Proceedings of ITC/ISHW2007

Effects of H-mode transition on plasma flow characteristics
in the helical divertor of the Uragan-3M torsatron
CHECHKIN Viktor V., GRIGOR’EVA Lyudmila I., SOROKOVOY Yegor L.,
SOROKOVOY Eduard L., BELETSKII Aleksej A., SLAVNYJ Aleksandr S.,
LAVRENOVICH Yurij S., VOLKOV Yevgenij D., BURCHENKO Pavel Ya.,TSYBENKO Sergej A., LOZIN Aleksej V., KULAGA Anatolij Ye.,
KURILO Dmitrij V., MIRONOV Yurij K., ROMANOV Vladislav S.
Institute of Plasma Physics, National Science Center“Kharkov Institute of Physics and Technology”, Kharkov 61108, Ukraine
In the l=3/m=9 Uragan-3M (U-3M) torsatron with an open helical divertor and RF produced and heated plasmas,effects of H-mode transition on (i) diverted plasma flow (DPF) magnitude in the spacings between the helical coilsand (ii) fast ion escape into DPF are studied by using arrays of plane Langmuir probes and electrostatic ion energy
analyzers. Data have been obtained on how the amount and energy of lost ions change with transition in several field
periods. A strong toroidal nonuniformity in ion loss has been observed. The island structure of the real U-3M magnetic
configuration and the locality of RF power injection into the plasma are considered as possible reasons for suchnonuniformity.
Key words: torsatron, divertor, H-mode, fast ions, ion loss, island structure

1. Introduction
In the Uragan-3M (U-3M) torsatron (Fig. 1,2) with anopen natural helical divertor the plasma is RF produced
and heated (multi-mode Alfven resonance: ω ≲ ωci(0)).
With this, a two-temperature ion energy distribution
arises with a tail of suprathermal ions (Fig. 3) [1]. The
faster ions (higher-temperature and suprathermal ones)
can undergo the neoclassical transport 1/ν.
One more U-3M feature is a spontaneous transition toan H-like mode (Fig. 4) where an edge Еr×B velocity
shear occurs (or is enhanced), resulting in suppression ofturbulence and turbulence-induced anomalous transport.
It is supposed [2], that the fast ion orbit loss is
responsible for Еr×B shear formation [3]. Also, thespecific for stellarators drift-orbit transport flux driven byhelically trapped ions [4] can contribute to this process.
We study effects of H-transition on (i) diverted plasma
flow (DPF) magnitude in the spacings between the helicalcoils and (ii) fast ion escape into DPF. Data have beenobtained on how the amount and energy of lost ionschange with transition in several field periods. A strong
toroidal nonuniformity in ion loss has been observed. Theisland structure of the real U-3M magnetic configurationand the locality of RF power injection into the plasma areconsidered as possible reasons for such nonuniformity.

2. Experimental conditions
Main parameters of U-3M:l = 3, m = 9, R = 1 m; a ≈ 0.12 m
ι( a ) ≈ 0.3; Bφ = 0.72 T
PRF≲0.2 MW, ne ~ 1018 m-3
Te(0) ~ 500 eV

author’s e-mail:chechkin@ipp.kharkov.ua

D2A2A3

D1naten
An

A1

1

D3III

III

D9A4II
A9

D4I
III

A5

D8A8III

I12II

11

D5A6

D7A7

D6Fig. 1. Helical coils I, II, III; indicated are symmetric
poloidal cross-sections A and D in all field periods 1-9(A1, D1, A2, D2,..., A9, D9).
1 – array of electrostatic ion energy analyzers (IEAs) inthe top spacing D5 (D5 top)
2 – IEAs in A6 top
3 – IEAs in D6 top
4,5,6 – divertor electric probe arrays in A7 (see Fig. 2)
7,8,9 – divertor electric probe arrays in D7 (see Fig. 2)
10 – IEAs near D7 top
11 – IEAs in A8 top
12 – CXN energy analyzer detecting neutrals from thespacing between A8 top and D8 top

