22P159-P
Enhancement of Ion Current in a Magnetron Ion Source for an Inertial
Electrostatic Confinement Fusion Device
慣性静電閉じ込め核融合駆動用マグネトロン放電イオン源の大電流化
Taiju Kajiwara, Kai Masuda, Yoko Kamiya, Yu Yamagaki, Genki Hashimotoand Kazunobu Nagasaki
梶原 泰樹, 増田 開，神谷 葉子，山垣 悠, 橋本 元輝, 長﨑 百伸Institute of Advanced Energy, Kyoto University
Gokasho, Uji, Kyoto 611-0011, Japan京都大学エネルギー理工学研究所 〒611-0011 京都府宇治市五ケ庄
A newly developed IEC device driven by a ring-shaped magnetron ion source showed a neutron yieldproportional to the square of the IEC grid current in low pressure D2 gas operation. However, the IEC gridcurrent in the RIS-IEC is limited to ~1 mA at 5 mPa. In order to study the new regime in the RIS-IEC in ahigher IEC grid current range, application of a thermal electron emitter to the magnetron ion source isstudied. Numerical simulations suggested a linear increase of the IEC grid current up to ~150 mA, whichmeets our requirement, as the thermal electron current increases to 120 mA. This thermal electron current
can be provided by use of commercially available thermal electron emitters.

1. Introduction
An Inertial Electrostatic Confinement (IEC)fusion device [1] is expected to be a compact
neutron source. A conventional IEC driven by atypically 0.1-5 Pa glow discharge consists of agrounded spherical vacuum chamber and a central
spherical cathode grid (an IEC cathode, hereafter).
An IEC device can accept larger input power thanexisting D-D or D-T neutron generators because thetarget of accelerated fusing ions is mainly neutral
gas or plasma in IECs, unlike the solid metal target
in the neutron generators. However, the ratio offusion output to input power is quite low in IECs,
and fusion output is limited by the capacity of thepower supply.
An IEC device newly developed at Kyoto
University aims at an operation under a reduced D2
gas pressure of units of mPa with the aid of aring-shaped magnetron ion source. This device
employs a negatively biased ring-shaped array ofpermanent magnets between the IEC cathode andthe anode, to produce ions in the vicinity of theanode by a magnetron discharge [2]. In thisring-shaped ion-source driven IEC (referred to as aRIS-IEC), the experimental neutron yield wasfound to be proportional to the square of the IEC
cathode current, unlike the linear dependence in any
experimental IEC devices so far. One possible
explanation for this newly observed phenomenon
might be fusion reactions between energetic ions(so-called a "beam-beam" fusion regime), which is
expected to increase the neutron output drastically,
thanks to the current-square dependence.
Through a comparison between the neutron

