J. Plasma Fusion Res. SERIES, Vol.3 (2000) 4G-50

Observation of the H-mode and Electron ITB by Heavy lon
Beam Probe on T-10 Tokamak
MELNIKOV Alexander*, ELISEEV Leonid, RAZUMOVA Ksenia, BONDARENKO lvan|), KRUPNIK Ludmilal)
KHREBTOV Sergeyl), KOMAROV Alexanderr), KoZACHEK Alexanderr) and T-10 teamRussian Research Centre "Kurchatov Institute", Moscow, 123182, Russiat)Ukrainian Scientiftc Cente "Kharkov Institute of Physics and Technology", Kharkov, Ukraine(Received: 18 January 2000 / Accepted: 12 May 2000)

Abstract
The Heavy Ion Beam Probe (HIBP) diagnostics on T-10 tokamak was modified to measure the localvalues of the plasma potential in the ECRH plasma. The Tl'ion beam with the energy up to 240 keV andintensity about a few dozens pA was used. The clear fall ofpotential in the range of-200 - -600 V wasobserved in the outward EC resonance during spontaneous improvement of plasma confinement. The fall
down of the potential accompanies the fall of Do and the density increase. The time evolution of thepotential profile demonstrates formation of the thin (about I cm) area near the limiter with strong electricfield E, (-300 V/cm). All this means that the H-mode with ECRH as the sole source of the auxiliary
heating in circular limiter tokamak was first obtained. The comparison with DIII-D and JFT-2M data
demonstrates the similarity of typical features of the L-H transition.
The measurements during the electron internal transport barrier (EITB) formation show theappearance of the local negative potential well about I kV in the region of the barrier. The local transient
values of the E, reach -1000 V/cm.

Keywords:
T-10 tokamak. ECRH. HIBP. H-mode. electron ITB

1. lntroduction

understand the underlying mechanisms that form
transport processes in plasma. It is known [5,6] that thechange of the electric field is fundamental for the Hmode. The study of E, in the L-H transition and the ITB
formation is the most desirable target for the SST
relevant research now. The Heavy Ion Beam Probe

One of the promising approaches to the steady state

tokamak (SST) is the electron cyclotron current drive(ECCD). Simultaneously EC waves can heat theelectrons and change the current profile, thus stabilize
the MHD activity. It would be beneficial to run the SST
with improvement of confinement (H-mode or ITB) [].

(HIBP) is the only diagnostics allowing us to investigate
directly the potential in the core and edge plasmas. The
local value of the plasma potential can be measured by
the change of the beam energy in the sample volume.
The intensity of the secondary beam indicates the local

Such regimes have been obtained in tokamaks withelongated cross section and divertor mainly. Recently
ITB was obtained in the circular limiter tokamak T-10
with ECRH [2,3].
The importance of the radial electric field E, for the
improved plasma confinement is well recognized nowt4,51. It is expected that knowledge of E, can help us to*density.

The first HIBP potential measurements near the@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchC o rre sp ondin g aut ho r' s e - mail : me lnik @ nfi . kiae. ru46

Melnikov A. et al., Observation of the H-mode and Electron ITB by Heavy Ion Beam Probe on T-10 Tokamak(full lines) in comparison with the reference one (dash).
The simultaneous effects: the rise of the line-averaged
density and the decrease of Do allow us to detect thetypical features of the H-mode. The total stored energy
14/ and soft X-rays emission increase correspondingly.
The difference of the two shots lies in the small
variation of /o.
HIBP observes the following:

separatrix during the sawtooth-triggered L-H transition
were perfonned in JFT-2M [7]. In T-10 the hints on theconfinement improvement were observed during theECRH pulse [2], but the improvement was notpronounced clearly enough. The HIBP measurementsperformed during the improvement of confinement in T10 show the strong fall down of the potential [8,9]. The
present work reports the first clear observation of the Hmode with ECRH as the sole source of the auxiliary

1. The plasma electric potential @ at r = 18.3 cm (.t = r/a = 0.73) rapidly decreases during the density increase (/= 800 - 850 ms). The absolute change ofthe potential isheating in circular limiter tokamak.

It also presents the first observation of the-600 V. Later on (t = 850 - 900 ms) the density rises
more slowly and the potential decreases slowly too.
After the end of the gyrotron pulse (900 ms) thepotential retums back to the initial Ohmic value.
2. Figure 2 shows the radial distribution of the extra

appearance of the local negative potential well in theregion of the electron ITB.

2. HIBP Diagnostic Set-up
To study the ECRH-ECCD plasmas on T-10 theHIBP diagnostic was improved [8]. The energy of Tl*beam increased up to 240 keV. The power supply
provides the measurements during the 7 ms every 20

potential over the initial steady state values. The Dostarts to drop at to= 78O ms. The time variation ofprofile shows three clear phases:a) preceding phase, 730 -:- 780 ms, when the innerregion with a small local potential well is formed;

ms.