Proceedings of ITC/ISHW2007

Fig. 2. Disposition of divertor electric probe arraysin the poloidal cross-sections A7 (left) and D7 ( right).

means that the fast ion outflow is also reduced [5,6] as apart of the total particle loss reduction.
In D7 (Fig. 5, right column) DPF increases on the ion∇В drift side (top spacing, (b)) and outboard spacing overthe midplane (d)), while it decreases in the bottom
spacing (c) and in the outboard spacing under themidplane (e), i.e., on the electron drift side.
The DPF vertical asymmetry in D7 increases after thetransition: from α ≈ 5 to α ≈ 9 for the top and bottom
spacings and from α ≈ 3 to α ≈ 4 for the top and bottom
legs in the outboard spacing. This indicates a rise of fast
ion loss in this cross-section after the transition [5, 6].
_ne (1012 cm-3)

216Ti2 = 326 eV0500 1000 1500 2000 2500 3000 3500 4000

perpendicular energy, eV

1_ne (1012 cm-3)

10

22030

405060(b)1020 203040

1020300.8405060(c)

50(c)

0.4
0.2
0.0
2

1020304050(d)05102030

40503(e) high energy CXN flux, W⊥=1350 eV10102030405040(c)5D7 bottom030 20304030(d)40

D7 outboard over midplane015 20304010(d)20

A7 inboard
30time (ms)

40

50(e)

D7 outboard under midplane
2030time (ms)40

Fig. 5. H-transition-induced changes in DPF magnitudein cross-sections A7 (left column) and D7 (right column)

60423060

2nd harmonic ECE
1D7 top

015 20

10

A7 bottom

010 20edge floating potential

0.6(b)20

501.0

ECE (a.u.)А7 top510

line-averaged electron density3Гn (a. u.)(b)

10Is (mA)

IRF (a. u.)

RF antenna current(a)4010151504Vf (V)4020Fig. 3. CXN energy spectrum corresponds to twotemperature ion energy distribution + supratermal tail.

03030

5Is (mA)

0line-averaged density

030 2025

Is (mA)

410(a)

1Is (mA)

STIIs (mA)_ne (1012 cm-3)

28line-averaged density

015 20Is (mA)__(a)112Is (mA)ln( 1E dd nE ) (a.u.)

Ti1 = 56 eV

60time (ms)Fig. 4. Some indications of H-mode transition

3. H-mode-induced DPF changes (field period 7)
The H-transition results in substantial changes in theDPF magnitude estimated as maximum ion saturation
current Is in the corresponding spacing between thehelical coils (Fig. 5).
In A7 (Fig. 5, left column) Is decreases in all spacings
while ne increases, evidencing a reduction of particle loss
in this poloidal cross-section. The vertical DPF
asymmetry decreases with transition: its degree is α ≈ 3
before transition and drops to α ≈ 2 after transition. This4 . Energies of ions outflowing to DPF
Cross-section A8, top spacing.
It follows from Fig. 6 that the fast ion loss roughly
follows fast ion content in the confinement volume(represented by Γn) and seems not to rise with transition.
The same is in A6 top.
The energy of ions escaping to DPF on the ion ∇B drift
side in A8 top (Fig. 7) and A6 top do not change withtransition and are minimum compared with D5 top, D6
top, D7 top (see below).
Cross-section D5, top spacing
Similar to A8 (and A6), the fast ion loss in D5 top
roughly follows Γn and seems not to increase withtransition (Fig. 8). The energies of ions escaping to DPF
on the ion ∇B drift side considerably exceed those in A8

Proceedings of ITC/ISHW2007

Also opposite to D5 top and D6 top, the energies ofescaping ions on the ion ∇B drift side increase withtransition (Fig. 11).
Effects of magnetic field reversal

1(a)line-averaged densityГn (a. u.)

