J. Plasma Fusion Res. SERIES, Vol. 4 (2001) 29-35

Potential structures and Flow Measurements with
Separatrix Biasing in the CASTOR TokamakvAN oosT Guido*, srocKEL Janl, HRON Martinr, DEVYNCK pascal2, DYABILIN Konstantin3,
GUNN JAMiE2, UONAiBT JANI, MARTINES EMiIiOA ANd TENDLER MiChAEI5
Department of Applied physics, Ghent University, Belgium
llnstitute of Plasma physics, prague,
Czech Republic' DRFC,

3Institute

CEA-Cadarache, Francefor High Energy Densities, Moscow, Russia'Consorzio RFX, Padova, Italy,

5A6tAn Inboratory,

Royal Institute of Technology, Stockholm, Sweden(Received: 5 December 2000 / Accepted:27 September 2001)

Abstract
The impact of sheared electric fields on the structure of edge fluctuations and plasma flows isinvestigated on the CASTOR tokamak. A non-intrusive biasing scheme is applied, whereby the electrodeis located in the SOL, but its top is just touching the separatrix. There is evidence of a strongly shearedradial electric field and formation of a transport barrier at the separatrix. The Z x B shear rate is larger
than for edge plasma biasing. It significantly exceeds the typical growth rate of unstable modes and has astrong impact on the turbulent structures, as demonstrated by Langmuir probe array measurements.
Plasma flows, measured by a fast rotating Mach probe, are strongly affected in the sheared region, inparticular in the perpendicular direction.

Keywords:
tokamak, separatrix biasing, plasma flows, electrostatic turbulence, potential structures.

1. Introduction

the sheared radial electric field is amplified between theelectrode and the separatrix. Although, this arrangement
has contributed significantly on different machines to

It has been demonstrated on many machines with avariety of discharge- and heating conditions as well asedge biasing schemes that E x B flow velocity shear is arobust and universal mechanism to reduce plasma
turbulence and to establish transport barriers. The
CASTOR edge biasing arrangement and novel
diagnostics have recently contributed to the improved

reveal the key role ofelectric fields, see e.g. [1-3], it canhardly be accepted in practice in large-scale experiments

CASTOR, the biased electrode can be located at two

with high thermal loads.
Biasing of separatrix: Alternatively, the biased electrodeis inserted just into the scrape-off layer (SOL) [4]. In
this case, the thermal load on the electrode should
certainly be smaller than in the previous case and suchdifferent positions with respect to the separatrix:

scheme can be employed as non-intrusive one. It was"Standard" configuration of biasing experiments: The
electrode is located deep in the edge plasma, i.e. in theregion with closed magnetic field lines. Consequently,

suggested [5] that separatrix biasing (i.e. imposing aboundary condition on the separatrix) is an efficient way

understanding of the physical mechanisms involved. On*Corresponding author's@2001 by The Japan Society of Plasma
Science and Nuclear Fusion Researche-mail: g.vanoost@fz-juelich.de29

Van Oost G. et al., Potential Structures and Flow Measurements with Separatrix Biasing in the CASTOR Tokamak

angle as the electrode, i.e. at the top of the torus.

to create strongly sheared electric fields, thereby
affecting plasma flows and turbulent structures. The
boundary condition is governed by the electric fieldwithin the SOL, and L-H transition is triggered provided
that the electric field on the separatrix has been raised

3. Diagnostics
The edge plasma is diagnosed on CASTOR byelectrical probes. Some of them (such as the rake probe
and the poloidal probe array) were already describedearlier [6]. Here, we present two novel diagnostics tosignificantly above the neoclassical value.
In this contribution we report on the sheared flow
decorrelation of turbulent structures by modifying theradial electric field in the edge plasma. The biasing set-

investigate plasma flows and properties of turbulent
structures in the radial and poloidal direction.

up and the novel diagnostics are described in Sec 2 and3, respectively. Plasma flow and turbulence measurements are discussed in Sec 4. Conclusions are drawn in
Sec 5.

2. Set-Up of Biasing Experiments
The impact of sheared electric fields on the
turbulent structures and plasma flows is investigated on
the tokamak CASTOR (R = 0.4 m, a = 0.085 m) at 8, =I T, Ir= 9 kA (i.e. high 4 operation) and line averaged
densities fr" = 1.101e m-3.\n the plasma edge, the density
is in the range of 9.5 2'1018 m-3 and the electron-

temperature is between l0 + 20 eV. A mushroom-likebiasing electrode (made of carbon) is inserted into theplasma from the top of the torus and biased with respect
to the vacuum vessel by a pulsed voltage.
It has been found on the CASTOR tokamak that
separatrix biasing is effective only ifthe SOL is broader
than the radial extent of the biasing electrode. For thispurpose, the plasma column is shifted downwards, as

schematically shown in Fig. l. Therefore, the minor
radius of the plasma column is not determined by theradius of the poloidal limiter, but is reduced to a - A. As
a consequence, the SOL becomes broader (width 2A) at
the top of the torus. The connection length in this region
is long compared to the circumference of the torus (2zrFig. 1 Poloidal cross section of the CASTOR tokamak

schematically showing the location of plasma
column and biasing electrode at the "standard"

and at the "separatrix biasing" arrangements,
respectively.

