y. He1, S.S. Wang9, Y.X. Sun1,2, X.H. Wang1,2
and he EAST Team1,a
1Ins i u e o Plasma Physics, He ei Ins i u es o Physical Science Chinese Academy o Sciences, He ei
230031, China
2Uni e si y o Science and Technology o China, He ei 230026, China
3Shiyan Key Labo a o y o Quan um In o ma ion and P ecision Op ics, School o Ma hema ics, Physics
and Op oelec onic Enginee ing, Hubei Uni e si y o Au omo i e Technology, Shiyan 442002, China
4Depa men o A omic, Molecula and Nuclea Physics, Uni e si y o Se ille, E-41012 Se ille, Spain
5Depa men o Applied Physics, Aal o Uni e si y, PO Box 11100, Helsinki FI-00076 AALTO, Finland
6VTT Technical Resea ch Cen e o Finland L d, Ki imiehen ie 3, Espoo, Finland
7Na ional Ins i u e o Fusion Science, Na ional Ins i u es o Na u al Sciences, Toki 509-5292, Japan
8SOKENDAI (The G adua e Uni e si y o Ad anced S udies), Toki 509-5292, Japan
9Qingdao Si ang SRI In ellec ual Technology Co., L d, Qingdao 266031, China
E-mail: [email p o ec ed] and [email p o ec ed]
Recei ed 12 Ma ch 2024, e ised 29 No embe 2024
Accep ed o publica ion 19 Feb ua y 2025
Published 3 Ma ch 2025
Abs ac
I is he i s ime ha as nega i e-ion loss signals a e obse ed by Fas -ion Loss De ec o
(FILD) and analyzed in EAST neu al beam injec ion expe imen . Veloci y-space dis ibu ions
o los as nega i e and posi i e ions econs uc ed by he FILDSIM code a e basically
consis en . O bi analysis back acked by he ASCOT code shows ha he as nega i e ions a e
gene a ed a he low- ield side (LFS) sc ape-o laye , whe eas he as posi i e ions a e
gene a ed nea he las closed lux su ace (LCFS) a bo h he high- and LFSs. The exis ence o
as nega i e-ion loss indi ec ly sugges s a e y high neu al pa icle densi y nea bo h he main
limi e and an enna limi e . The di e ing bi h loca ions o as posi i e and nega i e ions esul
in dis inc loss beha io s du ing ELM H-mode and n=3 o a ing RMP expe imen s. The
asymme y o neu al pa icle densi y in okamak sugges ha p e ious s udies may ha e
unde es ima ed he ac ion o cha ge exchange (CX) losses due o he assump ion o a uni o m
neu al pa icle densi y ou side he LCFS. The e o e, simula ions e alua ing he CX loss
ac ion in he u u e should accoun o a h ee-dimensional dis ibu ion o neu al pa icle
densi y, including local gas pu ing and he neu al pa icle densi y a ound a ious limi e s.
Keywo ds: as nega i e-ion loss, EAST okamak, FILD, eloci y-space dis ibu ion
(Some igu es may appea in colou only in he online jou nal)
aSee Gong e al 2024 (h ps://doi.o g/10.1088/1741-4326/ad4270) o he
EAST Team.
∗Au ho s o whom any co espondence should be add essed.
O iginal con en om his wo k may be used unde he e ms
o he C ea i e Commons A ibu ion 4.0 licence. Any u -
he dis ibu ion o his wo k mus main ain a ibu ion o he au ho (s) and he
i le o he wo k, jou nal ci a ion and DOI.
