Influence of electrode and limiter biasing on the ISTTOK boundary plasma C. Silva*, H. Figueiredo, J.A.C. Cabral, B. GonÁalves, I. Nedzelsky, C.A.F. Varandas

Associação Euratom/IST, Centro de Fusão Nuclear, Instituto Superior Técnico

1049-001 Lisboa, Portugal

Abstract

In this contribution limiter and electrode biasing experiments are compared, in

particular in what concerns their effects on the edge plasma parameters. For positive limiter

bias, an increase in the average plasma density is observed, although, without a significant

increase in the gross particle confinement. In spite of the lower density, particle confinement

is higher for negative polarization due to low turbulent transport.

For electrode bias changes in the plasma parameters are more significant. For positive

bias, a substantial increase (>50%) in the average plasma density is observed, without

significant changes in the edge density, leading to steeper profiles. The ratio ne/Hα also

increases significantly (>20%), indicating an improvement in gross particle confinement. The

plasma potential profile is strongly modified as both the edge Er and its shear increase

significantly. Good correlation between confinement modifications and ExB shear has been

found.

1. Introduction:

In tokamaks the radial electric field (Er) and the ExB velocity shear at the edge region play an

important role in plasma confinement and transport. A technique used for the modification of

the radial electric field consists in biasing plasma facing components like limiters or inserting

biased electrodes in the edge plasma [1].

Biasing experiments performed in several devices (CCT [2], TEXT [3], DIII-D [4],

TEXTOR [5,6], CASTOR [7] and ISTTOK [8,9]) in the past decade have shown that biasing

can modify the edge electric fields allowing the control of not only important aspects of the

plasma-wall interaction as edge plasma profiles, particle exhaust and impurities retention, but

also global properties as particle confinement. Furthermore, biasing has contributed

significantly to the understanding of the improved confinement regimes as the H-mode. H-

mode has been triggered using biasing [2,6], proving the importance of the radial electric field

in the transition to improved confinement regimes.

* corresponding author e-mail: csilva@cfn.ist.utl.pt

Although, both electrode and limiter biasing have proved to introduce substantial

modifications in the edge plasma, experiments have demonstrated that electrode biasing

introduces more substantial changes in energy and particle confinement [1,2,6].

Limiter and electrode biasing experiments have been performed on tokamak ISTTOK

in order to control the edge radial electric field and investigate its impact on confinement. In

addition to the constant bias voltages used in most biasing experiments [1-8], alternating bias

has also been investigated on ISTTOK. In this paper, two different bias configurations, limiter

and electrode, are compared in the same device, in particular in what concerns their effects on

the edge plasma parameters.

Limiter biasing, LB, has been investigated in detail on tokamak ISTTOK [8,9]. LB has

the advantage of using existing plasma facing components in the device contrary to electrode

bias where an object must be inserted into the plasma, which is not compatible with plasma

operation in large devices. However, the modifications in Er for limiter biasing are, in general,

limited to the scrape-off-layer. Electrode bias has the advantage of forcing the electric field in

the edge plasma. It seems more appropriate as a mean of elucidating the importance of Er and

the ExB shear velocity.

2. Experimental Setup ISTTOK is a large aspect ratio circular cross-section tokamak (R=46 cm, a=8.5 cm, BT~0.6 T,

∆Φ=0.22 Vs), which has two small localized limiters, one at the horizontal external and the

other at the vertical up positions.

An array of Langmuir probes, toroidally located at about 180h from the limiter, has been

used to study the influence of biasing on the boundary plasma. This array consists of two sets

of three Langmuir probes, radially separated by 4 mm. Two tips of each set of triple probes,

separated poloidally (3 mm), were used to measure the floating potential, Vf. The third tip was

biased at a fixed voltage in the ion saturation current regime, Isat. Measurements were taken at

different radial positions in a series of similar and reproducible plasma shots.

A movable electrode has been developed for biasing experiments on ISTTOK. In figure 1

the electrode head is depicted schematically. The electrode head is mushroom shaped and

made of 2-D Carbon composite, having a height of 6 mm and a diameter of 20 mm, screwed

to a stainless steel shaft, that is protected by boron nitride as insulating material to be exposed

to the plasma. The overall stroke length of the electrode is ≈80 mm (-50<relect-a<30 mm). It

has been observed that for relect-a>-25 mm the electrode does not become a limiter and is

capable of sustaining the heat load from the discharge.

