J. Plasma Fusion Res. SERIES, Vol.5 (2002) 404-408

Fine Structure of Divertor Flow Distributionsin the I = 3 Uragan-3M Torsatron
VOLKOV Yevgen, CHECHKIN Viktor, GRIGOR'EVA Ludmila, SMIRNOVA Marina,
SOROKOVOY Eduard, NAZAROV Nikolaj, LOZIN Aleksej, TSYBENKO Sergej,
LITVINOV Anatolij, SLAVNYJ Aleksandr, ADAMOV Igor', KULAGA Anatolij,
MIRONOV Yurij. MIZUUCHI Tohrur, MASUZAKI Suguru2 and YAMAZAKI Kozo2
Institute of Plasma Physics, National Science Center "Kharkov Institute of Physics and Technology",
Kharkov 61108, Ukrainellnstitute of Advanced Energy, Kyoto [Jniversity, Uji 611-0011, Japan
2National Institutefor Fusion Science, Toki 509-5292, Japan(Received: 7 January 2002

/ Accepted: 5 July 2002)

Abstract
An up-down asymmetry is inherent to the spatial distributions of divertor plasma flows in the l=3
Uragan-3M (U-3M) torsatron, similar to the l=2 Heliotron E heliotron. This asymmetry is attributed toconvective losses bf charged particles. Measurements of poloidal distributions of divertor flows carriedout with sufficiently high resolution also reveal some other specific properties of these distributions,
namely, (1) the existence of a pair of non-ambipolar flows of opposite sign corresponding to the basic
ambipolar flow in a divertor leg; Q) a splitting of ambipolar flow; (3) the presence of a comparatively
large non-ambipolar flow with an excess of electrons outflowing near the torus midplane; (4) acomparatively high ion saturation current and corresponding negative current to a grounded electric probe
in the divertor private region. Physical mechanisms are discussed, which could result in the above
mentioned structural features of divertor flows in U-3M.

Keywords:
heliotron, torsatron, helical divertor, up-down asymmetry, ambipolar flow, non-ambipolar flow, plasmadrift

1. Introduction

and medium levels of heating power (ECH, NBI,NBI+ECH), and in U-3M, with a fixed regime of RF
plasma heating, a non-ambipolar flow with an excess ofMeasurements of poloidal distributions of plasma

flows in the natural helical divertors of the l=2
Heliotron E heliotron (H-E) tl-41 and the l=3 Uragan3M (U-3M) torsatron [5] have revealed a strong updown asymmetry of these distributions. The main
characteristics of the asymmetry are (l) a many-folddifference in the ambipolar particle flow magnitude in
the divertor legs symmetrically positioned in the top andbottom parts of the torus and (2) opposite polarity ofcorresponding non-ambipolar flows. In H-E, with lowions corresponded to the larger ambipolar divertor flow.
Both these flows were directed with the ion toroidal B x
VB drift. Basing on this, the observed asymmetry isrelated to direct (non-diffusive) charged particle losses
from the confinement volume. Such a relation has been
confirmed by results of numerical simulations of fast
particle losses in U-3M [5] and H-E [6]. The simulations@2OO2 by The Japan Society of Plasma

Corresponding author's e-mail: chechkin@ ipp.kharkov.ua404

Science and Nuclear Fusion ResearchVolkov Y. et al., Fine Structure of Divertor Flow Distributions in the / = 3 Uragan-3M TorsatronA-A (0 = 0o)have really shown a strong up-down asymmetry ofpoloidal distributions of these losses with a predominantescape of fast ions in the ion toroidal B x VB drift
direction.
In both devices the diverted plasmas were detectedby arrays ofplane electric probes, and the ion saturation
current (ISC) 1, was taken as a measure of plasma flow
magnitude. Such an estimation is valid, if the flow hits
an isolated probe (or target plate). In this case, the flow
is ambipolar and the total electric current to the probe is
I = I+ + 1- = 0, where I+ = 1". However, if a plasma flow
hits the chamber wall or a probe (target plate) shortcircuited to the wall, then the flow can be essentially.{..1...
BOTTOM

D-D 14 = 26;

non-ambipolar. In this case, the probe currentcm("grounded pnrbe current", GPS) 1r, its sign and value,
can be taken as characteristics of flow non-ambipolarity.

