23P163-P
Improvement of Neutron Production Rate at Inertial Electrostatic Confinement
Fusion by Use of a Multi-Stage Feedthrough
多段電圧導入端子を用いた慣性静電閉じ込め核融合装置の中性子発生率の向上に関する研究
Yu Yamagaki, Kai Masuda, Taiju Kajiwara, Genki Hashimoto,
Yoko Kamiya and Kazunobu Nagasaki
山垣 悠, 増田 開，梶原泰樹, 橋本元輝, 神谷葉子, 長﨑百伸
Advanced Particle Beam Energy Research Sec
Inst. Advanced Energy, Kyoto Univ, Gokasho, Uji, Kyoto 611-0011, Japan
京都大学・エネルギー理工学研究所 〒611-0011 宇治市五ヶ庄
In our inertial electrostatic confinement fusion (IECF) device, the maximum applied voltage was limited to-80 kV. In order for an enhancement fusion rate, we have designed a 5-stage feedthrough aiming atapplying higher voltage up to -200 kV as well as at modifying spherical symmetry of the electric field in
the device, by numerical simulations. Based on the numerical design, we have constructed the experimental
IEC device employing a 5-stage feedthrough. We successfully applied a voltage exceeding the maximum
voltage of the original single-stage device, and have achieved -180 kV so far.

1. Introduction
In a conventional glow-driven IEC devices, therange of typical operating gas pressure ranges areseveral tens of mPa to 1 Pa. In those devices, theion energy is rapidly lost by charge exchange
collisions with background neutral gas molecules.
To cope with this problem, we have developed a
new IEC scheme driven by a built-in ring-shaped
magnetron ion source. This device can provide a
new IEC operation mode driven by the ion source(RS-MIS), which extended the accessible lowpressure limit very much down to 5 mPa, as wellas the conventional glow mode under several
hundreds mPa [1].
For both the operation modes in this newly
developed IEC device, the maximum applied
voltage is limited to -80 kV. Especially in theglow mode, the averaged energy of ions is known
to have about one third of applied voltage
because of the frequent collisions with thebackground neutrals. In this every range, i.e.
20-30 keV, the cross section of D-D fusion
increases rapidly as the ion energy increases. So
we can expect a significant enhancement offusion rate by increasing the applied voltage. In
this research, our aim is to apply -200 kV.
Meanwhile, the importance of ion beam optics
in terms of ion recirculation was increased in theion-source driven mode, because, for tens keVand 5 mPa, the mean free path for deuterium ion
charge exchange is of order of 10 m, which ismuch longer than the anode diameter. Therefore,
unlike the glow mode where charge-exchange

loss of ion is significant, field asymmetry inducedby the high-voltage feedthrough to the central
cathode grid may limit the ion recirculation(oscillatory motion of ions within the anode, prior
to striking the feedthrough rod).
In order to cope with these two requirements, a5-stage high-voltage feedthrough to be employedin the IEC device was designed. Figure 1 shows
the schematics. The numerical simulations of theion trajectories showed that, the averaged
recirculation number of injected ions is 3 times aslarge as that in the present experimental device[2]. Also the design showed that the maximum
electric field at the central cathode bias of -40 kV
on each electrode surfaces to be less than 9
MV/m, within the acceptable electric field limit[2].
We have then developed a 5-stage feedthroughbased on this design. In this paper, we describe
the IEC device employing the new feedthroughsystem and the preliminary experimental results.

Fig.1. Schematics of the 5-stage feedthrough device
2. Experimental Device
Figure 2 shows photos of the experimental IEC
device employing the 5-stage feedthrough. The

diameters of the original device were 445 mm and80 mm, respectively. In this research, we expanded
the size of both the anode and the cathode, i.e. thediameters of the new anode and cathode are 560mm and 200 mm.
The anode is made of 15 rings made of SUS316.
Diameters of these rings are decides to make thediameter of anode 560 mm (see Fig.2.a).
The cathode is made from molybdenum. This
cathode and the anode are set concentric. This
cathode is also connected to a cylindrical
feedthrough rod. The diameter of this rod is 10 mm,
which is to be biased at a high negative voltage up
to -200 kV.
Each of the 4 intermediately biased electrodesconsists of 4 parts, namely ring-shaped plates madeof alloy of ferrous, nickel and cobalt, a stainlessflared tube, 6 stainless rods and a duralumin rings.
The flared tube is connected to the alloy plates. 6
stainless rods are set on the end of the flared tube
inside the chamber. The duralumin ring is put on
the terminations of the 6 stainless rods (see Fig.2.aand b).
The 4 intermediately biased electrodes and thecentral feedthrough rod are insulated from eachother by 5 cylindrical ceramics. The diameter ofthese ceramics is 200 mm and the length is 120 mm.
The alloy plates of the intermediately biased
electrodes are fixed between these ceramics. We put
4 resistances between every intermediately biased
electrode outside the vacuum in order to divide theapplied bias voltage equally into the 4 intermediate
biases to the electrodes (see Fig.2.c).

(a)

(b)

3. Result of Experiment
We applied a high negative voltage to the device
in a vacuum. Figure 3 shows the relation between
the condition time and the applied voltage. We haveoperated the device about 80 hours so far. The
dotted line shows the maximum applicable voltage
in the original single-stage device. In the newly
developed device, we could reach this voltage very
quickly in less than 1 hour. And so far we havereached temporarily -180 kV. This is 90% of thetarget voltage of -200 kV.

Fig.3. Conditioning Data
4. Summary and Conclusion
We constructed an IEC device employing a
newly designed 5-stage high voltage feedthrough inorder for an improved fusion rate by applying ahigh bias voltage up to -200 kV and by modifying
spherical symmetry of the electric field. We haveconducted a high-voltage conditioning of thedeveloped device under a vacuum. So far, we
successfully achieved a bias of -180 kV, which ismuch higher than the maximum limit of -80 kV by
the use of the original single-stage feedthrough. In
the near future, we plan to apply a high voltage
under the glow mode by feeding a fuel deuterium
gas.
References[1] K. Masuda et al., Plasma Phys. Control. Fusion, 52,095010 (2010).
[2] Kai Masuda et al., Fusion Science and Technology,
vol.60, 2011, p.625(c)Fig.2. Photos of the IEC device employing the5-stage feedthrough:(a) the anode, the cathode and the intermediatelybiased electrodes, (b) the intermediately biased
electrodes, (c) the ceramic tubes and the resistances.