J. Plasma Fusion Res. SERIES, Vol.3 (2000) 63-66

First-Step Experiments of Neutral Beam Iniection

into an FRC Plasma
ASAI Tomohiko*, YONEDA Tomohiro, KODERA Fuji, OKUBO Mamoru, OKADA Shigefumi and GOTO Seiichi
Plasma Physics l-aboratory, Graduate School of Engineering, Osaka University,
2-I Yamada-oka. Suita. Osaka 565-0871, JAPAN(Received: 18 January 2000 / Accepted: 12 May 2000)

Abstract
We describe a novel neutral beam (NB) injection system for extremely high beta Field-Reversed
Configuration (FRC) plasmas, and show the initial results of preliminary experiments performed on theFRC Injection Experiment (FIX) device. An oblique injector with a set of three concave electrodes forbeam extraction is used to focus the beam enabling it to pass through a ndrow port. The beam target is alarge bore FRC plasma contained in a mirror field with a mirror ratio of 2-9. The results make it clearthat the beam improves the plasma performance, increasing the configuration lifetime by more than200Vo compned with no NB injection under the same conditions.

Keywords:FRC, high-beta, neutral beam injection, confinement1. lntroduction

have installed an NB injector at an angle of 19'to theA Field-Reversed Configuration (FRC) plasma [1]has attracted attention as a p - I fusion reactor coregeometric axis of the FIX. The ratio of the verticalvelocity components to the parallel one of the beam ionsis chosen to be outside the loss cone of the mirror field
at a mirror ratio of Ry-10 in the vacuum case. Thus, wecan use a high power NB source with several hundred
kW of power.
In this paper, we will describe the outline of theNB injection system, and show the initial result ofexperiments performed on the FIX device. The
experimental result is discussed and compared with thecalculations of injected beam particle orbits.

because of its high beta nature and engineering
simplicity. Unfortunately, present FRCs are operated
only in a pulsed fashion. For the steady state operationof FRCs, it is necessary to sustain this confinement
structure. Neutral beams may be used to drive current,
and to modify the internal structure and bulk parametersof the plasma, as is the case for tokamaks.

In the conceptual reactor "ARTEMIS" [2], neutralbeam (NB) injectors are installed perpendicular to thegeometric axis, for the purpose of driving toroidal

2. Experimental Apparatus

current. In the FIX (FRC Injection Experiment) device

Experiments were performed on the FIX device.

[3], high-energy beam ions injected perpendicularly

The FIX device, shown in Fig. l, consists of a formation
region and a confinement region. The FRC is producedby the field reversed theta pinch (FRTP) method withdeuterium puffing system. The FRC plasma iswould merely pass through the core plasma region andstrike the chamber wall because of its low confinement
magnetic field. However, beam particles injected
obliquely to the geometric axis can move between themagnetic mirrors even in the low confinement field. We*

Corre sponding author' s e-mail : tomohiko@ ppl.eng.osaka-u'ac,jp63@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchAsaiT. et al., First-Step Experiments of Neutral Beam Injection into an FRC Plasma

instantaneously injected along a magnetic guide fieldcopper, have 218 mm of effective diameter with 1628
apertures of either a 4 or 4.5 mm diameter. They areconcave with a curvature radius of 800 mm. The
configuration of neutral beams is focused at 800 mm on
the beam axis as shown in Fig. 2. The power supplies

into the confinement chamber, which is made ofstainless steel and serves as a flux conserver. After somebouncing motions between the mirrors, the injected
plasma reforms to a quasi-steady equilibrium state. The
applied typical confinement magnetic field is 0.04 T.
The mirror coils are spaced 3.4 m apart; the mirror fieldstrength is 0.08-0.40 T, there so the mirror ratio can bechosen to be 2-10. Typical plasma parameters in theconfinement region are; electron density n"-2.0 x lotem-3, total plasma temperature Tbt-150 eV, separatrix
radius r"-0.2 m, and separatrix volume y-0.4 m3. The

