Plasma and Fusion Research: Rapid Communications

Volume 4, 039 (2009)

Improvement of Field Accuracy and Plasma Performance in the
RT-1 Device
Yoshihisa YANO, Zensho YOSHIDA, Junji MORIKAWA, Haruhiko SAITOH, Hiroyuki HAYASHIand Tatsunori MIZUSHIMA
Department of Advanced Energy, University of Tokyo, Kashiwa, Chiba 277-8561, Japan(Received 19 June 2009 / Accepted 3 July 2009)
To improve the accuracy of the magnetic field of Ring Trap-1 (RT-1), we have constructed a system ofcorrection coils to cancel the geomagnetic field and control the attitude of the floating magnet. Without thegeomagnetic field canccellation, the floating magnet tilts about 1.4 degrees. The previous prototype correction
coils have been replaced by new coils that are much larger and farther from the chamber, so the error field dueto the multipole components of the correction field is reduced by a factor of 30 (from 2.6% to 0.1% of theconfinement field near the edge region). A significant improvement in plasma confinement has been observed(the stored energy of the plasma has been increased by a factor of 1.5).
c 2009 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: levitated dipole confinement, field accuracy, geomagnetic, correction coils, high-beta plasma
DOI: 10.1585/pfr.4.039
The Ring Trap-1 (RT-1) device has a magnetosphericconfinement field generated primarily by a superconducting ring magnet floating in a vacuum chamber [1, 2]. The
lifting coil is located at the top of the chamber. The motion of the magnet has six degrees of freedom consistingof the position of the center of the mass (x, y, z), the attitude (θ x , θy ) and the rotation θz . In RT-1, the magnet isunstable with respect to vertical (z) motion, while it is stable with respect to sliding (x, y) and tilting (θ x , θy ) motions.
We suppress the vertical instability of the magnet by a PID
feedback control system, and regulate the displacement in z
within 0.1 mm. Although the tilting motion is stable, small
nonuniformities of the mass distribution of the ring magnet as well as the geomagnetic field yield a finite tilt andslow oscillations of the attitude. The tilt due to the mass
unbalance is reduced to less than 0.05 degrees (from 0.88
degrees initially) by adjusting the mass distribution. The
aim of this development is to minimize the attitude-angleof the floating magnet and improve the plasma confinementby canceling the geomagnetic field with a suﬃciently small
error field.
The RT-1 experiment began operation with smaller
prototype correction coils that were installed near thechamber. Using these coils, we tested the attitude control
and proved that the magnet can be made level when weapply a 0.3 G horizontal magnetic field, which just cancels
the horizontal component of the geomagnetic field in Eastern Japan [3]. The new and prototype correction coils arecompared in Figs. 1 and 2. The correction coils are composed of two Helmholtz coils crossing at right angles. Theprototype coils are located at r = 1.4 m from the center ofFig. 1 Photograph of the RT-1 device and the correction coils.
The prototype coils have dimensions of 1.2 m × 0.8 m.
The new correction coils are made of aluminum selfstanding frames with a measurements of 4.5 m × 4.5 m× 4.0 m.

the chamber (about 0.5 m from the edge of the plasma region) so the error field (multipole component produced by
the correction coils) is considerably strong. The new system of correction coils was designed to reduce the error
field less than 0.1% of the mean field at the edge region.
Numerical estimates of the error fields of both correction
coils are shown in Fig. 3, where we assume a 0.3 G geomagnetic field to the north; the correction coil current is
set to make the average field at r = 0.25 m (magnet position) to cancel the geomagnetic field. The prototype coils
produce a maximum error field of about 1.3 G, which isauthor’s e-mail: yano@ppl.k.u-tokyo.ac.jp039-1

c 2009 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Rapid Communications

Volume 4, 039 (2009)

Fig. 4 Demonstration of attitude control. The vertical positionof the floating magnet is measured at three points (P1,P2, P3) on the upper surface of the coil casing (shown inupper figures). The tilt angle is estimated by the diﬀerences among these three positions (shown in lower figures). In these plots, the signals have been averaged over
30 seconds to reduce high-frequency fluctuations causedby micro-scratches on the magnet’s surface. (a) Left:
Without the correction field, the magnet is tilted about 1.4
degrees. (b) Right: With the optimum correction field,
the tilt is decreased to 0.05 degrees.

Fig. 2 The arrangement of the RT-1 device and the correction
coils. The prototype coils are not parallel to the geomagnetic field, while the new coils are set approximately parallel to the geomagnetic field. The EW coils are installed
for the fine adjustment of the magnetic field.

Fig. 3 Calculation results of the strength of the error field causedby the multipole component of (a) the prototype (upperfigure) and (b) the new correction coils (lower figure).
The distribution of the field is shown on the mid-plane (z= 0). Field strength is given in gauss.

Fig. 5 Comparison of the diamagnetic signals for plasmas withthe prototype and new correction coils. The diamagnetic
signal of 1 mWb corresponds to about 0.13 of the local
beta [4]. The diamagnetic signal in RT-1 depends mainlyon the filling gas pressure and the heating power, and it
has a large variation when we take the pressure on thehorizontal axis. The lower diamagnetic signals with thenew correction coils in this graph are measured at thebeginning of the experiment, when outgassing from thechamber wall is thought to be large.

2.6% of the ambient field at r = 1.0 m. In contrast, the newcoils reduce the maximum error field to 0.05 G, which is
0.1% of the ambient field.
A typical experimental result for the attitude control
is shown in Fig. 4. The magnet rotates like a pendulum,
because the heaviest point oscillates around the minimum
potential of the error field (see Fig. 4). Without attitudecontrol, three vertical positions (P1, P2, P3) of the magnet(measured by laser sensors) have appreciable diﬀerences,which correspond to about 1.4 degrees of tilt of the magnet.
When we apply the correction field, the tilt is reduced toless than 0.05degrees.
With the new correction coils, we have observed appreciable improvement of the plasma confinement. The
diamagnetic signals have been increased more than 10% incomparison with the previous data with the old prototype

correction coils [4]. The improvement is more significant
in operation with a lower RF power (Fig. 5). The diamagnetic signal is enhanced by a factor of 1.5 in plasma with5 kW RF power.
In conclusion, we have succeeded in improving theplasma confinement of RT-1 by constructing new fieldcorrection coils and reducing the field error.
[1] Z. Yoshida et al., Plasma Fusion Res. 1, 008 (2006).
[2] Y. Ogawa et al., Plasma Fusion Res. 4, 020 (2009).

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Plasma and Fusion Research: Rapid Communications

Volume 4, 039 (2009)[3] Kakioka magnetic observatory Japan meteorological agency
website, http://www.kakioka-jma.go.jp/en/index.html[4] Z. Yoshida et al., High-Beta (Hot Electron) Plasma in

Ring Trap 1 (RT-1), 22th IAEA Fusion Energy Conference,
Switzerland, 2008, EX/P5-28.

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