R).

Sliding
Contacts

As seen from the figure, the mushroom electrode ishidden in the SOL and its top is just touching theseparatrix. The location of the separatrix (the last closed
flux surface) is not determined by magnetic diagnostics
on CASTOR, but is taken as the position of the velocity
shear layer (VSL) where the radial electric field and thepoloidal flow velocities are measured to be zero. The
VSL position is deduced from the floating potential
profile, measured by the rake probe in every shot' The
radial position of the separatrix can vary within several

0Plamm, depending on the discharge conditions.
Because of the strong poloidal asymmetry of thescrape-off layer (SOL), the main diagnostics of the

'

\1/
\/

Copper Rings

Fig. 2 Drawing of the rotating Mach probe.

biasing experiment are located at the same poloidal
1nVan oost G. et al., Potential structures and Flow Measurements with Separatrix Biasing in the cASToR Tokamak

3.1 Flow Measurements

two flat collectors separated by an insulator have been

The perpendicular and parallel flows are measured

simulated. A comparison between experimental Machby a rotating Mach probe [7], which provides the firstprobe data and PIC code results, displayed in Fig. 3,

opportunity to study in detail the angular dependence of
the ion flows with respect to the magnetic field lines,
and to validate existing models [8]. The construction of
the probe head is apparent from the schematic picture

shows a good agreement over a large angular range.

3.2 2D Matrix of Langmuir Probes
The probe array consists of 64 tips arranged into 8
rows and 8 columns [4]. The distance between the rows,
d, = 4.5 mm, defines the radial resolution. The distanceshown in Fig. 2. It consists of two back-to-backrectangular plates (3 mm x 5 mm), parullel to each otherand spaced by 2 mm of insulating material. The probe

between the columns is do = 6 mm. The carbon tips
(diameter 2 mm) Ne fixed in an insulating plate made ofboron nitride. The individual tips measure either thefloating potential or the ion saturation current. The modeof operation can be switched between shots. The data
are digitized at a rate of I sample/ps.
The probe head itself is made of stainless steel, butcovered by a layer of boron carbide (BoQ to improve its

head rotates by means of an electric drive located within
the diagnostic port. The duration of one revolution is Z
> 3 ms, which determines the temporal resolution. The

inclination of the plates is monitored by an auxiliary
contact (C), which marks a defined angular position (90' 29o).The probe is located at the top of the torus andradially movable on a shot-to-shot basis.
The temporal evolution of the ion saturation current
measured by a plate during a single revolution provides

compatibility with the plasma and to reduce its
conductivity. The probe head is mounted on a

its angular variation. A typical modulation of the probe
signal is apparent from Fig. 3. The minima correspondto the orientation of the plate parallel to the magnetic
field lines, while maxima are close to the perpendicular

manipulator allowing its radial motion on a shot-to-shotbasis.

The position of the probe head in a poloidal planeof the CASTOR tokamak is shown schematically, bur inscale, in Fig. 4. It is immersed into the edge plasma sothat the magnetic field lines are perpendicular to theprobe matrix, being located at the top of the torus (135"
toroidally away from the ion side of the main poloidal
limiter).
In the same figure, a snapshot of the distribution of
the floating potential in the poloidal plane is shown as
well. The dark regions in the plot denote the floatingorientation.
There are fluid [7,9] as well as kinetic models [10]to interpret the ion collection by orientable probes inmagnetized plasmas in the presence of perpendicularZ x B drifts. In the following we will concentrate on the
2D kinetic model (PIC code), which has been adapted tosimulate the CASTOR probe geometry. Rather than acylindrical probe (circular boundary in the 2D plane),?

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Fig. 3 Angular variation of the ion saturation current on a rotating plate - left. Polar diagram of the ion saturation current- right. Experimental data are fitted to the plC model.

3lVan Oost G. et al., Potential Structures and Flow Measurements with Separatrix Biasing in the CASTOR Tokamak

Potential structures

Eas

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Poloidally 42mm

Fig. 4 Position of the 2D matrix in the poloidal plane of the CASTOR tokamak (in scale) together with a snapshot of
potential structures. Only six rows of tips were active in this discharge.