for the beam sources are capacitor banks that areswitched with Insulated-Gate Bipolar Transistor (IGdT)
units. Using this system, neutral hydrogen beams with
320 kW of power (14 keV,23 A) and a l0 ms pulse
width are produced. The DC level outputs for thesepower supplies are listed in Table l. Shown in Fig. 3 are
the degree of neutralizations and the current density
profile of the neutral beam measured at 600 mm on
beam axis for a beam energy of 5 kV. The injected
neutral beam has a77Vo degree of neutralization and aplasma diagnostics consist of diamagnetic probes andnoninvasive optical systems. An array of 35 magnetic
probes is located just inside the flux conserver wall tomeasure the axial magnetic field Bw. An excluded flux
analysis, made with these probes, is used to determine
the separatrix shape r"(z) of the FRC. A multichord CO2
laser interferometer (1O.6 pm) is installed parallel to the5.3 mm radius at the focal point.

In our experiment, we mounted the neutral beam
injector as shown in Fig. 1. The injector is installed at
an angle of l9o to the geometric axis so that the injectedbeam particles do not strike the chamber wall and are

y-axis at z = 0.6 m.
The bucket-type ion source is employed in the NB
injection system. The electrodes, made of oxygen freeoutside the loss cone of the minor field in the case of

RM-10 for the vacuum case.
Table 1 DC level output of power supplies.

vortageH#r #f;

Power l5kv 40A
1Oms
-3kV
Power4A20ms
Arc Power
300V 500A 40ms
Filament Powerl2V 1000A lOs

Accelerating

Fig. 1 Schematic diagram of the FRC confinement
chamber with NB injection system.

Decelerating

Dccclcrating clccFode
Gar Inl6t

I
Neutral Beam-Beam Axie

; 0.6q()o.4IL

ArcPh*maChdr

O
locclcc*ing Ebclrrdc

oF

o.2
0-40-20

rImm]
Fig. 2 Schematic view of the ion source; it consists of anarc plasma chamber and three electrodes. Theelectrodes have an effective diameter of 218 mmFig. 3 Radial distribution of the beam current density atz = 600 mm in case of 5 kV of accelerating voltage
and 3.0 A of current.

and a radius of curvature of 0.8 m.
64

AsaiT. et al., First-Step Experiments of Neutral Beam Injection into an FRC Plasma

3. Galculation of Beam Orbit

cation is obtained from a 2-D numbrical FRC equilibrium in the strong mirror field [5], which reflects the experimentally observed FRCs confined in the FIX device.

To consider the experimental phenomena, it isnecessary to understand the behavior of the injected
high-energy beam in detail. Then we calculated theThe beam particles ionized inside the separatrix aretrapped by a comparatively low external mirror ratio
where the particles can quickly escape because they arein the loss cone at the case of Rla - 4 of simple straight
mirror field. Typical calculated ion trajectory is shown

trajectories of ionized beam particles using the method
described later, with equilibrium configurations thathave the parameter of the FIX-FRC.

The equation of motion: mdvldt = q(E + v x B) issolved by the numerical method to obtain the
trajectories of the charged particles. Here m, q, v, E andin Fig.4.

B are the mass, the electric charge, the velocity of theparticle, the electric field and the magnetic field,

4. Experimental Results

respectively. In this calculation, because quasi-neutral
condition is assumed, the electric field E is neglected.
Since the FRC consists only of an axially symmetric
poloidal magnetic field and has no toroidal field, B isbeen performed. Figure 5 shows the difference in thecalculated from the poloidal flux function y incylindrical coordinates (r, 0, z) as follows.

which obtained by alternate FRC shots with and without
NB injection. In the beam injection experiment here, theinitiation of the beam pulse so early prior to the FRC
formation time (some I ms before an FRC is formed)
results in the output of the pulse becoming thedesignated and stable DClevel. The formation process'(r,,) (;';)

