Plasma and Fusion Research: Regular Articles

Volume 11, 2402110 (2016)

Development of Magnetic Flux Surface
Measurement Method on TOKASTAR-2∗)
Yusuke SHIMOOKA, Hideki ARIMOTO, Takaaki FUJITA, Atsushi OKAMOTO,
Tatsuya SAKITO, Hiromasa ITOU, Kenji MURAOKA, Ryoichi SUGIOKA,
Kouhei YASUDA1) and Ryoma YOKOYAMA1)
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan1)
School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan(Received 30 November 2015 / Accepted 28 April 2016)
The measurement method of magnetic flux surfaces on TOKASTAR-2 was developed. The electron gun
was designed and fabricated. Calculation of the orbit of electrons showed that low energy of injected electrons,
typically less than 50 eV, was proper for the measurement to suppress large deviation of the electron orbits fromthe magnetic flux surfaces. The preliminary experiment for the method using a fluorescent screen showed thatthe acceleration voltage of 80 V was required and we need significant improvement of the electron gun to apply
the method using a fluorescent screen to TOKASTAR-2. The experiment for the method using a probe showedthat the required acceleration voltage was 10 V so we found that the method using a probe was proper for a small
plasma confinement device with weak magnetic field like TOKASTAR-2.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: TOKASTAR, tokamak, stellarator, magnetic flux surface, electron gun
DOI: 10.1585/pfr.11.2402110

1. Introduction
Plasma confinement in a stellarator device relies onformation of closed magnetic flux surfaces by externalcoils. Therefore, measuring magnetic flux surfaces experimentally is significant for stellarator experiment.
TOKASTAR-2 is a plasma confinement device which
has two coil systems, one for tokamak and the other forstellarator [1]. The purpose of TOKASTAR-2 is to study
the influence of helical magnetic field application to thetokamak plasma and the influence of the plasma current tohelical plasma.
Figure 1 shows the schematic view of the TOKASTAR-2 coil systems. The TOKASTAR-2 coil systems consist of eight Toroidal Field (TF) coils (50 turns each), twoPulsed Vertical Field (PVF) coils (20 turns each), threeblock Ohmic Heating (OH) coils, two Helical Field (HF)
coils (98 turns each), two Additional Helical Field (AHF)
coils (126 turns each ) and two Vertical Field (VF) coils(100 turns each). The VF coils are installed outside of
the vacuum vessel and the other coils are inside it. The
TF coils, the PVF coils and the OH coils are connected
to pulsed power supplies with capacitor banks while theAHF coils, the HF coils and the VF coils are connected
power supplies with steady state operation. The tokamak
coil system and the stellarator coil system can be operatedindependently.
The stellarator coil system was designed to makeauthor’s e-mail: fujita@ees.nagoya-u.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

Fig. 1 Schematic view of the TOKASTAR-2 coil systems.

closed magnetic flux surfaces without any plasma current.
However, due to the influence of manufacturing errors, installation errors of coils and error fields, the location andshape of the helical magnetic flux surfaces can be deviated from the ideal ones. Generally, detection of electrons
injected by an electron gun is used to measure vacuum
flux surfaces. The deviation of electron orbits from flux
surfaces becomes larger for larger electron energy withlarger drift velocity and hence lower acceleration voltage
is preferable. The allowable acceleration voltage would
be lower especially in a small device with weak magnetic
field. On the other hand, the electron current injected byan electron gun would be smaller for a lower acceleration voltage. Therefore the acceleration voltage should be
carefully determined for flux surface measurement in smalldevices. The required electron current would depend on
the method for detecting electrons. The object of this paper is feasibility study to measure the magnetic flux surfaces experimentally showing how to decide the acceleration voltage and the method for detecting electrons to mea-2402110-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2402110 (2016)

Fig. 2 Schematic view of the method using a probe.

sure magnetic flux surfaces in a small device with weakfield.