yields by the RIS-IEC mode and the glow-driven
mode in the present device, it is found that theneutron yield in the new RIS-IEC mode would
exceed that of the conventional glow-driven mode
if the IEC cathode current would increase to 4 mA.
The IEC cathode current in the RIS-IEC mode is,however, limited to 1 mA in the present device.
Also, a stable glow-mode operation can hardly bemaintained below 10 mA, which makes a directcomparison of neutron yields between the twomodes at the same IEC cathode current difficult.
In this paper, we discuss an upgrade of theRIS-IEC device to meet the IEC cathode current
requirement in order to make clear the possible
fusion rate enhancement by the new IEC regime.
2. Theory
The present self-sustained operation of themagnetron ion source in the RIS-IEC can bedescribed as follows. The ring-shape array ofmagnets induces orthogonal magnetic and electricfields between the anode and the negatively biased
magnet array.
Either thermal electrons, electrons byfield-emission or secondary electrons due to ions’impact on the magnet array tend to drift because of
the cyclotron motion in the crossed electric andmagnetic fields. The electrons thus collide to thebackground neutral molecules many times andproduce ions and electrons by ionization, before
reaching the anode. The produced ions areaccelerated toward the negatively-biased magnets.
Some of them go through the spacing of themagnets and are accelerated toward the IECcathode to gain a fusion relevant energy. The otherions hit the magnets to emit secondary electrons.
The electron and ion currents are thus multiplied
through the ionizations and the secondary emissions,and a magnetron discharge will start up.
In the present magnetron discharge, thechain-reaction ionization by the electrons, and thesecondary electron emission by the ion collision to
the magnets balance, to lead to a specificsteady-state IEC cathode current dependent on theapplied bias voltage and the gas pressure.
We plan to place a thermionic electron emitter
attached to the magnet array to provide a muchhigher electron current, e.g. 100 mA, than in thepresent self-sustained magnetron discharge.
Numerical simulations are presented in the next
section, to see if this system can lead to an
enhanced IEC cathode current and to meet therequirement of 10 mA.
3. Simulation
We carried out trajectory simulations of ionsand electrons in the present magnetron discharge ina cylindrically symmetric system, by use of a2-dimendional code [3]. This code is aparticle-in-cell code based on the Finite Element
Method taking the space-charge effect into accountself-consistently. It also handles atomic andmolecular collisions among electrons, ions andbackground neutrals.
In this study we neglected the secondary
electron emission from the magnet array (themagnetron cathode), though the code can handle it.
Instead, we gave constant electron currents of 0.1,0.3, 1.0 and 100 mA from the magnetron cathodesurface as the input conditions of the simulations,which is thus regarded as the sum of thermal andsecondary electron emission currents. We fixed thegas pressure and the applied bias to the magnet
array to 5 mPa He and -10 kV, respectively.
The simulation results are summarized in Table
1, showing the incident electron currents from themagnet array and the resultant He+ ion current to
the magnetron cathode. The ratio of electron andion current is found to be roughly 3:10 independentof the incident electron current. The ratio is foundto be reasonable as the secondary yield coefficient
ranges 0.3-0.5 in the present energies of He+ ion
impact on stainless steel (the magnets in theexperimental device are covered by stainless steel
for shielding from the incident ions).
An important suggestion from the simulation
results is that a linear increase of the ion supply isexpected up to an incident thermal electron currentof 120 mA. With an incident thermal electron

current of 120 mA, the resultant ion current and thetotal magnetron discharge current of 310 mA and
430 mA, respectively as shown in the table. Though
the present simulations do not handle the ion
current extraction from the magnetron ion source,
the expected IEC cathode current turns out to be~150 mA, according to the experimental ratio of themagnetron discharge current and the extracted IEC
cathode current.
4. Conclusions
In order to enhance the IEC cathode current in
the RIS-IEC device, the application of thermal
electron emitter to the magnetron ion source isdiscussed in this paper. Numerical simulations
suggested a linear increase of the IEC cathodecurrent up to ~150 mA by the use of a thermal
electron emitter, where the current-square
dependent neutron yield in the RIS-IEC regime isexpected to exceed that in the glow-driven mode.
We plan to introduce an electron emitter to theexperimental RIS-IEC device. As a candidate
emitter, a directly heated tungsten dispenser
cathode is under consideration, which iscommercially available and can provide a thermal
electron current of ~20 A in a vacuum. The
fundamental electron emission characteristics
measured in helium, hydrogen and deuterium gases
as well as in a vacuum will be presented in theconference.
References[1] R. L. Hirsch: J. Appl. Phys. 38 (1967) 45422.
[2] K. Masuda, T. Nakagawa, J. Kipritidis, T. Kajiwara,
Y. Yamagaki, H. Zen, K. Yoshikawa and K.
Nagasaki: Plasma Phys. Control. Fusion 52 (2010)
095010.
[3] K. Masuda and K. Yoshikawa: Fusion Science andTech. 52 (2007).
Table 1. Currents of incident electrons and resultant ions.
Incident
electron
0.11
0.33
1.11
120
current Ie [mA]Resultantion
0.37
1.13
3.92
310
current Ii [mA]
Ie : Ii
3: 10 3 : 10 3 : 10 4 : 10