Proceedings of ITC/ISHW2007

Time Evolution of Spatial Structure of Radial Electric Field inTohoku University Heliac
H. Aoyama, S. Kitajima, M. Sasao, A. Okamoto, T. Kobuchi, Y. Tanaka, H. Utoh, H. Umetsu,
K. Ishii, H. Takahashia
Department of Quantum Science and Energy Engineering, Tohoku University, Sendai 980-8579, JapanaNational Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan

Radial electric field is closely related with the confinement of plasma. In Tohoku University Heliac (TU-Heliac),
the electrode biasing experiments were carried out to control the radial electric field actively by ramping up/downthe electrode current. An emissive probe array consisting of three filaments was designed as new measurement
equipment. With this probe, time evolutions of the spatial structure of the radial electric field were measured in thebiasing experiments. There was the tendency for the radial electric field to extend from the inner region to the outerregion while the electrode current was ramping up. In the plasma inner region, the radial electric field wasmaintained the longer time compared with the outer region while the electrode current was ramping down. The
radial electric field, poloidal flow of plasma and fluctuation level of the ion saturation current changed appreciably
in the period when the electrode voltage showed nonlinearity against the electrode current. Therefore, the plasma
nonlinearity period was the transition region to the improved mode.
Keywords: radial electric field, emissive probe, electrode biasing experiment, LH transition, fluctuation

1. Introduction
The neoclassical theory describes that the ion
viscosity has a local maximal value against the poloidal
rotation velocity [1]. The rapid increase of the poloidal
rotation arises when the driving force in the poloidal
direction exceeds a critical value. At that moment, it isconsidered that the plasma transits into the improved
mode. One of the characteristics of the transition into theimproved mode is sudden growth of the radial electricfield [2]. Therefore, it is important to investigate the time
evolution of the spatial structure of the radial electricfield to comprehend the detailed transition mechanism.
The radial electric field can be controlled actively by theelectrode biasing experiments. In fact, the electrodebiasing experiments have been done in various devices [3,4]. In addition, the shear of the radial electric field plays
an important role in the improvement of the plasma
confinement with suppression of fluctuation level [5, 6].
In TU-Heliac, the electrode biasing experiments
using a hot cathode made of LaB6 have been carried out
to control the transition into the improved mode [7, 8].
We tried the forward/reverse transition experiments by
ramping up/down the electrode current. In thoseexperiments, the negative plasma resistance and thehysteresis between the radial electric field and the storedenergy were observed in the transition region [9]. The
nonlinear change of the electrode voltage against theauthor’s e-mail: hiroshi.aoyama@ppl2.qse.tohoku.ac.jp

electrode current is one of the characteristics of thetransition. To understand the behavior of the radial
electric field in the transition region, it is important tomeasure the time evolution of the spatial structure of theradial electric field around the period when the electrodevoltage shows nonlinearity against the electrode current.
For this purpose, we developed the emissive probe arrayto define the radial electric field. Using this probe, wemeasured the distributions of the radial electric field andcompared them with the results of a mach probe and thefluctuation level of the ion saturation current.

2. Experimental Setup
TU-Heliac is a small heliac device with three sets ofmagnetic field coils. Top view of TU-Heliac is shown inFig. 1. The size of this device is as follows: major radius R= 0.48 m, and average plasma radius a ∼ 0.07 m. In
electrode biasing experiments, the target plasma was He
plasma produced by alternative ohmic heating with f =18.8 kHz, P ~ 35 kW and the discharge time was 10 ms.
Electron temperature Te, electron density ne, floating
potential Vf and their fluctuation level were measured by
the triple probe placed at toroidal angle φ = 0 °.
The hot cathode made of LaB6 was used as anelectrode in the electrode biasing experiments. The hot
cathode was a cylindrical shape with a length of 17 mm
and a diameter of 10 mm. This cathode was inserted intoProceedings of ITC/ISHW2007
measurement of the Vs compared with other Langmuirprobe methods. In this experiment, the electron density
was ne ∼ 1012 cm-3, the filaments were heated to 2000 K
or less by the heating current about 2 A to 2.5 A, and thefloating potential was almost saturated in this condition.
This probe is able to measure the Vs in three points at thesame time. Therefore, the coincident measurements of theEr at three points can be carried out.