1741-4326/25/046001+12$33.00 P in ed in he UK 1 © 2025 The Au ho (s). Published by IOP Publishing L d on behal o he IAEA
Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
1. In oduc ion
Good con inemen o as ions plays an impo an ole in
usion powe gene a ion o u u e usion eac o s, such as
ITER and CFETR. Howe e , as -ion losses can occu due o
a ious issues, such as MHD ins abili ies [1], esonan mag-
ne ic pe u ba ions [2,3], o oidal ield ipple [4], and edge-
localized modes (ELMs) [5], e c. Fas -ion losses no only
deg ade he co e pe o mance, bu also could cause damage
o plasma- acing componen s (PFCs). Scin illa o -based Fas
Ion Loss De ec o (FILD) is a c ucial diagnos ic and widely
used o in es iga e as -ion losses in many magne ic con ine-
men de ices, including JET [6], DIII-D [7], ASDEX Upg ade
[8], KSTAR [9], NSTX [10], MAST-U [11], HL-2A [12], LHD
[13] and W7-X [14]. I can p o ide bo h eloci y-space dis i-
bu ion (pi ch angle and gy o adius) and absolu e luxes o as -
ion losses [15]. Including he measu emen esul s o he FILD
de ec o , he ull-o bi codes such as ASCOT [16] and SPIRAL
[17] ha e been used o ack he los as ions o unde s and he
loss mechanism [2,18]. FILD has also been applied on EAST
o in es iga e he as -ion loss beha io in neu al beam injec-
ion (NBI) plasma [19], wi h he co ec ness o he eloci y-
space dis ibu ion o as -ion losses analyzed by FILDSIM and
ASCOT [20].
In his pape , using FILD he los as posi i e and nega i e
ions a e simul aneously de ec ed and dis inguished o he i s
ime wi h he de ailed analysis shown in he con en . He e, he
gene a ion o as nega i e ions may be a ibu ed o cha ge
exchange (CX) eac ions. Fas posi i e ions can unde go a
CX eac ion wi h backg ound neu als, he eby gene a ing as
neu als (D+
+M→D0
+M+, whe e D+
ep esen s as pos-
i i e ions, Mdeno es backg ound neu als, and D0
e e s o
as neu als) [21]. The as neu als unde go second-s ep CX
eac ions ei he wi h backg ound cha ged pa icles o become
as posi i e ions (D0
+D+
s→D+
+D0
s, whe e D+
s ep esen s
backg ound cha ged pa icles) [22–26], o wi h backg ound
neu al pa icles o gene a e as nega i e ions (D0
+M→
D−
+M+, whe e D−
ep esen s as nega i e ions). The mul i-
s ep CX p ocess esponsible o gene a ing he as nega i e
ions may be analogous o he mul i-s ep in e ac ions be ween
sola wind and backg ound neu als om plane s and come s
in space physics [27,28].
The pape is o ganized as ollows: sec ion 2desc ibes he
expe imen al se up and he p inciple o obse ing he loss o
as nega i e ions by FILD. Sec ion 3p esen s he eloci y-
space dis ibu ion o he los as nega i e ions obse ed in he
NBI expe imen . Sec ion 4p esen s he bi h loca ion and o i-
gin o as posi i e and nega i e ions de e mined h ough o bi
analysis. Sec ion 5discusses he in luence o he bi h loca ion
o as posi i e and nega i e ions on hei loss beha io in he
expe imen s. Finally, in sec ion 6we p esen conclusions and
u u e wo k.
2. Expe imen al se up and me hods
EAST is equipped wi h ou neu al beam injec o s (NBIs)
which use deu e ium as he wo king gas o auxilia y hea ing
Figu e 1. Layou o he auxilia y hea ing sys em on EAST, showing
he ela i e posi ion o he FILD diagnos ic ( op iew).
[29]. The NBI sys em consis s o beamlines NBI1 loca ed
a he A po , and NBI2 loca ed a he F po , as shown in
igu e 1. Beamlines NBI1R and NBI2L p o ide pe pendicu-
la injec ions in he co-Ipand c -Ipdi ec ions, espec i ely,
while NBI1L and NBI2R a e angen ial injec ions in he co-
Ipand c -Ipdi ec ions, espec i ely. The main limi e (ML) is
ins alled be ween he G and H po s o p e en con ac be ween
he co e plasma and he acuum wall. Addi ionally, limi e s a e
ins alled on he ho izon al sides o he LHW an ennas (E and
N po s) and he ICRF an ennas (B and I po s) o shield he
an ennas om excessi e he mal loads.