12

Electrode ot r" = 59 --'

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time [ms]time [ms]Fig. 5 Evolution of polarized discharges at "standard" (left) and "separatrix biasing" (right). From top to bottom:

electrode current and return current to the poloidal limiter together with the evolution of the plasma current;
intensity of the H" spectral line; line averaged density; relative improvement of the global particle confinement
time.

32

Van Oost G. et al., Potential Structures and Flow Measurements with Separatrix Biasing in the CASTOR Tokamak
100

potential structures (potential valleys and hills) differingfrom the mean value. Even a visual inspection of such a
map indicates that turbulent structures are smaller thenthe dimension of the array but larger then the distance
between the tips. Pioneering research with a similararray was performed on the CALTECH tokamak [11].
The area of the probe head represents a non-t'. -_-_
rl
I

,\] \

A

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Eiosing.{,'1"'"i. l!'-inegligible part of the the poloidal cross section.
However, the radial profiles of the floating potential,
measured at several radial positions of the probe headoshowed that the shadowing due to the array does not
dominate the edge plasma.

20

ooroo,," 1..;

uo

ao

1oo

Fig. 6 Comparison of the radial profile of the floating

potential with and without biasing (Ua= +100 Vl,
plotted for two positions of the biasing electrode:foo" = 60 mm - "standard" biasing scheme (solidlines), measured by a single floating probe on a
shot-to shot basis; fo." = 68 mm - "separalrix"
biasing (dashed lines), measured by a rake probe.

4. Experimental Results
4.1 Evolution of Polarized Discharges
The evolution of the global plasma parameters at
both the biasing schemes is compared in Fig. 5 forsimilar discharge conditions. The electrode is positively
biased (U5 = +2OO V).

As seen in Fig. 5, a current of about 35-40 A isdrawn by the electrode during the biasing period. In
spite of the different biasing configuration, the fractionof the electrode current flowine to the limiter iselectric field is highly sheared in that region and affects

significantly the plasma flows and the turbulent
structures. The strongest shear appears at the innermost

comparable.

side. The flow and fluctuation measurements described
below have been performed with separatrix biasing.

The line averaged density increases substantiallywith biasing in both cases. However, the evolution of
the Ho spectral line intensity 1g" exhibits different

4.2 Flow and Fluctuation Measurements in
Sheared Electric Fields

shapes. For the "standard" configuration, the intensity

Figure 7 shows radial profiles measured during
separatrix biasing, (Uu = +150 V, the electrode position
at r = 70 mm). The top panels show the potential/electricfield profiles measured by the rake probe. The bottom

drops immediately with biasing, which evidently implies
a reduction of recycling and results in improvement of
the global particle confinement (more than 50 Vo) [6].
On the contrary, at separatrix biasing, the Ho emission
slightly increases during the initial phase of the biasing
period. Nevertheless, the improvement of the global
particle confinement time is of the same order as in thecase ofthe standard biasing, as seen in the bottom panelof Fig. 5. However, for a precise comparison of particle
confinement improvement, a detailed bookkeeping ofrecycling at the limiter, the wall and the electrode ispanels show the parallel and perpendicular Mach
numbers.

It is evident from the figure that the layers of thelargest perpendicular flow velocity shear and thesheared region of the radial electric field appear tocoincide (r = 69 + 72 mm). Moreover, the values of the.E x B shear rate, deduced from the radial electric field
profile and the shear of the flow velocity voor derived
from the Mr profiles are comparable in this region

required [1].
A substantial difference between the radial profileof the floating potential, Up1-, for both biasing schemes

Lv6r6/A,r = LvrotlLr - 5.106 s-r

is apparent from Fig. 6. At "standard" biasing, thepotential of the whole plasma column is shifted by -Uu

and significantly exceed the typical growth rate ofunstable modes. The growth rate is estimated fromfluctuation measurements as the reciprocal value of the
autocorrelation time, T= llre - 5.104 - 2.105 s-r. It is
worthwhile to note that the character of the electrostatic
fluctuations in the edge plasma of CASTOR is similar tothat observed in large - scale experiments [6].

and, consequently, the radial electric field E, isamplified between the electrode and the separatrix. On
the other hand, at separatrix biasing, the potential isaffected only in a relatively narrow region near the
separatrix and the radial electric field is amplified atboth sides of the electrode [4]. Consequently, the radial
JJ

Van Oost G. et al., Potential Structures and Flow Measurements with Separatrix Biasing in the CASTOR Tokamak# e824

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Fig. 8 Spatial-temporal correlation functions of thepotential fluctuations in the poloidal direction for

-20
0.4

several radial positions of the poloidal probe arrayc = ploteI = plote 2(indicated in the y-axis at the right hand side).
Position of the separatrix - r" = 67.5 mm. Leftcolumn - ohmic phase, right column - polarizedphase of the discharge (separatrix biasing, U, =1

o.2

r-"brtFl. 0.0= -o.2ocol.