Two different bias configurations have been studied on ISTTOK: Limiter bias (LB) and

Electrode bias (EB). Both DC (from -300 V to + 300 V) and alternating (50 Hz, 80-130 p.V)

bias voltages have been used, though, this work concentrates on the latter. A. C. bias has the

advantage of not only allowing the investigation of the effect of polarization in a single shot,

but also determining biasing thresholds for confinement modifications. In the experiments

reported here, the bias voltage is applied between the limiters/electrode and the vacuum vessel

and the electrode tip is situated at r=7 cm (1.5 cm inside the limiter). A transformer is used to

provide the alternating voltage and the phase is not synchronised with the discharge timer.

3. Experimental results:

Taking a typical ISTTOK discharge (Ip~4-8 kA, τD~30-40 ms, ne(0)~5-10x1018 m-3,

Te(0)~150-250 eV, τE~0.5 ms, β~0.5 %, q(0)~1, q(a)~5) as a reference, we have made studies

concerning the influence of limiter and electrode biasing on plasma confinement and stability.

3.1 Limiter biasing A comparison of the main plasma parameters during alternating limiter and electrode bias

discharges (Vbias=120 p.V) is presented in figure 2. Limiter biasing experiments have been

already described in detail before [8,9]. Analysis of a set of discharges with LB has shown

that during the positive (negative) bias half-cycle, we may observe: (i) - a decrease (increase)

of the plasma current, and of the electron temperature, (ii) - an increase (decrease) of the line

averaged electron density, ne, and of the intensity of the Hα radiation. The drop in the plasma

current and electron temperature observed for positive bias is possibly a consequence of the

rise in the radiation losses induced by the large increase in electron density and by the

accumulation of impurities. The transition from a positive biasing half-period to a negative

one impairs the discharge parameters, specially the plasma density, which drops considerably

afterwards.

Changes in the gross global particle confinement time, τp, are inferred from the ratio of

the line-averaged density to the Hα intensity, which is a rough estimation of τp. Although ne

rises for positive bias, both the Hα radiation and the turbulent particle transport also increase

significantly, leading to very limited modifications in particle confinement. In spite of the

lower density, τp is higher for negative polarization due to the low transport. These results are

in agreement with previous findings [3,4], where particle confinement has been observed to

increase for negative LB.

With the Heavy Ion Beam Diagnostic the radial profiles of the product neσeff (where

σeff is the effective cross section of the ionisation process) were measured. The amplitude of

these profiles show good temporal correlation with that of ne. We observe the gradual passage

from a large amplitude peaked profile (positive bias) to a much lower amplitude and flatter

profile (negative bias).

Analysis of the Langmuir probe data has shown that in the region around the limiter

the edge plasma potential follows the applied voltage with the consequent modification of Er

from ≈3 kV/m for positive bias to ≈1 kV/m for negative bias. The behavior of the cross-field

particle flux ( BEnExB /~~θ=Γ , where Eθ is the poloidal electric field and B the toroidal

magnetic field) during biasing is similar to that of Isat. Particle losses in the main plasma rise

for positive applied voltages and decrease for negative ones.

3.2 Electrode biasing 3.2.1 Main plasma

As shown in figure 2, for electrode bias the changes in the plasma parameters are more

significant. The time evolution of the plasma current and temperature is very similar to that

observed in LB experiments. However, EB effects on the density and particle confinement are

much larger than with LB. For large positive applied voltage, V>100 V, a substantial increase

(>50%) in ne is observed, without significant changes in the edge density, leading to steeper

profiles. These results are corroborated by the Heavy Ion Beam diagnostic data, which shows

steeper neσeff profiles for positive bias when compared with negative bias, particularly at the

plasma edge. Although the Hα radiation increases, the ratio ne/Hα also rises significantly

(>20%) indicating an improvement in gross particle confinement. Both the average density

and the particle confinement decay rapidly after the applied voltage starts to decrease and do

not vary significantly for negative bias.

3.2.2 Edge plasma Figure 3 shows the time evolution of bias current together with ne, Isat, Vf and the cross-field

turbulent flux, for five discharges, corresponding to different radial probe positions. The

signals have been shifted in time so that they became synchronised with the biasing voltage.

Thus, discharges should only be compared in the region between 10 and 30 ms. The floating

potential near the electrode follows the applied voltage while close to the limiter the opposite

behaviour is observed. The Vf profile is strongly modified by electrode bias. For Vbias<80 V,

Vf is a monotonously decreasing function of the probe penetration into the plasma, while for

Vbias >80 V a clear inversion is observed.

Figure 4 illustrates the radial profiles of Vf, Vp and Isat at different applied voltages

taken from figure 3. The plasma potential is derived from Vp = Vf +3Te. For most of the

discharges no temperature measurements with high temporal resolution are available.