The scin illa o -based FILD is used o de ec los as ions
and is ins alled on he J po o he EAST okamak [19]. In he
poloidal c oss-sec ion, he FILD is loca ed nea he EAST mid-
plane a z∼0.224 m. The FILD consis s o six main compon-
en s: he exchange box, long sha , p obe head, elay op ical
sys em (ROS), CCD sys em, and PMT sys em, as illus a ed
in igu e 2(a). A acuum isola ion al e is posi ioned be ween
he exchange box and he po lange, allowing he scin illa o s
in he p obe o be eplaced wi hou comp omising he in eg i y
o he p ima y acuum. The long sha connec s he p obe o
he d i e mo o s, which include bo h linea and o a y ypes
ha con ol he adial mo emen (>2 m) and o a ional mo ion
a ound i s axis, espec i ely. The p obe is p ima ily composed
o a s ainless-s eel shield, a scin illa o (4 cm ×4 cm, ZnS:Ag,
decay ime: 70 ns, emission peak: 450 nm), and collima o s.
The scin illa o is moun ed on he ea side o he s ainless-
s eel shield, and he p obe is blackened o minimize he impac
o in e nal su ace e lec ions on he signal. In o de o enhance
he collec ion e iciency o he ligh emission o scin illa o , a
ROS is employed. The op ical sys em includes se en lenses,
h ee o which a e ield lens g oups. The e ec i e ocal leng h
o he ROS is 358.4 mm, wi h a dis ance o 680 mm be ween
he scin illa o plane and he i s lens. The ligh emission pa -
e n o scin illa o is ansmi ed h ough he op ical pa h in
he long sha o he op ical spli e , whe e hal o he ligh
2
Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 2. (a) Schema ic o he FILD diagnos ic on EAST, showing
he elay op ical sys em, which includes op ical 1, op ical 2, op ical
3, and he op ical spli e . (b) Co espondence be ween he CCD
image o he ligh emission plane o scin illa o and he 25 op ical
ibe a ays.
Figu e 3. Fas posi i e ions (in g een lines) and nega i e ions (in
ed lines) in he e -B ope a ing egime. (a) Schema ic o he en y
o as ions in o he FILD p obe head h ough he collima o (in
blue). (b) Illus a ion o as ions s ike he scin illa o (in whi e).
is e lec ed o he CCD sys em and he o he hal is di ec ed
o he PMT sys em. The CCD sys em uses a high-speed C-
MOS ype Phan om V2010-96GB came a (1280 ×800 pixels)
om Vision Resea ch, Inc. The PMT sys em employs a mul i-
channel PMT a ay (Hamama su Pho onics, No. H10492-
003, sampling a e: 2 MHz) o synch onously collec sig-
nals, wi h each da a acquisi ion sys em ope a ing independ-
en ly. The ligh emission pa e n o scin illa o is collec ed
by a uni o mly dis ibu ed a ay o 25 op ical ibe s (5 ×5),
which a e hen ansmi ed o each PMT subsys em [30].
The co espondence be ween he ligh emission pa e n o
scin illa o and he 25 op ical ibe a ay is illus a ed in
igu e 2(b).
The FILD has a p obe head geome y wi h op and bo -
om collima o s, allowing i o de ec bo h los as posi i e
and nega i e ions. As shown in igu es 3(a) and (b), in he
e -B (wi h Ipand B di ec ed coun e clockwise om he op
iew) ope a ing egime [31], los as nega i e ions a e expec-
ed o s ike he scin illa o in he FILD p obe head h ough he
op collima o , while as posi i e ions a e expec ed o s ike
h ough he bo om collima o . In con as , los as nega i e
ions a e expec ed o s ike he scin illa o h ough he bo -
om collima o , while as posi i e ions a e expec ed o s ike
h ough he op collima o in he no -B ope a ing egime (wi h
Ipdi ec ed coun e clockwise and B di ec ed clockwise om
he op iew), espec i ely.
3. Cha ac e is ics o los as nega i e ions
Figu es 4(a)–(c) show he ime e olu ion o he line-a e aged
densi y, NBI powe , and gap-ou ( adial wid h o sc ape-o
laye (SOL)) o discha ges #93723 and #94994 on EAST.