02 20

30

1.0mc
0

Ii(U)/Ii(0)_ne (1012 cm-3)

240(b)

130

0.5

30

40

5060

A8 TOPTop spacing A8Ii (mA)

Ii (mA)
0.0

40050 100 150 200 250 300 350 400 450 500retarding voltage, U (V)U=400 V(c)Ii (a. u.)

20

1.6

high-energy CXN flux

01 20

100.8

0.4

0.40Ii(U)/Ii(0) to the collector of IEA
203040versus retarding voltage U in thetime (ms)
top spacing A8.
↑ Fig. 6. Time evolution of density ne (a), high energy

0.8200400(b)

1.2

0.8

↑ Fig. 7. Normalized ion current

fast ions in A8 top(a)1.2← Fig. 12.

1.6

0200

4000CXN flux Γn (b) and fast ion flow outflowing to DPF Ii (c)in the vicinity of H-transition in A8 top.

D7 TOP
0.8(a)

0.4

0.4

0.2

0.20(b)

0.6

0.6

200

4000200

400_ne (1012 cm-3)2(a)line-averaged density30

0.4
Ii(U)/Ii(0)Гn (a. u.)

03 20

40

0.5ms(b)high-energy CXN flux

Ii (a. u.)

02 20

30

00.0

40

030

30

405050 100 150 200 250 300 350 400 450 500
retarding voltage, U (V)40time (ms)↑ Fig. 8. Same as in Fig. 6 for the top spacing D5

Гn (a. u.)

02 20

130(b)
high energy CXN flux

Ii (a. u.)430(c)

340

U=500 V

fast ionsin D7 top

212030time (ms)40mc0102030

40

50

60

0.5Top spacing D7
0.005 20040

1.0

Ii(U)/Ii(0)_ne (1012 cm-3)2(a)line-averaged density

00.2

0.1

0.1
200

U (V)

400(b)

0.3

0200

400

U (V)

The Ii versus U plots measured at opposite directions ofthe magnetic field demonstrate that the fast ion loss
dominates on the ion ∇B drift side, being at least partiallya real reason for the observed DPF vertical asymmetry ina heliotron/torsatron device like U-3M [5,6].

6. Summaries and discussions(and A6) but also do not change with transition (Fig.9).
The same is in the top spacing D6.
Top spacing in a cross-section close to D7 (∆φ = 50).

10.4

0.2
060

↑Fig. 9. Same as in Fig. 7 forthe top spacing D5.

(c)fast ions in D5 top
2020Fig.8

U=500 V

1010(a)0.3

Top spacing D521(a) – beforetransition;(b) – aftertransition

D5 TOP

1.01○ – positivefield (ion ∇Bdriftupward);, - negativefield

50 100 150 200 250 300 350 400 450 500
retarding voltage, U (V)↑Fig. 11. Same as in Fig.7 forthe top spacing of a crosssection close to D7.

↑ Fig. 10. Same as in Fig. 6 for the top spacing of across-section close to D7.
Opposite to D5 top (Fig. 8) and D6 top, in the vicinityof D7 top the fast ion loss increases with transition (Fig.
10c) synchronously with a decrease of fast ion content inthe confinement volume (Fig. 10b).

The DPF characteristics such as flow magnitude, itsvertical asymmetry, energy of ions in DPF are distinct bya considerable toroidal nonuniformity both in the initialstate (i.e, before transition) and by the character ofchanges induced by H-transition. Juxtaposing the data ontime behavior of ne , Γn, DPF magnitude (current Is) andcurrent Ii of fast ions in DPF, we note the following.
1. In all spacings A7 the transition entails a DPF
reduction. Taking the ne and ECE increase into account,
this should be associated with a total particle loss
reduction in the vicinity of A7. In a similar manner thefast ion component in DPF drops in A6 and A8 on the ion∇B drift side. At the same time, the fast ion content
represented by Γn decreases in the confinement volume
too. Therefore, strictly speaking, we can only presume
that the transition does not result in a fast ion loss
increase in the vicinity of A6 and A8.
The energies of ions outflowing to DPF in A6 and A8
do not change with transition. A valuable contribution of
fast ion loss to DPF vertical asymmetry obviously
follows from comparison of Ii(U) plots measured in thetop spacings of A6 and A8 at opposite magnetic fielddirections.

