J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 481484

Development of Gompact Toroid Injector for Large HelicalDevice and New Goncept of Burning Compact Toroid
MIYAZAWA Junichi*, YAMADA Hiroshi, YASUI Kouji, KATO Shinichi, FUKUMOTO Naoyukir,
NAGATA Masayoshir and UYAMA Tadaor
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

IHimeji Institute of Technology, Himeji, Hyogo

671-2201, Japan(Received: 18 January 2000 / Accepted:27 May 2000)

Abstract
Compact Toroid Injector named SPICA mk. I (SPheromak Injector using Conical Accelerator) has
been developed and successfully assembled on March 1999. Main power source of 40 kJ for CTformation will be installed on March 2000. Simulation result shows that CT kinetic energy of 7 kJ isachievable with this bank in one-stage operation. As CT density is more than a hundred times larger thanthat of LHD plasmas, fast ions can effectively react with this dense plasmoid. Investigation result on thisburning CT concept is also given.

Keywords:
compact toroid, SPICA, point-model simulation, glow discharge, burning CT, advanced fusion

1. lntroduction
Compact toroid (CT) injection experiment isintended to fuel dense plasmoid into large and hot

to obtain high acceleration efficiency [3]. SPICA mk. I
was successfully assembled on March 1999. Main
power source for CT formation, which consists of 20
kV/40 kJ capacitor bank, will be installed on March

plasma of Large Helical Device (LHD). CT is dense
plasmoid usually generated as a spheromak, and can be2000.

accelerated electro-magnetically to several hundreds ofAs CT density is more than a few hundreds times
larger than that of conventional fusion plasmas, fast ionstrapped inside the main plasma can effectively reactkm/s []. High-speed CT can penetrate into core region
due to the large kinetic energy, which is comparable tothe magnetic potential energy at the core region. CT
magnetic field decays resistively inside the main plasmaand dense plasma confined inside CT is then released.
Computed trace of an injected CT inside LHD is three-with this dense plasmoid. If one chose so-calledadvanced fuels such as 3He, 6Li, and llB together withdimensional reflecting the helical structure [2]. This canhydrogen isotopes, it is possible to provide large fusionenergy as the kinetic energy of charged particles, notneutrons. High-energy charged particles generated bybe utilized for momentum injection because CT can fuelthis beam-CT fusion can heat the main plasma, andits contents along with the magnetic field.

strong core heating has been often related to improved
confinement.
In this paper, simulated waveforms of 20 kV/40 kJ

A compact toroid injector named SPICA mk. I(SPheromak Injector using Conical Accelerator) has

capacitor bank applied to SPICA mk. I are given
together with some results of the glow discharge

been developed. Conical shape is adopted in the designof inner- and outer-electrode at acceleration part toadiabaticaly compress the CT. Attention has been paid

@2000 by The Japan Society of Plasma*C o rre sp ondin g autho r' s e - mail : miy azaw a @ LH D. nifs. ac.j p481

Science and Nuclear Fusion ResearchMiyazawa J. er a/., Development of Compact Toroid Injector for Large Helical Device and New Concept of Buming Compact Toroid

one-stage operation has merits in its simple trigger
system and reduced cost. Calculated waveforms for onestage operation in SPICA mk. I are shown in Fig. l.
Efficiency defined as the ratio of CT kinetic energy toexperiment. Investigation results on beam-CT fusion(burning CT) are discussed in the section prior tosummary.

2. Simulated Waveforms of SPICA mk. Iand Glow Discharge Experiment
A capacitor bank of 20 kVl4O kJ will be installedbank energy depends on CT mass, circuit resistivity, andexternal inductance of the circuit. Most of all, CT mass
is controllable as it depends on the gas injection rate

into SPICA mk. I on this March. This bank is designed
for CT formation and another CT acceleration bankshould be prepared after the installation of formation

into the formation part. In this case, the optimum CT
mass is 0.2 mg, of which waveforms are drawn in Fig.