The pho omul iplie s (PMTs) signal o he FILD shown in
igu es 4(d) and (e), which can be used o de ec as ion
loss signals a di e en posi ions on he scin illa o . I can
be obse ed ha as ion loss signals a e gene a ed only
du ing he NBI injec ion. Fas nega i e-ion losses ha e been
obse ed in a a ie y o plasmas, co e ing a wide ange o
pa ame e s, including plasma densi y ( om 2.5 ×1019 m−3
o 5 ×1019 m−3), plasma cu en ( om 0.4 MA o 0.5 MA),
and gap-ou ( om 4.5 cm o 7 cm). No ably, he in ens-
i y o as posi i e and nega i e ion loss signals shows a
weak posi i e co ela ion wi h elec on densi y in discha ge
#93723. Fu he mo e, he as posi i e and nega i e ion loss
signals exhibi nega i e and posi i e co ela ions, espec -
i ely, wi h he adial wid h o he SOL, pa icula ly a he
powe s ep ( ∼3.51 s). A de ailed discussion is p o ided in
sec ion 5.
Scin illa o signals om he FILD we e ex ac ed a wo
ime slices om wo di e en sho s o he analysis o as
ion loss cha ac e is ics. The eloci y-space dis ibu ion o
as ion loss was ob ained using FILDSIM code [32] which
inco po a es o bi ajec o ies wi hin he geome y o he
FILD p obe head. The FILDSIM code maps he eloci y-
space g id on o he FILD signals. FILD obse ed wo main
popula ions o beam ion losses du ing NBI2L injec ion, as
shown in igu e 5(a). The esul s indica e ha he pi ch (a c-
cos( /// )∼68◦) and gy o adius (RL∼3 cm) a e nea ly
iden ical o bo h popula ions o beam-ion losses. The beam-
ion popula ion in he uppe hal o he scin illa o co esponds
o as nega i e-ion losses, while he popula ion in he lowe
hal co esponds o as posi i e-ion losses. FILD obse ed
h ee dis inc beam-ion loss popula ions du ing NBI1R injec-
ion, as shown in igu e 5(b). One o he as nega i e-ion
popula ions is consis en wi h he eloci y-space dis ibu ion
o as posi i e-ion losses, as de e mined by pi ch (a ccos(-
/// )∼70◦) and gy o adius (RL∼2.5 cm). Ano he as
nega i e-ion popula ion wi h a pi ch o ∼45◦is expec ed o
co espond o he loss dis ibu ion o as posi i e ions wi h he
same eloci y-space dis ibu ion. Howe e , as posi i e ions
wi h pi ch o ∼45◦canno be de ec ed, as hey a e ou side he
ange o he FILD.
3
Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 4. Time e olu ion o discha ges #94994 and #93723 o (a)
he line-a e aged densi y, (b) NBI powe , (c) gap-ou ( adial wid h
o SOL), (d) as posi i e-ion loss and (e) as nega i e-ion loss.
Figu es 5(c)–( ) show he eloci y-space dis ibu ion o
as nega i e and posi i e ions, ob ained by pe o ming i-
angula in e pola ion on he s ike map. The e ical axis
is con e ed om gy o adius RL o beam ene gy Eusing
E=(e·Z·RL·BFILD)2/(2·MD), whe e Zis he ion cha ge num-
be , BFILD is he magne ic ield a he FILD p obe head, and
MDis he mass o he Da om. As shown in igu es 5(c)
and (e), he ene gy o he los as nega i e and posi i e ions
is app oxima ely 50 keV, which is consis en wi h he main
ene gy o he NBI2L beamline. Figu es 5(d) and ( ) show ha
he ene gy o he los as nega i e ions wi h a pi ch angle o
45◦ma ches he main ene gy o he NBI1R beamline, while
he ene gy o he los as posi i e and nega i e ions wi h a
pi ch angle o 70◦is app oxima ely 65 keV. Addi ionally, he
as ion ene gy in igu es 5(d) and ( ) ex ends up o nea ly
100 keV, due o he limi ed size o he FILD collima o , which
es ic s he esolu ion o he de ec o in he La mo adius
[20,32]. The dis ibu ion measu ed on he scin illa o can be
conside ed a dis o ion o he eloci y space, esul ing om he
ini e esolu ion o he de ec o ape u e. Fas ion loss signals
om he NBI1R beamline may include ene gy componen s o
E/2 and E/3, bu hese canno be dis inguished by he FILD
p obe head.