The T-10 HIBP measures the plasma electric
potential in two main modes:
Shot by shot measarements prodlce the time evolutionof the plasma parameters in every desired point of thedetector grid.
Scanning along the detector line in the single shot produce a set of plasma parameter profiles.
The energy resolution of the analyzer with parallel
plates was tested by the calibration [10]. The analyzer-15xtto?- 0E r.r
2osensitivity to the variation of the potential during theshot depends on the beam intensity. In the discussed
measurements it is less than 20 V.
The spatial resolution determined as the radial sizeof the sample volume is 5-10 mm. The temporal
resolution was limited by the data acquisition systemwith sampling time lG-64 ps.

; 1.2
1.05
.100

g-0-400o.E?

3. Experimental Results

GThe search of the ITB on T-10 (R = 150 cm, a = 30cm) was made by the variation of safety factor q(r)using EC power deposition near the rational 4 values[2,3]. The off-axis ECRH experiment was done with Bs= 2.28T, Ip = 150 kA, n" = 1.2 x 1013 cm-3, Pu. - 0.5
MW, a1i^ = 25 cm. The electron temperature T"(O) = 1.7
keV was measured by Thomson scattering and the ion=oo?rocxat,jOl. . | . . . . |500.

. . | .

600

700. . ' ..

Eoo' . | .r..1.8900

time, mstemperature Tr(0) = 0.5 keV was derived from NPA
data. The EC-resonance lies at r = 14 cm. The outer area(r > l8 cm) was achievable for HIBP.
Figure I presents the time history of the typicalshot with the spontaneous improvement of confinement

Fig. 1 Time traces of some plasma parameters in the
shot with H-mode, #23702, lp= 152 kA (full lines),
and with the L-mode one, #23706, L = 158 kA(dashed lines).

^1Melnikov A. et al., Observation of the H-mode and Electron ITB bv Heavv Ion Beam Probe on T-10 Tokamak

N23724

N23724

H = 2.28 T, to - 780 ms,fo, = 160 kA, a,., = 25 cm

H = 2.28 T, to - 780 ms,lo, = 160 kA, dri. = 25 cm---r-t=735ms--f-t=854ms---4-t = 874 ms
22r, cmr, cm
Fig. 3 The temporal evolution of the relative beam
current. Indication of the formation of the density
barrier in the H-mode.

Fig. 2 The temporal evolution of the potential profiles inthe shot with H-mode: circles - before transition,
squares - the transient stage, triangles - aftertransition.

region (Fig. 4). The depth of the well is about I kV, thelocal E, value is -1000 V/cm.

b) transient phase, 780 + 870 ms, when the innerpotential falls down to -300 V and the limiter potential
remains the same. This forms the layer (about I cm)
with strong E. (-300 V/cm). This layer does not change
during the Dodrop.
c) the final phase, 870 + 900 ms, when the potential
profile reaches the steady state.

4. Discussion
Comparison with H-modes on JFT-2M and DIII-D

The observed potential drop during theimprovement of the plasma confinement in T-10 is very

3. HIBP is sensitive to the plasma density. Thesimilar to the potential change at the L-H transition inJFI-2M [7]. In both cases the rise of negative potential

secondary ion beam current is proportional to the local
plasma density I - nf(T") (the attenuation factors can beindicates the improvement of the electron confinement.
HIBP density data is also in a good accordance with theneglected here). The time evolution of the relative value

JFT-2M ones. In both cases / at the edge drops down to-307o simultaneously with Do, while the inner signal
remains almost the same. The range of these phenomena
in both cases are very similar, particularly:i) the extra potential well with the flat bottom -300 Vof 1 with respect to the initial steady state value 1s ispresented in Fig. 3. It shows the rapid increase of thedensity gradient during the transition. It indicates theformation of the edge transport barrier.

4. In the experiments presented in Figs. 1-3, the ECresonance (r = 14 cm) was outside the area available forHIBP. The decrease of the toroidal field down to B =and the steep walls;ii) the width of the gradient layer is about I cm, thevalue of the extra E, = 300 V/cm;

2.14 T leads to overlapping of these two areas. The
achievable area for HIBP became 13 < r < 20 cm while
EC-resonance moves to r =17 cm). The EITB was
obtained with 1, = 290 kA, n" = 1.2 x l0r3 cm-3, Pu. =0.5 MW, a1;^= 3O cm [3]. The local negative potential
well appears simultaneously with EITB formation in itsiii) the time scales of the phenornena and Do decay arethe same.
The features of the edge transport, particularly thetransport barrier formation in divertor (JFT-2M) andlimiter (T-10) plasmas may cause the difference in thetime scales of the decav.
48

Melnikov A. et al., Observation of the H-mode and Electron ITB bv Heavv Ion Beam Probe on T-10 Tokamak
307o. This comparatively small values coincide with thelimiter H-mode data in JFT-2M [ll,l21 where theelongation ft is varied. When ft decreases from ft = 1.3 tok = 1.O3, the improvement in the energy confinement
time decreases from 30Vo to l0vo llll. So, the smallTt-' Ivalues of the H-factor looks to be typical for the limiter

H-modes obtained till the moment. This feature does notdepend on the heating method: NBI that heats solely theions, ICRH that heats mainly the electrons in JFT-2M

0n[2] and ECRH that heats solely the electrons on T-10,
lead to the similar features of the H-modes.