The spatial distributions of divertor flows in U-3M weremeasured with a higher resolution as compared with HE. Owing to this, some new peculiar features of theseFig. 1 Lay-out of electric probe arrays in the symmetric
poloidal cross-sections of the U-3M torus A-A andD-D. Shown are the helical coils, I, II, Itr anddistributions have been observed. These featuresindicate an obvious non-uniformity of plasma flow
characteristics even within one divertor leg, some ofthese features being presumably related to plasma drift
effects in the SOL and divertor region. The account ofsuch a non-uniformity may appear necessary, when
designing the divertor facility for a large device ofPoincare plots of edge field lines. The probe
numbers N in A-A: 1-17 (top), 1-15 (bottom, in DD'.'l-23 (outboard)

spacings between the helical coils beyond the X-point at
the minor radius 0.27 m in two half period-separated"symmetric" poloidal cross-sections of the torus withheliotror/torsaff on type.

2. Experimental Conditions

different configuration of field lines in the divertor
region: 4 = 0" (cross-section A-A) and Q = 20" (crosssection D-D) as is shown in Fig. l. From the viewpointof the present work, an important feature of the crosssection D-D is the existence of the outboard spacing

The U-3lvI device is an I = 3/m = 9 torsatron (Ro =I m, a = 0.12 m, Bo = 0.72 T, t(a) = 0.4). The wholemagnetic system is enclosed into a large vacuum
chamber, with the minor radius of the helical coilcasings being 0.19 m, so an open helical divertor isrealized in this; device. Diverted plasma parameters aremeasured with Br directed both counterclockwise

between the helical coils with two divertor legs
symmetric about the midplane. As it will be shown
below, however, the structural features of the plasma
flows in these legs occur quite different. The spacing
between adjacent probes (0.1 cm) is considerably lessthan the probe size in the poloidal direction (1.25 cm).

("positive field", the ion toroidal B x YB drift isdirected upward) and clockwise ("negative field"). A
hydrogen plasma is RF produced and heated (ar =0.8ar"r(0)), with the irradiated RF power P = 200 kW in
the 25 ms pulse as a standard operating regime. The
electron density is n" = 2 x l0l8 m-3. The electron (ECE)

3. Peculiarities of Divertor Flow Structure
3.1 Cross-section A-A

For the standard regime of device operation,
poloidal distributions of ambipolar divertor flows in thetop and bottom spacings between the helical coils arepresented in Fig. 2(a) as current 1. versus probe number
Nplots (positive magnetic field). Within the accuracy ofprobe array adjustment and the array resolution, thepositions of 1" maxima (N = 3 and N = 11 at the top, N =7 and N = 15 at the bottom) correspond to the calculatedand ion (CX analyzer) temperatures are Z" = 0.3 keV
and, Tt = 0.1 keV, respectively. Also, minor groups ofsuprathermal electrons (2 I keV at the confined plasmaperiphery) and ions (0.2-0.3 and 0.6-0.8 keV) aredetected.

To detect the diverted plasma, 7E plane 1.25 x 0.8
cm2 electric probes are used. The probes are grouped in

6 arrays. The arrays are arranged poloidally in the405

Volkov Y. et al., Fine Structure of Divertor Flow Distributions in the / = 3 Uragan-3M Torsatron

positions of the divertor legs (cf. Fig. l). The ((M1the top and bottom. At the top, these additional maxima

distributions are distinguished by an up-down
asymmetry [5], with the d maximum in the inner leg atof negative polarity are shifted poloidally clockwise
relative to their main (positive) maxima, when being
seen in the positive magnetic field direction. At thebottom, the additional maxima of 1, > 0 are shifted
counterclockwise relative to the main (negative)
maxima. The adjacent plasma currents of opposite sign
exist with all levels of heating power.
With magnetic field reversal, together with reversalof the asymmetry of ambipolar divertor flows in their

the top being 2.4 times higher than that at the bottom.