4. O igin and bi h posi ions o as nega i e ions
4.1. Ini ial deposi ion loca ions o los beam ions
The o igin o he los beam ions can be de e mined by analyz-
ing he cha ac e is ics o he as ions. The cha ac e is ic ime
o he i s -o bi as -ion losses is on he o de o ∼10−5s (by
eco ding he ime aken o as ion losses o he i s wall
using o bi - ollowing code) in EAST. In his imescale, col-
lisions be ween backg ound and as ions can be neglec ed
(τD∼10−3s,ne∼1019m−3,Te∼1keV, τDis deu e ium ion
collision ime [33]). The e o e, ene gy can be ea ed as a con-
se ed quan i y [34]. Mo eo e , he gy omo ion o as ions
ollows he conse a ion o magne ic momen due o he slow
a ia ion o magne ic ields bo h in ime and space. Since bo h
ene gy and magne ic momen a e conse ed du ing he gy o-
mo ion o he as ions, he ela ionship be ween he pi ch θo
he beam-ions losses and he adial ini ial deposi ion posi ion
Rcan be de i ed as ollows:
θ(R) = a c an√1
E
µ(R)BFILD
−1.(1)
Eis he neu al beam ene gy (eV). µ(R) is he dis ibu ion
o he magne ic momen along he adial ini ial deposi ion pos-
i ion o he beam lines, which can be calcula ed by conside ing
he geome y and magne ic ield dis ibu ion o he beamline.
BFILD is he magne ic ield s eng h (T) a he loca ion o he
FILD p obe head.
Based on equa ion (1), he ini ial deposi ion posi ion o he
neu al beam can be de e mined by obse ing he eloci y-
space dis ibu ion using he FILD. Figu e 6(a) shows he ela-
ionship be ween he ini ial deposi ion posi ion o he neu al
beam om di e en injec o s on EAST and he pi ch angle o
beam-ion losses. I can be seen ha beam-ion losses o igina e
om he ini ial deposi ion o he beamlines nea he las closed
lux su ace (LCFS). No ably, beam ions wi h pi ch angles o
∼45◦and ∼70◦, gene a ed by NBI1R, o igina e om he ini-
ial deposi ion o neu al beam on he high- ield side (HFS)
and low- ield side (LFS) o he SOL, espec i ely. Howe e ,
he calcula ion in equa ion (1) does no accoun o he sign
o he o beam ions cha ge, and hus i can only indica e ha
he o igin o as nega i e ions is he neu al beam deposi ed
nea he LCFS. Fu he analysis using ull-o bi ollowing is
equi ed o de e mine he speci ic loca ions whe e he as neg-
a i e ions a e gene a ed. Based on he geome y and beam
ene gy o di e en beamlines, he ini ial pi ch angle o beam
ions can be de e mined as a unc ion o he ini ial deposi ion
loca ion o he neu al beams. The ini ial pi ch o he los beam
ion can be ob ained by calcula ing he adial deposi ion loca-
ions o di e en loss beam-ions using equa ion (1), as shown
in igu e 6(b). The ini ial pi ch o he los beam ions gene a ed
by NBI2L is 73.98 ◦, and he ini ial pi ch o he los beam ions
gene a ed by NBI1R a e 61.48 ◦(HFS) and 70.79 ◦(LFS),
espec i ely.
4
Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 5. S ike map o beam-ion losses measu ed by FILD p obe head wi h (a) #[email p o ec ed] s (b) #[email p o ec ed] s. (c) and (e)
Co espond o he eloci y-space dis ibu ion o los as posi i e and nega i e ions om NBI2L. (d) and ( ) Rep esen he eloci y-space
dis ibu ion o los as posi i e and nega i e ions om NBI1R.