l, and 17 Vo of bank energy is translated into CT kineticenergy (CT kinetic energy of 7 kJ is obtained). This
kinetic energy corresponds to the potential energy of 1.4
T magnetic field (when CT volume is 5 x l0-3 m3).
Glow discharge (GD) cleaning is a powerful wallconditioning technique and adopted in SPICA mk. I. As
SPICA is co-axial magnetized plasma gun, one canperform co-axial GD with magnetic field in the axial
direction. Stable helium GD is obtained in the pressure
ps of l5-2O Pa, without bias magnetic field. GD plasma
breaks down at voltage of about I kV, and in the stablephase, voltage drops to 250-300 V with current of up to0.5 A (constant current operation). Gap length d
between inner- and outer-electrode of formation part is
0.065 m, and therefore psd for GD break down is aboutI m.Pa. or 7 mm.torr. After GD breaks down. axialbank and initial formation experiments. Although
SPICA is equipped with two inner-electrodes isolated
from each other, it is possible to connect themelectrically and form one-stage plasma gun. This onestage operation is intended for the initial experimentusing formation bank only, and detailed consideration isgiven in ref. 3. There will be some problems expected inthis one-stage operation, such that the blowby
phenomena [3], the quality of CT, which might beelongated because of the long acceleration time in onestage operation, and small acceleration efficiency due tothe loss in CT formation process, etc. On the other hand,magnetic field is gradually increased. When themagnetic field is increased to about 50 gauss, visible
light emission from GD decreases, and localized
emission is observed at the plasma periphery (Fig. 2).

200Ee.
r0o020-..'4--/

Localized emission seems unstable, although applied

Io"(t)

0voltage shows no arcing behavior. If one assume He ion
temperature of l0 eV, ion Larmor radius is calculated to400+va6(kV)

300

d - " "'-o""-.-a-'t0-._200'too :;500-5-10040?
+Er",(kJ)

'61l'

ut0u"

t0t(uw)l5Fig. 1 Simulated waveforms of SPICA mk. l. Point-model[3] is solved using the fourth order Runge-Kutta
method. CT mass of 0.2 mg, circuit resistivity of
20 mO, and external inductance of 0.48 pH areassumed. As for the electrode geometry, see ref.

Fig. 2 Localized emission is observed at the periphery ofco-axial GD plasma with axial magnetic field of 50gauss.

3.

482

Miyazawa J' et al', Development of Compact Toroid Injector for Large Helicalbe 0.065 m, which equals the gap length d. This meansthat GD plasma is affected by the magnetic field. Morethere are many unsolved problems related to burning
plasmas, i.e., a particle confinement and its heating

detailed measurement of the localized emission andhydrogen GD experiment are now in preparation.

efficiency, instabilities like TAE, radial electric field
due to the orbit loss of a particles, and accumulation ofcold He ash, etc. LHD is equipped with two lg0 keV
neutral beam injection systems (NBI) and ion cyclotron

3. Discussion on Burnable CT Goncept
Fusion experiments with less emission of neurons
and radioisotopes are desired especially in LHD, whereheating systems (ICH). These heating devices can afford
high-energy particles. Therefore, if the density of target
particle like boron is large enough, fusion reactions will

large amount of super conducting materials are used andwell public acceptance is not yet obtained. There exist

occur in measurable scale.
Note that even advanced fuels cannot be free fromthe neutron and radioisotope emission. There are someunfavorable side-reactions as below;
6Li + p -) y +7Be+ 5.606 MeV,
(4)
7Li + p-+ n + 7Be

some reactions suitable for these requirements, i.e.,

6Li + p -+ 3He + a + 4.O2Mey,
7Li + p -+ 2a + 17.348

MeV ,"B * p --> 3a + 8.682 MeV.

Device and New Concept of Buming Compact Toroid

(l)
(2)
(3)

These fusion reactions involve neither deuterium nor- 1.644 Mev,+rrc + 8.691 Mev ,tritium and emit neither neutrons nor radioisotopes.

toB * p -+ yFusion reactor scenario that involves reactions above
has been investigated from the early stage of fusion
study [4]. Such a scenario, however, inevitably needs'oB * p -+ a+7Be + 1.146 MeV,
T +'2C + 15.957 Mev,
"B +p)

lrc-2.765Mey .
"B * p -r n +Cross-sections of above reactions are small comparedwith reactions (1)-(3) (see Fig. 3). Only endothermic
reactions emit neutrons (see eqs. (5) and (9)) andradioisotopes produced by these reactions are lesshigh temperature, high density, and long energy
confinement time. This is because fusion cross-sectionsof (l)-(3) are small in the temperature range of less than
0.1 MeV as shown in Fig.3 [5]. Although it seems
difficult to use these advanced fusion reactions forreactor design, it is still meaningful to carry out fusion
experiments with less neutron emission. This is because

harmful as they have short half-life (7Be: 53.3 days, rrC:20.39 min.).
It seems better to chose boron for burning CT
investigation because the maximum cross-section ofreaction (3) is larger than that of others, and neutrons

are emitted only in high temperature range. To useboron, of which the boiling point is extremely high(3931 K), as a working gas to form CT, there are at least,r DT
i_.''i:'i-;'!
.ii

j!,,:''
10{'0,0l

other is to use the diborane gas (B2H6). The better is thedecaborane vapor because the ratio of boron ion density

n.il t ,@ 7'i,DDI:.rt;.'ri,;
I:,:
TIi.'
ta-! L,Itwo ways. One is to use the vapor of decaborane(BroHra), of which the boiling point is 486 K, and the',"ril.l='-DtHe

I

(5)
(6)
(7)
(8)
(e)

to electron density (0.156) is larger than that of thediborane gas (0.125) when they are fully ionized. Notethat the ratio in case of pure boron is 0.2.