0The main difference between the discussed
experiments and the previous ones is the time scale ofthe L-H transition. Put the time duration of the Do decay
as a time scale of the transition, assuming the following
phase with the low level of Do as the H-mode. The timescale of the transition on T-10 is 40-90 ms, while it is-n3o{x)0.2 ms in the sawtooth-triggered NBI H-mode in JFT2M. In both cases "fast" Do decay time coincides with-r(no-1zIto1o r, crn n

the inner potential drop and the strong E, layer
formation (transient phase in Sec. 3). The later steady3l)

state phase shows the slow Do decay and the same forFig. 4 The electron temperature profiles (a) and theplasma potential (b) in the similar shots #24264
and #24273 with internal transport barrier (lTB); l)

the potential. The papers which described the limiter Hmode do not focus on the duration of the L-H transition.

On the basis of the presented time traces of the Do wecan estimate this time from less than a few ms in NBI orICRF heated limiter and divertor plasmas in JFT-2M

t = 753 ms. before lTB, ll) t = 853 ms, during ITB
formation, lll) f = 893 ms, after ITB formation.

tll,l2l and DIII-D [5] to l0 ms in NBI heated limiter
plasma in JFT-2M [3]. These values are a few times

Analyzing the direct measurements on JFT-2M Ul
we can conclude that the fast potential drop near theedge and the formation of the thin layer with the strong
electric field is the characteristic feature of the H-mode.
Indirectly the E, can be derived from the spectroscopic
measurements of the rotation velocity and the ion
momentum balance equation. Such measurements in theH-mode were done in DIII-D and JFT-2M for NBI
divertor plasmas. Their main features are:i) the width of the E, shear layer just inside theless than the L-mode energy confinement time 26. On
the contrary, the transition to the ECRH divertor Hmode in DIII-D takes 40 ms that is comparable with theL-mode 16 = JJ+$ ms [14]. Therefore, the slow L-H
transition in a time scale of diffusivity looks to be acharacteristic of the ECRH H-mode and it does notdepend on the limiter or divertor configuration. Thisconclusion looks even more robust if we take into
account that in T-10 we are very close to the power
threshold. while DIII-D data obtained from the exact

separatrix is l-2 cm,ii) the values of the extra E, = -300 V/cm in DIII-D [5]threshold to the large extra power.

and E, = -100 V/cm in JFT-2M [6],5. Conclusionsiii) the time scales of the phenomenon and Do decay are

Simultaneous increase of the density and the total

the same.
They all corespond to the direct measurements of [7].
The observation of the similar phenomenon on Tl0 leads us to the conclusion that in the regime with offaxis ECRH we meet the H-mode.

stored energy, decrease of Do, the formation of the thinedge layer with the strong electric field and the edge
density barrier demonstrate the establishment of H-modein the limiter circular tokamak T-10.
The similar scale machines T-10 and JFT-2M andthe bigger DIII-D show the very similar scale of thephenomenon: E, -300V/cm in the -1 cm layer at theThe features of the H-mode on T-10

The improvement in confinement varies within l049

Melnikov A. et al., Observation of the H-mode and Electron ITB by Heavy Ion Beam Probe on T-10 Tokamak

plasma edge, while the former has a limiter and otherControl. Fusion, Maastricht, 23J,301 (1999).
t3l V.V. Alikaev, A.Ya.Kislov, K.A. Razumova et al.,submitted to Plasma Phys. and Control. Fusion.
[4] S-I ltoh, K.Itoh., Phys. Rev. Lett. 60,2276 (1998).
t5l K. Burell et a/., Plasma Phys. Control. Fusion. 31,

ones have the divertors. The similar value of the E, canindicate to the similar physics, for example thesuppression of the same instability by E x B shear flow.

The local transient negative potential well appears
during EITB formation in its region with the depth ofabout I kV.
The further measurements are necessary to know
the interconnection between the internal (ITB) andexternal (H-mode) transport barriers in ECRH plasma tocontribute the SST optimal scenario.

1649 (1989).

t6l K. ldaet al., Phys. Rev. Lett. 65, 1364 (1990).
t7l Y.Hamada et al., l7th IAEA Fusion Energy Conference, Yokohama (1998).
[8] A.V. Melnikov et al.,26 EPS Conf. Plasma Phys
Control. Fusion, Maastricht, 23J,829 (1999).
[9] A.V. Melnikov and L.G. Eliseev, Czechoslovak J.
Phys.49 (Suppl. 35) 35 (1999).

Acknowledgment
This work was supported by Russian Basic
Research Foundation, Grant No 99-02-18457. We are
grateful to Dr. S. Lysenko for the manuscript

ll0l A.V. Melnikov et al.,Rev. Sci. Instrum.6E (l) 308(1997\.

Il]H. Matsumoto et al., Nucl. Fusion. 27, llSlpreparation.

( I e87).

|21H. Matsumoto et al., 14 EPS Conf. Plasma Phys
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