4.2. Compa ison o ajec o ies o as posi i e and nega i e
ions
In o de o in es iga e he bi h loca ion o as nega i e
ions, hei ajec o ies a e acked backwa ds based on hei
eloci y-space dis ibu ion using ASCOT [16], which is a ull-
o bi ollowing code using a Mon e Ca lo app oach. In he
equilib ium magne ic ield, as nega i e and posi i e ions can
be ollowed backwa ds in ime om he FILD p obe posi ion,
hen he ajec o ies and ini ial bi h posi ions o as posi i e
and nega i e ions can be a ained. Ma ke s o igina ing om
he espec i e beam lines we e ixed wi h a single pi ch angle
and main ene gy, as shown in igu e 5(c), whe e he o bi s
a e ollowed backwa ds in ime om he FILD posi ion un il
hey o e lap wi h he co esponding beamlines. Figu es 7(a)
and (c) show ha he as posi i e ions deposi ed by NBI2L a
R∼2.2 m a e apidly los o he FILD p obe head.
Howe e , o as nega i e ions, e e se- ollowing om he
FILD p obe posi ion only back aces i o he i s wall o he
de ice, as shown in igu es 7(b) and (d). The signi ican di -
e ence in he loss ajec o ies be ween as posi i e and neg-
a i e ions is due o he ac ha he di ec ion o he magne ic
ield g adien d i and magne ic ield cu a u e d i eloci y
depends on he cha ge sign o he pa icles. I is clea ha
as nega i e ions canno o igina e om he i s wall, meaning
ha hey mus be p oduced a a loca ion be ween he i s wall
s ike poin and he FILD p obe posi ion. Figu e 7(b) illus-
a es a ypical ajec o y o as posi i e ions ansi ioning
in o as nega i e ions. The as posi i e ions (in g een line,
R=2.2 m, pi ch =73.36◦) mo ion owa d he SOL, whe e
hey unde go CX eac ions wi h backg ound neu al pa icles
a a ce ain posi ion (in ed ci cle), acqui ing wo elec ons and
ans o ming in o as nega i e ions (in blue line). Ul ima ely,
he as nega i e ions a e los o he FILD p obe. By a ying
he CX loca ion o as posi i e ions and subsequen ly ollow-
ing hei ajec o y un il o e lapping wi h he beamline, he
app oxima e o oidal bi h loca ions o as nega i e ion we e
de e mined o be be ween φ∼309◦and φ∼21◦, as shown in
igu e 7(d).
4.3. Compa ison o as nega i e ion ajec o ies o igina ing
om HFS and LFS
To illus a e he p ocess by which neu al beam, ini ially
deposi ed on he HFS, is con e ed in o as nega i e ions los
5
Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 6. (a) The pi ch o he beam ion losses and (b) beam-ion
ini ial pi ch, o igina ed by di e en neu al beam injec o s as a
unc ion o he ini ial deposi ion adial posi ion o he neu al beam.
o he FILD p obe head, an o bi analysis o as nega i e ion
loss o igina ing by NBI1R was conduc ed in he no -B ope a -
ing egime. The popula ion o as nega i e ion loss obse ed
by he FILD wi h a pi ch angle o 45◦and an ene gy o 65 keV
o igina es om he ini ial deposi ion o neu al beam inside he
HFS LCFS, wi h he bi h posi ion loca ed ou side he LFS
LCFS, as shown in igu e 8(a). Figu e 8(c) illus a es he o -
oidal bi h posi ions o as nega i e ions wi h a pi ch angle o
45◦, which a e app oxima ely be ween φ∼297◦and φ∼19◦.
Figu e 8(b) shows he o bi s o as nega i e ion loss wi h a
pi ch angle o 70◦and ene gy o 65 keV, indica ing ha hey
o igina e om he ini ial deposi ion o neu al beam ou side
he LFS LCFS. The app oxima e o oidal bi h loca ions o as
nega i e ions wi h a pi ch angle o 70◦we e de e mined o be
be ween φ∼297◦and φ∼18◦, as shown in igu e 8(d). The
bi h posi ions o he as nega i e ions a e also loca ed ou side
he LFS LCFS, bu he o oidal bi h posi ions a e close o
he FILD o oidal posi ion. Thus, i is impe a i e o conside
he ini ial deposi ion o neu al beam in he SOL in o de o
accu a ely alida e he eloci y-space dis ibu ion o bo h as
posi i e and nega i e ions measu ed by he FILD. I should
be no ed ha he po en ial bi h loca ions o as nega i e ions
gene a ed by NBI1R a e con ined o a speci ic egion be ween
he FILD p obe posi ion and he i s wall s ike poin (such
as he di ision o he as nega i e ion ajec o y in o blue and
g ay lines in igu e 8(a)). This limi a ion a ises om he ac
ha when he CX posi ion o he as posi i e ions alls wi hin
he g ay line egion, hey a e unable o ollow o he posi ion
o e lapping wi h he beamline.