Preliminary experiments using the NB system havetime evolution of volume between FRCs with andwithout NB injection, in the case of mirror ratio of 8.
Each curve corresponds to an average of over 14 shots,(1)

Here the magnetic flux function y is obtained ftom a 2D equilibrium calculation. The relation between the nor-lo
0.3

malized, dimensionless parameters and the original parameters is given byv,(i)=+ ,) (,,)*(

v,,{.50.2

1.0Fig. 4 Typical trajectory of an injected beam ion in therv'-v;Vconfinement chamber simulated numericaly
forthe case of E, = 19 kev, BM = 4.0.

vz

and t=ffi,.

0.5{ooQ)

Here r,, B, and \rw are the wall radius, the magnetic
field and the flux function at the chamber wall, respectively. If the initial conditions of position (X(0), f(0),Z(0)) and velocity (Vr(O), Vy(0), Vz(O)) for the ionization point are given, the particle trajectory can be obtained by using the Runge-Kutta method. Adaptive step
size control [4] is used to obtain results of the intended

0.4

with NBI(rv-230ps)

F o.r

1 /".
;..
y''/

€ 0.20.t

precision. As additional constraints, the canonical angular momentum (pe = )Lldd = mr26 + qYlc) and Hamiltonian H of the particle are conserved. Here, H is repre-

01002N300/t00

Time [ps]sented as:

u = J- (\ m'i' + m'2m'.7w/o NBI(tv'95Ps) r'qY)'.
i' \| *( P'-2mr'Fis 5 Time evolution of separatrix volume in the case ofmirror ratio Br-8. The characteristic decay time rnof the volume increase from 95 ps to 230 ps with
NB injection.

€)The value of the flux function t1(r, z) at the particle's lo65

AsaiT. et al., First-Step Experiments of Neutral Beam Injection into an FRC Plasma

itself is not influenced by this beam operation. Just after
translation at about 80 ps (t = 0 refers to the preheating
capacitor bank discharge time), the FRCs in the twocases have almost the same volume and separatrix

comparatively low mirror fields (Ry = 4). The trappedions bounce between the magnetic mirrors while also

radius. They, however, decay at drastically different
rates. Here, we define the confinement lifetime as the efolding time Tv of the volume decrease just after

internal structure and bulk parameters of the FRC.

gyrating and circulating about the geometric axis. These"spiraling-ions" may prove useful in modifying theIn the future, we will explore the possibilities ofusing NB injection to drive seed currents, to heat theelectrons, and to modify the confinement structure.

translation. We find that the configuration lifetime
increases by about 240Vo with an injection power of 320

kW (14 keY,23 A) compared to the case with no NB

Acknowledgment

injection.

The authors are grateful for the assistance of themembers of the FIX group. The first author would liketo acknowledge Dr. A. Martin for helpful suggestions
and discussions. We thank Mr. Y. Suzuki for his support
on the numerical work.

5. Summary
The first experiment of NB injection into FRC
plasmas is performed on the FIX device. The NB
injector is driven at nearly maximum power; 320 kW(14 kV, 23 A). In this case, we find that theconfiguration lifetime increases by about 240Vo as

References

tll M.Tuszewski, Nucl. Fusion 2E,2033 (1988).
tzl H. Momota et al., Fusion Technol. 21,2307 (1992).
t3l A. Shiokawa et al.,Phys. Fluids 5, 534 (1993).
t4l W.T. Vetterling et al., NUMERICAL RECIPES Ex-

compared with the case of no NB injection. The result
demonstrates the successful application of NB injection
into the FIX experiment, and marks one of the first key
step in research into the steady state operation of FRCs.

ample Book ICl (Cambridge University Press,

Trajectories of beam ions are also evaluatednumerically. Injected fast beam ions are trapped by1992).

t5l Y. Suzuki et al., submitted to Phys. Plasmas.

66