2. Method for Measurements
Two methods are commonly employed to detect electrons and measure magnetic flux surfaces experimentally.
One is a method using a fluorescent screen and the otheris a method using a probe [2–4]. In both methods electrons emitted from an electron gun are detected in vacuum,
without plasma. These methods are based on the property
that injected electrons travel along the field lines. Figure 2
shows a schematic view of the method using a probe. The
electron gun and the probe are located at diﬀerent toroidal
positions. The position of the probe is scanned horizontally and vertically at a fixed toroidal position. If the point
where electrons were injected and the position of the probe
are on the same magnetic flux surface, injected electrons
hit the probe and generate the probe current. The shape ofthe magnetic flux surface is obtained connecting the probe
positions where the probe current is detected.
Figure 3 shows a schematic view for a plan to apply
the method using a fluorescent screen to TOKASTAR-2.
The electrons are injected by the electron gun at a certain point and circle along the field lines just like in themethod using a probe. A highly transparent fluorescentmesh screen is installed on a side of one of the TF coils.
A part of electrons hit the fluorescent screen while most of
the electrons do not hit the fluorescent screen but keep oncircling around the torus since the screen is highly transparent. The electrons hit the fluorescent screen at the otherlaps and the points hit by the electrons emit fluorescentlight. The image of fluorescent light is observed by a CCD
camera.
These methods need development of an electron gunand analysis of the orbit of electrons commonly. On theother hand, the method using a probe takes longer timethan the other since the measurement should be repeatedby changing the probe position horizontally and verticallycovering the measurement area inside the walls. It also
needs a mechanical system to move the probe horizontallyand vertically. Thus, the method using a fluorescent screen

Fig. 3 Schematic view for a plan to apply the method using afluorescent screen to TOKASTAR-2.

is basically superior to the method using a probe. Then,
we designed the electron gun and analyzed the orbit ofinjected electrons first. After that, we examined whether
the method using a fluorescent screen can be applied toTOKASTAR-2. In examination, we focused on estimationof the acceleration voltage required for fluorescent lightemission.

3. Design of the Electron Gun
We designed and fabricated the electron gun for measurements. Figure 4 shows the assembly drawing of theelectron gun. The electrons are emitted as thermion fromthe tungsten filament and injected through a 1 mm aperture in the filament holder surface. There were four important points in designing this electron gun. First, a partnear the filament will become hot and hence the electricalisolation parts were made from ceramic. Second, we decided the diameter of the holder to be 7 mm so that the injected electrons will not hit the holder when they circle thetorus. The vertical distance between the point where electrons are injected and the point where the electrons comeafter circling the torus once was evaluated to be 3.7 mm ina typical magnetic field configuration by a field line tracing code HSD [5]. Third, the radial position of the gun
can be adjusted by using a bellows between R = 0.12 m
and R = 0.16 m, where R is the major radius. Finally, thestainless-steel filament holder can be turned to adjust theinjection angle between −15 degrees and +15 degrees fromthe equatorial plane keeping the vacuum of the device.

4. Analysis of the Electron Orbit
We calculated the orbit of electrons in the typical magnetic configuration condition using HSD. Figure 5 shows
the result of the calculation. The outermost line indicatesthe last closed magnetic flux surface in this condition andthe other lines indicate the orbits of the guiding center ofthe electron with several energy. The electrons were in-2402110-2

Plasma and Fusion Research: Regular Articles

Volume 11, 2402110 (2016)

Fig. 6 Layout of the preliminary experiment for the method using a fluorescent screen.

Fig. 4 The assembly drawing of the electron gun.

Fig. 7 Conditions for detecting fluorescence in the preliminaryexperiment.

Fig. 5 Orbit of electrons in the typical coil current condition.

jected parallel to the equatorial plane at R = 0.159 m, Z =0 m, on the last closed magnetic flux surface. The electrons
will be injected at this point in this coil current condition in
the measurements. The higher the energy of electrons was,the more largely the orbit of electrons deviated from themagnetic flux surface. When the energy of electrons was
50 eV or more, a part of the orbit was outside of the magnetic flux surface. The deviation between the last closed
magnetic flux surface and the orbit of electrons became
smaller on the outer side, around R = 0.16 m. The deviation is in inverse to the local rotational transform. Since
the helical coils are located at R = 0.24 m, the local rotational transform is larger on the outer side than in the otherareas and hence the deviation is smaller. From the analysisof electron orbits, we found that the lower energy of theelectrons, typically less than 50 eV, was proper to measure
the magnetic flux surfaces in TOKASTAR-2.