5. Discussion
When back acking o he posi ion whe e he as ions o e -
lap wi h he beamline, he de e mined ini ial deposi ion posi-
ions and pi ch angles a e gene ally consis en wi h hose cal-
cula ed based on he conse a ion o ene gy and magne ic
momen , u he alida ing he accu acy o he o bi analysis.
This also implies ha ega dless o he no -B o e -B ope -
a ing egimes, o whe he he as nega i e ions o igina e om
he HFS o he LFS, he FILD can only obse e as nega i e
ions bi h in he SOL nea he FILD p obe head (i.e. ML (φ
∼325◦) and ICRF limi e (φ∼350◦and φ∼10◦) in EAST).
The possible ac ha he limi e s a e loca ed close o he
LCFS esul in highe neu al pa icle densi y nea hem.
As desc ibed in sec ion 3, igu e 4demons a es ha he
in ensi y o as posi i e-ion loss signals shows a posi i e co -
ela ion wi h elec on densi y and a nega i e co ela ion wi h
he adial wid h o he SOL in discha ge #93723, which can
be a ibu ed o wo main ac o s. Fi s ly, lowe elec on dens-
i y leads o a highe ac ion o neu al beam deposi ion in
he co e, he eby educing he as posi i e-ion loss ac ion
[29]. Secondly, he inc easing o as posi i e-ion losses may
be a ibu ed o he educ ion o SOL adial wid h and he sim-
ul aneous NBIs (NBI1R +NBI1L) a =3.51 s, wi h he
o me b inging as posi i e ions close o he PFCs nea he
LFS LCFS and he la e inc easing he densi y o as ions di -
ec ly. On he con a y, he in ensi y o as nega i e-ion loss
signals is posi i ely co ela ed wi h bo h elec on densi y and
he adial wid h o he SOL. Figu es 9(a) and (b) show he
c oss sec ions o he eac ion p ocesses ha gene a e as neg-
a i e ions ( he ‘sou ce’ p ocesses) and hose ha annihila e as
nega i e ions ( he ‘sink’ p ocesses), espec i ely, as lis ed in
able 1. The eac ion a es (R) o he ‘sou ce’ and ‘sink’ p o-
cesses a e posi i ely co ela ed wi h he densi ies (n) o he
eac ing species. A dec ease in plasma densi y weakens he
‘sou ce’ p ocess, as he densi y o as posi i e ions in he SOL
dec eases. The educ ion in he adial wid h o SOL leads o he
mo ion o as nega i e ions in o he LCFS, he eby enhancing
he ‘sink’ p ocess due o inc eased backg ound ion densi y.
Fu he mo e, he educ ion in he adial wid h o SOL esul s in
mo e as posi i e ions s iking he limi e s, which s eng hens
local pa icle ecycling and enhances simul aneously bo h he
‘sou ce’ and ‘sink’ p ocesses due o inc eased local neu al
pa icle densi y. As shown in igu e 9, he c oss sec ion o he
‘sink’ p ocess ( eac ion 8) is signi ican ly highe han hose
o o he ‘sink’ p ocesses and he ‘sou ce’ p ocesses. The es-
ul s sugges ha he ‘sink’ p ocess ( eac ion 8) is he p ima y
cause o he obse ed educ ion in as nega i e-ion loss in ens-
i y, pa icula ly a powe s ep ( =3.51 s). In conclusion, he
in ensi y o as nega i e-ion loss is go e ned by he compe i-
ion be ween ‘sou ce’ and ‘sink’ p ocesses, and he ‘sink’ p o-
cesses seem dominan a powe s ep. The e ec s o SOL adial
wid h and plasma densi y on as nega i e-ion losses will be
u he sys ema ically in es iga ed in u u e expe imen s.