Fig. 1 Top view of TU-Heliac and cross section ofmagnetic surface at φ = 270˚.

Fig. 2 Schematic of the emissive probe arraythe plasma horizontally at φ = 270˚. In the electrodebiasing experiments, the electrode was negatively biased
against the vacuum vessel by a current control power
supply.
The plasma poloidal flow relates closely to the radial
electric field Er. The mach probe was used to compare theEr with the plasma poloidal flow. It was inserted verticallyinto the plasma from the upper port at φ = 159˚.
We designed the emissive probe array shown in Fig.
2. It had three filaments made of 1% thoriated tungsten(Th-W) with a diameter of 0.1 mm. Th-W is very usefulmaterial to emit the electron at low temperature comparedwith pure tungsten. The intervals of each filament were 5mm. The Th-W wires were 30 mm in length and bent intoa circle with a diameter of 1 mm. The both ends of thewires were connected with the tungsten wires with adiameter of 0.55 mm in a doubly drilled ceramic tube.
This probe was inserted vertically into the plasma fromthe upper port at φ = 270˚. The all filaments were set
facing toward the magnetic axis. This setting was adopted
not to intersect the plasma flow and the magnetic fieldline. When the filaments are heated, they start to emit
electrons and the floating potential measured by theemissive probe shifts to the plasma space potential Vs .
The advantage of emissive probe is the direct

3. Experimental Results
Figure 3 shows the typical time evolution of theelectrode voltage VE and the current IE. The IE wascontrolled such that (a) the I E was ramped up to -4 A for 6ms starting from 4 ms or (b) the IE was maintained at -4 A
at first, and was ramped down to 0 A for 6 ms. In theseexperiments, the hot cathode was located at ρ = 0.18 ∼0.62, where ρ is the normalized minor radius defined byρ = <r>/a, <r> is the average radius of the flux surface.
The VE shows nonlinearity against the IE in the period
surrounded by broken lines.
The radial profiles of the Vs measured by the #1
filament in the emissive probe array are shown in Fig. 4.
The each profile was obtained from the results of 18
discharges. The broken lines in Fig. 4 are corresponding
to that drawn in Fig. 3, and the solid line indicates thestart time of ramping up/down. As can be seen in Fig. 4(a), the Vs became deeper negatively, starting from theFig. 3 Typical time evolution of V E and IE; (a) in thedischarge of the IE was ramped up, (b) in thedischarge of the IE was ramped down.

Proceedings of ITC/ISHW2007

Fig. 4 The plasma space potential Vs measured
with the emissive probe array in the discharge of(a) IE ramped up and (b) IE ramped down.

plasma inner region when the IE was ramped up. In Fig.4(b), when the IE was ramped down, the Vs changed intoshallow level from the plasma outer region. The similarresults were also obtained with the other two filaments #2and #3. The Vs near the magnetic axis (ρ < 0.35) could
not be measured with the emissive probe array, because
the probe scanned the plasma along the code that was
off-center to the magnetic axis.
Figure 5 shows the time evolution of the spatial
structure of (a) the Er, (b) the current ratio of the machprobe and (c) the fluctuation level of the ion saturation
current in the discharges of IE ramped up, which iscorresponding to Fig.3 (a). The Er was calculated fromthe differential of the Vs measured with filaments of #1
and #3. In Fig. 5 (b), Iup and Idown indicate the ion
saturation currents, which flowed into pins at upstreamside and downstream side. It can be considered that thecurrent ratio of the mach probe shows the plasma flow,which has a close connection to the Er. Moreover, thesuppression of fluctuation level also relates to the Er andits shear. In this experiment, the fluctuation level was theintegral of power spectrum from 0 to 100 kHz. As can beseen in Fig. 5 (a), the strong Er arose in the plasma inner
region, and diffused to the plasma outer region. Figure 5(b) indicates the poloidal plasma flow in the electron
diamagnetic direction, which is the same direction as theExB poloidal flow. The high-speed flow spread frominside to outside. Figure 5 (c) shows that the fluctuation