The direction of the larger divertor flux outflow(upward) corresponds to that of the toroidal ion B x VB

drift. The largest absolute values of GPC in thecorresponding 1r(M) distributions (Fig. 2(b)) fal at theprobes, recording the d maxima, and the GPC maxima
have opposite polarity at the top and the bottom. With
this, the maximum of GPC with an excess of ions (1, >0) corresponds to the higher ISC maximum (at the top),
thus confirming the effect of the toroidal ion B x VB
drift on the up-down divertor flow asymmetry.
Also, a more fine structure of the (N) distributions

magnitude, a change of GPC sign occurs incorresponding non-ambipolar flows, including the signof 1, in the adjoining flows. This means that theobserved polarization of flows and occurrence ofadjoining flows could be related to a charged particle
drift across magnetic field.

can be seen in Fig. 2(b), where a small 1, maximum ofopposite polarity adjoins to each main maximum both at

BOTTOM

TOP
2525

20

20
F

l5

l5ool02 4 6 8t0t2t4t6
probe number, N1l

15105
probe number, N5101520

6-<I-lv

probe number, N!a5101520
probe number, Nlo,o5

R e-10EExv

ElroEO
L=oDe -30

probe number, NFig. 2 (a) lon saturation current l" versus probe number
N in the top and bottom spacings between the he-Fig.

3 (a) lon saturation current l" versus probe number N in the outboard spacing between the helical coils in the cross-section D-D with P = 200
kW (on the left) and P = 80 kW (on the right);(b) same as (a) for the grounded probe current
In. The vertical axis position corresponds to thatof the torus midplane.

lical coils in the cross-section A-A;(b) same as (a) for the grounded probe current ls.
In(3) = 13.8 mA. The vertical axes correspond tothe minor radii, passing through the X-points.

406

Volkov Y. et al., Fine Structure of Divertor Flow Distributions in the I = 3 Uraean-3M Torsatron

3.2 Cross-section D-D

substantial current 1, in the private region is notconnected directly with a plasma drift across magnetic

The 1.(1V) distribution in the outboard spacing of theD-D cross-ser:tion is shown in Fig. 3(a) (positivemagnetic field) for P = 20O kW (standard regime) and p= 80 kW. With the lower heating power, the positions of
1" maxima, N = 6 and N = 17, are close to the calculated

4. Summary and discussion
In addition to a distinctly pronounced up-down

positions of diyertor legs (cf. Fig. 1). For this case, theasymmetry of divertor flows in the value of 1, and thedegree of asymmetry of ambipolar particle flows, whensign of 1, [5J, some other structural features of thediverted plasma flows contributing to the up-down

field.

estimated as the ratio cr of maximum values of (, iscomparatively [ow, cr = 1.4, with the larger particle flux

asymmetry have been revealed.
1. In the top and bottom spacings of the cross-section
A-A, two adjacent plasma currents with opposite signs

outflowing ovcr the torus midplane, that is, on the ion
toroidal drift side. A pair of adjacent non-ambipolar
fluxes corresponds to the larger ambipolar flux (Fig.
3(b), on the right) with an excess of ions in one flux (N= 6) and of electrons in the other flux (N = 8) andcurrents 1, conrparable in their absolute values. To thehave been observed in each divertor leg. A polarizationof diverted plasma flow due to the cenrifugal drift ofcharged particles moving along the bended field lines
beyond the X-point seems to be a plausible explanation

ambipolar flux outflowing under the midplane afor this effect. (A qualitative picture of such a drift

comparatively high maximum of negative current 1,
corresponds (A'= 18), while no adjoining maximum of

could be presented more evidently, when going into theplane perpendicular to the helical direction with thethe positive plasma current is observed.

magnetic field component in this plane B - (Ben)e6, e6
being the unit vector in the helical direction, similar towhat has been done in the analysis of the LHD helical