Proceedings of ITC/ISHW2007

2. Similar to A6 and A8: (i) the transition induces aDPF reduction in D5 and D6 on the ion ∇B drift side.
This is consistent with the Гn reduction and allows one topresume fast ion loss not to increase with transition; (ii)the energies of ions outflowing to DPF do not increase in
D5 and D6, but they are considerably higher than in A6and A8 both in the initial state and after the transition;(iii) the Ii(U) plots in the top spacings of D5 and D6 whenmeasured at opposite directions of the magnetic field
evidence a substantial contribution of fast ion loss to the
DPF up-down asymmetry.
3. The H-transition-induced reduction of Гn indicates arise of fast ion loss. Among all poloidal cross-sections
having been explored, only in D7 on the ion drift side theH-transition-induced increase of DPF and of fast ion
outflow occurs synchronously with the Гn decrease.
Hence, toroidal segments can in principle arise in U-3M
where fast ion loss increases with H-mode transition, thisbeing one more manifestation of a strong toroidal nonuniformity of some plasma properties in U-3M.
The data available are not sufficient to clear up
distinctly the factors responsible for the non-uniformities
observed. Only some general considerations seem to bepossible at present.
1. It has been observed experimentally [7] andconfirmed numerically [8] that an island structure isinherent to the real magnetic configuration of the U-3M
torsatron, that includes two main chains of magnetic
islands at ι = 1/4, each of them rotating poloidally whengoing round the torus and thus causing a differencebetween magnetic configurations in the same poloidal
cross-sections belonging to different field periods. As ismentioned in [8], the real magnetic system of U-3M has
actually only one field period instead of 9 ones in theideal system, and we should not expect an exact
reproducibility of all plasma characteristics from period
to period.
2. Helically (locally!) trapped ions should be highly
sensitive by their loss to the magnetic distortions in thefield period where they are trapped. The 1/ν transport(effective ripple εeff3/2) significantly increases near theislands [8]. Therefore an enhanced escape of fast ions
into separate components of DPF is possible in principle(D5 top, D6 top).
3. In U-3M the ion orbit loss [3] and the drift-ion-orbit
flux [4] are expected to be possible mechanisms driving
the polodal velocity Er×B to bifurcation with formation ofgradients high enough to suppress the edge turbulence
and turbulence-induced anomalous transport. The islandcaused nonuniformity in fast ion loss distribution over theU-3M torus can presumably result in a nonuniformity information of edge plasma layers and in appearance ofzones where the Er profile is not favourable for fast ion
confinement. Possibly, this could explain the occurrenceof zones of enhanced fast ion loss after the transition (inparticular, in the vicinity of D7 top).
4. The non-uniformity in toroidal distribution of someDPF characteristics could also originate from the local
character of RF power injection into the U-3M plasma(RF antenna disposition in one field period only, RFpower injection through only one spacing between thehelical coils, embracing of the plasma by the antenna
only under one pair of the helical coils) and RF power
deposition at the plasma edge. In these conditions wecannot exclude a nonuniformity in fast particle generation
both along the torus and over the minor azimuth.
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1992), Vol. 2, IAEA, Vienna, 679 (1993).
[2] V.V. Chechkin et al., Plasma Phys. Control. Fusion 48
A241 (2006).
[3] K.S. Shaing, E.C. Crume, Phys. Rev. Lett. 63 2369.
[4] K.S. Shaing, Phys. Rev. Lett. 76 4364 (1996).
[5] V.V. Chechkin et al., Nucl. Fusion 42 192 (2002).
[6] V.V. Chechkin et al., Nucl. Fusion 43 1175 (2003).
[7] G.G. Lesnyakov et al., Nucl. Fusion 32 2157 (1992).
[8] V.N. Kalyuzhnyj, V.V. Nemov, Fusion Science andTechnology 46 248 (2004).