!
|

Before going into details of beam-CT fusiont:l.{investigation, we should note that <ov> shown in Fig. 3

are obtained by averaging ov with the Maxwellian
distribution while assuming the same temperature of|
0,1both particles in each reactions. In our case, high-energy
component generated by NBI and/or ICH should havemuch higher effective temperature T, than that of theT. (I6v)

Fig. 3 Fusion cross-section as a function of .iontarget (CT) plasma Is. When applying Fig. 3 in thiscase, we should use reduced temperature I. defined astemperature [. Number indicates reaction typeand identical to the equation number in thearticle. Cross-sections of eqs. (4), (6) and (8) aresmall (< t6'zs 6s/s at ?1 < 2 MeV) and not

below;,,=hro+ftr,,illustrated.
483(10)Miyazawa J . et al. , Development of Compact Toroid Injector for Large Helical Device and New Conceptof Burning Compact Toroid

the kinetic energy of a particles is as large as 0'118
MW/m3. This is the same order as NBI heating power
density. In LHD, deuterium-deuterium (DD) experimentis supposed to produce neutrons at a rate of 2.4 x 1016
neutrons/sec, or considering LHD plasma volume of -26
m3,9.2 x l01a neutrons/(m3s) in average. Same neutron
production rate is only obtained in the high temperature
range of T1 > I MeV in p-B reaction. Moreover, as theCT volume of about 5 x l0-3 m3 in our case is verysmall compared with the LHD volume, total neutron
emission rate of burning CT is five thousands times
smaller than that of DD experiment. Fusion energy

where ms is mass of target particle, ltr1 is mass of highenergy particle, and mr is reduced mass (m. = momtl(morrB reaction T, (llll2) Tl in+ m)).In the case of p =the limit of Ts = 0, for instance.

To estimate the order of fusion energy density
released from beam-CT fusion, we assume boron
density ns = lo22 m-3, and density of high-energy
protons np* = l0t6lTr m 3 (Zl in MeV). This assumption

makes it possible to determine the optimum Iy in
constant pressure of the high-energy component. Thismodel also approximate steady-state LHD plasma
heated by 7.5 MW150 keV NBI, where zo* is estimateddensity released from DD plasma as kinetic energy ofcharged particles is about 720Wlm3 and more than ahundred times smaller than that of burning CT, on theto be of order l0l7 m-3 with the slowing down time of0.01 sec. Production rate of a particles per unit volume
as a function of 71 is calculated for p-B reactions and7Be and neutrons.
shown in Fig. 4, together with that ofrrB and 20 Vo ofNatural boron, which contains 80 7o ofr0B, is assumed to contained in the target CT. Note that
if one separates I rB from natural boron, the radioisotope
emission can be largely decreased. As can be seen in thefigure, 3 x 1017 m-3s-l of a production rate is obtained at
Tr = 20O keV. Higher Z1 is not necessarily effective toincrease o production rate in this model. Averaged aparticle energy is given as 2.46 MeV (from eqs. (3) and(7)), and therefore the fusion energy density released asother hand.

4. Summary
Discharge simulation result of SPICA mk. I
energized by 20 kY 140 kJ capacitor bank shows that CT
kinetic energy of 7 kJ can be obtained. This bank will be

installed on this March. Initial test of helium glow
discharge has been carried out and localized emission isobserved when 50 gauss of magnetic field is applied. Asa new scenario for confinement improvement by CT
injection, other than density peaking induced by center
fueling or momentum injection effect, the burning CT
concept was examined. Fusion energy density released
as kinetic energy of charged particles from the burning

CT is more than a hundred times larger than DD
experiment in LHD, and the order is the same as NBI
heating. Meanwhile, neutron emission is negligibly
small compared with DD experiments. Fusion related
experiment with less neutron emission is possible using
this burning CT in LHD.

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ostllr0'"!

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tzl J. Miyazawa et al.,Jpn. J. Appl. Phys.37 (1998)
6620.
t3l J. Miyazawa et a/., NIFS-614 (NIFS ResearchReport, 1999); J . Miyazawa e/ a/. , Fusion Eng. andq6o..

t4lDesign, to appear.
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t5l W.A. Fowler et al., Ann. Rev. Astron. Astrophys.
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from p-B reactions. For each particles, two

ORNL/TM-6914.

extreme cases of % = 7i and Io = 0 are shown.
484