At a higher heating power (Fig. 3(a), on the left,
the standard regime), the difference between themaximum valur:s of .1, in the divertor legs increases (ct =divertor, see ref. [8] and Fig. I therein). At present, theabsence of more detailed data on parameters of diverted
plasma does not allow us to make quantative estimationsof the centrifugal drift effect on the space structure ofplasma flow.
The splitting of the larger ambipolar flux in thecross-section D-D, with occurrence of the pair of2.3), similar to what has been observed earlier in H-E[3]. Also, a qualitatively new element of asymmetry

occurs, nameiy, splitting of the larger flow andappearance of lwo 1. maxima (N = 6 and N = 9) in thevicinity of the c:alculated divertor leg position. The sameeffect has been observed earlier in the Uragan-3
torsatron [9], the U-3M predecessor. A substantial
current 1" in the private region (N = I l-15) is one more

oppositely directed 1, maxima can also supposedly result

from a flux polarization due to the centrifugal drift ofpeculiarity of the ((M; distribution occurring at thehigher heating power. As follows from the comparison

ions and electrons beyond the X-point.

With such an interpretation of the observed finestructure of plasma flows in the divertor region, thisstructure seems to arise independent of the up-down
asymmetry of the divertor flows, though both theseof the upper antl lower plots in the left column of Fig. 3,
to the splitted ambipolar plasma flow in the divertor leg

over the midplane two adjacent maxima of GPC
comparable in their absolute values correspond, witheffects are caused by similar factors, i.e., particle drifts
across magnetic field.
2. As it follows from the calculations of direct electron
losses [5], in the positive magnetic field case, the maximum of the angular distribution of these losses falls at adomination of ions in one maximum (N = 7) and ofelectrons in the other maximum (N = 8). Similar to thelower heating power case, the non-ambipolar plasma
flux outflowing below the midplane is characterized bya large negative 1, maximum (N = 19) without any
substantial adjoining flux with 1, > 0. In the privare
region (N = I I -15), the plasma current is negative. In

narow interval of 0 adjacent to the midplane under it on
the inner side of the torus (see Figs. 7 and 8 in ref. [5]).
The velocity distribution of escaped electrons is also
highly peaked. Most of them leave the confinement volume moving with the magnetic field (see Fig. 9 in ref.

the negative rnagnetic field, the splitted higher 1,maximum is observed in the divertor leg under the torus

midplane. The structure of poloidal distributions ofcurrent 1, shown in Fig. 3(b) also reverses in the vicinityof divertor legs. However, the sign of 1, in the private[5]). Crossing the LCFS on the inner torus side and possessing a large parallel velocity and, consequently, alarge poloidal rotation velocity, the lost fast electrons inregion does not change. This might signify that athe process of their drift in the SOL can approach the X40't

Volkov Y. et al., Fine Structure of Divertor Flow Distributions in the I = 3 Uragan-3M Torsatron

Acknowledgements
This work was in part supported by Fundamental
Research Foundation of Ministry of Education andScience of Ukraine (Project No. 02.52100028) andcarried out within the research collaboration by theLIME Program (National Institute for Fusion Science,point on the outboard torus side and escape to thedivertor region along the nearest divertor leg. Such anassumption is consistent with the observation of a large
negative plasma current in the non-ambipolar plasma
flux outflowing under the midplane (Fig. 3). (A similar
behavior of fast electrons has been also observed experimentally in the edge plasma of a tokamak with the sym-Japan) and by the Agreement between IPP NSC KIPT

metric single-null divertor [10]).
3. The occurrence of a substantial (as compared with
the A-A cross-section) current 1. and a corresponding
negative current 1, in the divertor private region at a
sufficiently high heating power means that a region of
reduced potential arises beyond the X-point on the outboard torus side. Such a potential drop can be presumably caused by a fraction of lost fast electrons being
trapped in the private region and bouncing between two
divertor legs.
4. The measured poloidal distributions of diverted
plasma flows in U-3M are distinguished with a width
multiply (up to 30 times) exceeding that of corresponding calculated magnetic divertor channels. At least partially, such a broadening can be caused by the physicaleffects having been considered in the present work, suchas plasma flow polarization, outflowing of fast particles
along the divertor channels and accumulation of electrons in the private region of the divertor. The possibility of divertor flow broadening should be taken into
account, when designing a closed helical divertor for aand Institute of Advanced Energy Kyoto University

heliotron/torsatron device.

[0] Yamamoto, S. et al., Nucl. Fusion 18, 205 (1978).

(Japan).

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