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Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 7. Typical ajec o ies o as posi i e and nega i e ions o igina ing om NBI2L. (a) Schema ic o he ajec o y o as posi i e ions
obse ed by FILD. (b) Schema ic o he ajec o y whe e a as posi i e ion in he SOL con e s in o a as nega i e ion obse ed by FILD.
The g een and blue lines indica e he ajec o ies o he as posi i e and nega i e ions, espec i ely. The ed ci cle indica es he posi ion
whe e as posi i e ions become as nega i e ions. (c) Is op- iew p ojec ion o he ajec o ies shown in (a). (d) Is op- iew p ojec ion o
he ajec o ies shown in (b). G ay a eas indica e possible bi h egions o as nega i e ions.
The los as posi i e ions obse ed by he FILD o igin-
a e nea he high and low ield side o he LCFS, while he
los as nega i e ions a e gene a ed exclusi ely in he low
ield side SOL h ough in e ac ions wi h he los as posi -
i e ions and neu al pa icles. The di e en bi h loca ions es-
ul in dis inc loss beha io s o as posi i e and nega i e ions
in ELMy H-mode expe imen s. Figu es 10(a)–(c) illus a e
he ime ace o Dαemission, line-a e aged densi y and as
ion losses o discha ges #93723 a 4.6–4.7 s. In his pe iod,
q95 =6.35, ELM ∼160 Hz and βN∼1.0. In igu e 10(a),
line-a e aged densi y is obse ed o d op a each ELM c ash.
I was obse ed ha each ELM c ash leads o an enhancemen
in as posi i e ion losses, as shown in igu es 10(b) and (d).
This phenomenon can be explained as he ELM c ash caus-
ing edge-pedes al collapse, u he esul ing in he anspo
o as posi i e ions om he edge-pedes al egion (nea he
FILD p obe head) in o he SOL [36–38]. Consequen ly, he
FILD p obe in he SOL de ec s an enhanced loss signal o
as posi i e ions. Howe e , ELM c ash leads o a educ ion
in as nega i e ion losses, as shown in igu es 10(c) and ( ).
O bi analysis indica es ha he obse ed as nega i e ions
a e gene a ed nea he limi e s on he LFS, ou side he LCFS.
Du ing an ELM c ash, as posi i e ions and o he cha ged
pa icles in he edge-pedes al egion (nea he limi e s) a e
di ec ly los o he limi e s. Consequen ly, he numbe o as
posi i e ions a ailable o gene a e as nega i e ions nea he
limi e s sha ply dec eases (sudden weakening o he ‘sou ce’
p ocesses). The educ ion esul s in a momen a y educ ion o
he as nega i e ion loss signal de ec ed by he FILD p obe.
Be o e he nex ELM c ash, spu e ing o he i s wall by as
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Nucl. Fusion 65 (2025) 046001 Z.X. Zhang e al
Figu e 8. Typical ajec o ies o as nega i e ions o igina ing om NBI1R. (a) Schema ic o he ajec o y whe e as posi i e ions
deposi ed on he HFS con e in o as nega i e ions in he SOL. G ay line indica es he ajec o ies in which he as nega i e ions will no
be bi h, and he same as below. (b) Schema ic o he ajec o y whe e as posi i e ions deposi ed on he LFS con e in o as nega i e ions
in he SOL. (c) Is op- iew p ojec ion o he ajec o ies shown in (a). (d) Top- iew p ojec ion o he ajec o ies shown in (b).
Figu e 9. (a) C oss-sec ion o cha ge exchange and elec on cap u e (‘sou ces’) in able 1. (b) C oss-sec ion o elec on s ipping (‘sinks’)
in able 1. C oss sec ion as a unc ion o he ela i e pa icle ene gy pe nucleon is iden ical o he eac ion p ocesses o hyd ogen iso opes.
A ypical 60 keV EAST deu e ium beam wi h i s ull, hal , and hi d ene gy componen s a e indica ed.
8