However, since the edge temperature does not change significantly during biasing, the

average Te profile is used in the evaluation of Vp. The plasma potential profile is very flat for

negative and zero applied voltage, while for positive bias a large electric field is observed just

inside the limiter (values of Er larger than 8 kV/m are measured) associated with a strong Er

shear (dvExB/dr≈3x106 s-1 at r-a=-5 mm). During bias, the radial electric field is not strongly

affected by temperature uncertainties because the contribution of the Vf gradient is larger than

that from the temperature gradient.

Although the average density increases strongly up to t=19 ms, Isat at the limiter

position is observed to decrease for t>14.5 ms. This behaviour seems to propagate inwards;

probes at a smaller radius observe this reduction later, leading to progressively steeper

profiles. In contrast, the turbulent transport is observed to reduce strongly in the whole edge

simultaneously, reaching its minimum at t=18.5 ms, when Er and the ErxB velocity shear

attain their maximum. Results suggest that positive bias creates a region with strong ExB

velocity shear inside the limiter, which may explain the strong reduction in turbulent transport

and the improvement in particle confinement observed during that period.

The plasma average density is only strongly modified for Vbias>≈50 V (t=16 ms),

suggesting that confinement modifications require a biasing voltage of that order. The

modification in ne is preceded by the reduction in turbulent transport, Isat at r=a and by a

significant modification in the Vp profile (dvExB/dr changes from ≈3x105 s-1 for Vbias=0 to

≈106 s-1 for Vbias=50 V).

Spectral analysis of both Isat and ExBΓ , in the main plasma shows a large decrease of

the turbulence level for positive bias, in particular for frequencies between 20 and 50 kHz,

when compared with results obtained for negative bias.

4. Plasma potential dependence on Vbias

An interesting observation is that the plasma potential profile is not significantly

modified by negative bias as expected from the Langmuir probe theory. This is illustrated in

figure 5, which shows the variation of the electrode current and plasma potential at the

electrode radial position with the applied voltage for three similar discharges with EB. For

negative applied voltage most of the applied potential falls in the probe sheath, while for

positive bias, the plasma potential follows the applied voltage.

Experimental data has been compared with results from a simple model developed to

evaluate the influence of bias on the plasma potential [10]. This model considers that a flux

tube is in contact with the bias structure at one end while the other end leads to a conduction

surface that is kept at constant potential. Simple calculations presented in Ref. [10] show that

the electrode/limiter current is given by [ ] [ ]ATeVTeVAjj ebiasebiassat +−= )/exp(/1)/exp(

]

, where

the ratio of the probe to the return areas (A) is taken from the experiment, whereas the

modification of the plasma potential due to the bias, ∆Vp, is given by

. The model predicts that ∆V[ )1(/))/(exp(ln ++=∆ AeATeVTV ebiasep p follows Vbias and that by

using negative biasing it is not possible to create a potential modification exceeding

Teln(A/A+1). The predicted dependence of j and ∆Vp with Vbias given by the model (A=35 for

the electrode) is also shown in figure 5. The agreement is reasonably good, confirming that it

is much more difficult to impose a negative voltage to the plasma than a positive one.

5. Discussion and conclusions The results presented have shown that both limiter and electrode bias can modify the

plasma behaviour, however, it is clear that the best confinement is achieved for positive

electrode bias. As expected, EB is more efficient than LB in modifying the radial electric field

and confinement. The stronger modification in Vp observed for EB results in larger electric

fields and consequently larger alterations in confinement. For the same applied voltage

(∆Vbias=240V), the modification in Vp imposed by EB (∆Vp=80V) is much larger than that for

LB (∆Vp=30V).

A good correlation between confinement modifications and ErxB shear has been found

for positive EB suggesting that confinement enhancement originates at the edge plasma as a

consequence of the formation of a particle transport barrier just inside the limiter. In these

bias experiments a radial electric field has been imposed in a controlled fashion and the issue

of causality between Er and confinement investigated in detail.

The radial electric field can be sustained by poloidal or toroidal velocity or by ion

pressure gradient. As confinement improves, edge pressure profiles became steeper. However,

the increase in Er is clearly not a consequence of the improvement in confinement because the

pressure gradient is negative, creating a negative Er, while the measured one is positive. The

observed Er is probably induced by the poloidal velocity, which results from the radial current

driven by the electrode.

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Figure 1: Schematic illustration of the electrode head.

Figure 2: Comparison of the main plasma parameters during alternating limiter and electrode bias

discharges (Vbias=120 p.V).

Figure 3: Time evolution of the edge plasma parameters for a series of discharges with

electrode bias.

Figure 4: Edge parameters radial profiles for a discharge with electrode bias.

Figure 5: Variation of the electrode current and plasma potential position with the applied voltage at

the electrode radial.