acc+150 V.

tr c

-0.4

-0.6

function. The bright patterns of elliptical shape are the0.4

regions of the highest correlation (> 50 7o). Theiro.2
Adimension in the y-direction corresponds roughly to thepoloidal extent of the turbulent structures (Lpo1 = | + 2cm), whlle the x-dimension corresponds roughly to their
lifetime (10 - 20 gs). The poloidal propagation velocityof the structures is deduced from the slope of thecorrelation patterns. A more precise interpretation of

0.0

L

I

8, .cc.3

-o.2

-0.4
-0.6

l'ao

a

I

60

65

70

c

75 80

85

90

such 2D correlation is given in [14].

rodius Imm]

The position of the separatrix is seen in the left
column at r = 67 .5 mm, where the reversal of poloidal
propagation is well apparent. The biasing electrode islocated approximately at the same radius. Thus, theregion with the highest shear flow is formed there
during biasing. A strong decorrelation of the turbulent
structures in the poloidal direction as well as in time due
to biasing is evident from a dramatic reduction of thesize of the correlation patterns in this range of radii.
A similar effect is observed also in the radial
direction U2,I3l using the data from the rake probe.
The simultaneous poloidal/radial correlation analysis at
separatrix biasing (using data from the 2D matrix) is in

Fig. 7 Radial profiles at separatrix biasing lUt= +150 Vl.

Top panels: floating potential and radial electricfield (E, = -YUtr, neglecting VI with biasing].
Bottom panels: parallel and perpendicular Mach
numbers deduced from both plates (plate 1. - o,plate 2. - o). The dashed vertical line denotes the
position of the separatrix in the non-polarizeddischarges.

To demonstrate the strong impact of sheared flows

on the edge turbulence, the data measured by thepoloidal probe array was used ll2,l3l. The spatialtemporal correlation functions of potential fluctuations
are calculated and the result is shown in Fig. 8, which
consists of 18 panels. Each row of the figure
corresponds to a single radial position of the probearray. The left panel of a row characterizes theturbulence in the OH phase of the discharge, the rightprogress [15].

5. Gonclusions
The biasing arrangement and the plasma flow andturbulence measurements on CASTOR offered a firstexperimental proof that imposing a boundary conditionby (non-intrusive) separatrix biasing is an efficient way

one coffesponds to the polarized phase (U6 = +2O0 D.
Each panel shows the spatial - temporal correlation
34

Van Oost G. et al., Potential Structures and Flow Measurements with Separatrix Biasing in the CASTOR Tokamak

to create strongly sheared electric fields, thereby(2000).

affecting plasma flows and decorrelating turbulent
structures, and creating an edge transport barrier in theproximity of the separatrix, which leads to improved[4] J. Stdckel et al., Proc. of 27th EPS Conf. Contrl.
Fusion Plasma Phys.,2O00, Budapest, p. 1032.
M.
t5l Tendler, Plasma Phys. Contrl. Fusion 39,8371,

confinement.
The rotatable Mach probe enabled to measure the(1997).

[6] J. Stttckel et al., Plasma Phys. Contrl. Fusion 41,

parallel and perpendicular plasma flows which are

As77 (1999).

strongly affected in the sheared region, especially in the

t7l K. Dyabilin et al., Proc. of 27th EPS Conf. Contrl.

perpendicular direction. To improve the temporal

Fusion Plasma Phys.,2O00, Budapest, p. 1653.

t8l J.P. Gunn e/ al.,Phys. Plasmas 8, 1995 (2001).
t9l H. Van Goubergen et al., Plasma Phys. Contrl.

resolution, especially for studying transient phenomena,a multi-pin Gundestrup system (t161, t8l) has been
recently tested on CASTOR [17].

Fusion 41,L17 (1999).

The E x B shear rate for separatrix biasing is larger

t10l J.P. Gunn, Czech. J. Phys. 48,293 (1993).
[11] S.J. Zweben and R.W. Gould, Nucl. Fusion 25, 17lthan for standard biasing schemes, and significantly
exceeds the growth rate of unstable turbulent modes.

(1985).

Probe array measurements demonstrated a strong impact[12] J. Stdckel et al.,26th EPS Conf. Contl. Fusion
Plasma Phys.,1999, Maastricht, p. 1589.
ll3l J. Sttickel et al., J. Tech. Phys. 41, Special Issue, 49(2000).

on the turbulent structures.

Acknowledgement:
Supported by Grant Agency of Czech Acad. Sci.,[4] J. Petrzilka and J. Stdckel, Conrrol. Plasma phys.

No. 1043002.

38S,74 (1998).
[15] E. Martines et al., to be published in Proc. of 28th
EPS Conf. Contrl. Fusion Plasma Phys., 20O1,

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35