5. Preliminary Experiment for theMethod Using a Fluorescent Screen
We performed preliminary experiment without magnetic field to estimate the electron energy required to observe the fluorescent agent emission. Agents P15 and P24(designation of Joint Electron Device Engineering Coun-cil, ZnO:Zn) were used because these agents have goodemission eﬃciency for low energy electrons [6]. A fluorescent mesh screen made of the stainless steel wires withdiameter of 0.2 mm was used. Figure 6 shows the layout of the preliminary experiment. The distance between
the filament holder and the fluorescent screen was 10 cm.
The electrons were injected by acceleration voltage, Va ,and emission of light was observed through the viewing
port by eye. The acceleration voltage and the power of thefilament were scanned.
Figure 7 shows the results of the preliminary experiment. As shown in Fig. 7, the electron energy larger thanabout 80 eV was required to observe the fluorescence ofP15 and P24. Since the deviation between the orbit ofelectron with 80 eV and the magnetic flux surface is large
as shown in Fig. 5, we concluded that significant improvement of the electron gun was needed to apply the method
using a fluorescent screen to TOKASTAR-2.
The required electron energy in this experiment isquite larger than the work function of ZnO:Zn of about
2.5 eV [7]. This is because the light emitted from the hot
filament and reflected on the fluorescent screen hinders detection of weak fluorescent light emission.

6. Preliminary Experiment for theMethod Using a Probe

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We performed preliminary experiment for the method

Plasma and Fusion Research: Regular Articles

Volume 11, 2402110 (2016)
two vertical dotted lines in Fig. 9 (a) was averaged for eachshot. Figure 9 (b) indicates 20 shots average of the averaged current as a function of the acceleration voltage. We
confirmed that the probe could detect the current for theacceleration voltage of 10 V and the pulse width of 200 µs
so we decided that the method using a probe was proper
for TOKASTAR-2.

7. Summary and Discussion

Fig. 8 Layout of the experiment for the method using a probe.

Fig. 9 (a) The time evolution of the probe current. (b) The observed current as a function of the acceleration voltage.

using a probe to estimate the lowest acceleration voltage
for the probe to detect the current by injected electrons.
The acceleration voltage was applied with a pulse circuit.
The pulse width was 200 µs. This corresponds to the flattop duration of TF coil current in TOKASTAR-2. Theprobe tip was in a square shape with 5 mm × 5 mm. The
electric power of the filament was 9 W. Figure 8 shows
the layout of the preliminary experiment. We measured
the voltage of 1 MΩ resistance connected to the probe forevaluating the probe current. Figure 9 (a) shows the time
evolution of the probe current. The probe current due toinjected electrons is indicated by the arrows. The large
spikes on the both sides of the flat part of the probe current
are noise; they appeared even when the filament current
was turned oﬀ. The current in the flat part between theThe method to measure magnetic flux surfaces onTOKASTAR-2 was developed. The electron gun was designed and fabricated. The orbit of injected electrons
was calculated showing that the low energy of the electrons, typically less than 50 eV, was proper to measure themagnetic flux surfaces in TOKASTAR-2. The acceleration voltage required to observe fluorescent agent emission light was investigated for the fabricated electron gun
and was found to be about 80 V, for which the deviation ofelectron orbits from the flux surface would be unacceptably
large, and then we concluded that significant improvementof the electron gun was needed to apply the method using a fluorescent screen to TOKASTAR-2. On the other
hand we confirmed that the probe could detect the electron
current for the acceleration voltage of 10 V and the pulse
width of 200 µs so we found that the method using a probe
was proper for a small device with weak magnetic field like
TOKASTAR-2.
The probe tip used in the preliminary experiment wasin a square shape with 5 mm × 5 mm. In the measurements,a smaller probe tip is proper considering the space resolution of the flux surface measurement and then the probe
current would be smaller. If the obtained current was insuﬃcient for a smaller probe tip, improvement of the electron gun, for example changing the tungsten filament tothoriated tungsten filament, would be required.

Acknowledgments
This work was performed with the support and under
the auspices of the NIFS Collaboration Research program(NIFS14KOAP027) including the NIFS Joint Use Programof Measurement Instruments.
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