Fig. 5 The time evolution of the spatial structure of (a)the Er, (b) the current ratio of the mach probe, (c) thefluctuation level of ion saturation current. All of themshow the data in the discharges of the IE ramped up.
Solid line indicates the start time of ramping up.
Broken lines are indicator of nonlinear period.

level was suppressed after the second broken line (t > 8.7ms), especially in the inner region (ρ < 0.65). In theregion of ρ < 0.7 and t > 8 ms, the region where thestrong Er was formed was corresponding to the highcurrent ratio region of the mach probe and the low
fluctuation region. In the period surrounded by broken
line in Fig. 5, the Er and the current ratio weresignificantly increased. And after the second broken line,
both of them were saturated at higher values. On thecontrary, the fluctuation level changed gradually in thisperiod and was suppressed remarkably after this period.
The results in the discharges of the IE ramped down
are shown in Fig. 6. In the plasma inner region the strong
Er was maintained the longer time compared to the outerregion. The plasma flow gradually weakened from theplasma outer region. The fluctuation level wascompressed at the low level during 2 < t < 6 ms. This
period was corresponding to that with the strong Er andthe large current ratio shown in Fig. 6 (a) and (b). This

Proceedings of ITC/ISHW2007
mode.
4. Summary
It was demonstrated that the emissive probe array
had enough capability to measure the time evolution of
the spatial structure of the plasma space potential in theelectrode bias experiments in TU-Heliac. The estimationof the spatial profile of the radial electric field from theresults of the plasma space potential was carried out. It
was confirmed that the strong radial electric field spreadfrom the inner region to the outer region in the dischargeof the IE ramped up, and was maintained for the longertime in the inner region in the discharge of the IE ramped
down. The characteristic region of the spatial and time
profile of the radial electric field was corresponding tothose of the current ratio of the mach probe and thefluctuation level of the ion saturation current, although
the radial electric field had weak correspondence to theplasma poloidal flow at the part of the plasma outerregion.

Fig. 6 The time evolution of the spatial structure of (a)the E r, (b) current ratio of the mach probe, (c) thefluctuation level of the ion saturation current. All ofthem show the data in the discharge of the IE ramped
down. Solid line indicates the start time of ramping up.
Broken lines are indicator of nonlinear period.

The Er, the flow and the fluctuation level changed
appreciably in the period when the bias electrode voltage
shows nonlinearity against the bias electrode current.
Therefore the plasma nonlinearity region was thetransition region to the improved mode.

Acknowledgement
This work was supported by a Grant-in-Aid from theMinistry of Education, Science and Culture of Japan (No.
17360439).

Referencesindicates that the Er suppressed the fluctuation level. In
the period surrounded by broken lines, the Er decreased
appreciably, and the fluctuation level had the maximum
value in the plasma outer region. Although the current
ratio did not change greatly in the outer region, the largechange was seen in the inner region (ρ < 0.6). After thesecond broken line (t > 8 ms), the Er became strong again
and the fluctuation level became lower. It is suggested
that the Er was connected with the suppression of thefluctuation.
In both of the discharges of the IE ramped up anddown (shown in Fig. 5 and Fig. 6), the Er was sustained athigh level although the current ratio was very small in thepart of outer region.
The strong Er, the high-speed flow and thesuppression of the fluctuation were observed at the periodof t > 8.7 ms in Fig. 5 and t < 6 ms in Fig.6. These results
suggested characteristics of the improved mode.
Therefore the plasma nonlinearity region surrounded bybroken lines was the transition region to the improved

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