J. Plasma Fusion Res. SERIES, Vol.4 (2001) 1gg_202

Increase of Hot Initial plasma Energy content in the Endsystem of AMBAL-M During Hydrogen puffing
AKHMETOV Timour*, BEKHER Sergei, DAVYDENKo Vladimir, KRIVENKoAleksandr,
MURAVIEV Maksim, REVA Vladimir and SOKOLOV Vladimir
Budker Institute of Nuclear Physics, Prospect lnvrentieva Il, Novosibirsk,630090, Russia(Received: 5 December 2OO0 / Accepted:27 August 2001)

Abstract
At the end system of the completely axisymmetric mirror trap AMBAL-M the experiments

oncreation and study of a hot initial plasma have been performed. In the experiments
a gas-box was used
for hydrogen supply into the hot startup plasma in the mirror trap to increase
the plasma density. The hot
initial plasma in the trap was produced by the trapping of a plasma stream with
developed electrostatic
turbulence generated by a gas-discharge source located outside the entrance
throat. It was found that inaddition to the increase in the plasma density by a factor of 2-3, hydrogen puffing
resulted in

unexpected nearly twofold diamagnetism increase. The gas puffing

andid not reduce the electron

temperature in the trap. Essential for explanation of the observed effectis the fact that with the gas
puffing the measured plasma potential in the trap increased. The increaseenhanced the trapping of the ion flow entering the trap and increasedin the plasma potential
the average energy of the electron

flow entering the trap. It was found that with the increasing hydrogen puffing rate plasma
the trap were saturated.

parameters in

Keywords:
magnetic mirror trap, plasma confinement, heating, gas puffing

1. Introduction
The ambipolar magnetic mirror trap AMBAL_M[1-3] is a completely axisymmetric machine, andexperimental investigations performed at it are offundamental importance for a future thermonuclear
reactor. Investigations at AMBAL-M are divided into
two stages. Since 1993 the experiments at the endsystem (Fig. 1) consisting of a simple mirror trap andsemicusp have been carried out. In this system, a hot30nl00l0

initial plasma was obtained using a gas-discharge
plasma source located outside a magnetic throat.

Fig. 1 Top view of the magnetic system and vacuum

Thermal insulation in the entrance throat and electron

chamber of the end system of AMBAL_M.
Magnetic field line starting from the plasma

heating in the mirror trap by the axial current flowing
along the plasma were studied. Consistent accumulation

source lz = -375, r = 6 cm) is shown. Below isgiven the magnetic field profile at the axis.

of fast ions in the mirror trap plasma during pulse*Corresponding

zrgm@2001 by The Japan Sociery of plasma

author's e-mail: akhmetov@ inp.nsk.su

Science and Nuclear Fusion Researcht99

Akhmetov T. et al., Increase of Hot Initial Plasma Energy Content in the End Systemof AMBAL-M During Hydrogen Puffing

n, 1d3crn-3

injection of powerful neutral beams was demonstratedand experiments on ICR heating of the initial plasma

Jwere performed.
However, the plasma density in the mirror trap waslimited by the value of the plasma flow produced by thesource. Therefore, in order to increase the plasma

21t0

density, additional fueling mechanisms were required'
One of the commonly used and predictable ways toincrease the plasma density is the use of gas puffing
during the pulse of the hot plasma, when ionization of-300 -200 -100 0z'cm

Uo,'V

this gas by the hot plasma leads to controllable-100

production of the additional plasma. In the present paper
recent experiments at the end system on enhancement of
the initial plasma parameters in the mirror trap duringhydrogen puffing through a gas-box are described'-150-200-250-300 -200 -100

2. Greation of the Initiat Plasma in the Trapand lts Parameters

0z,cFig. 2 Axial distributions of the plasma density andplasma Potential.

The initial plasma in the end system of AMBAL-Mis created by a gas-discharge plasma source which

the plasma potential and e is the absolute value of theelectron charge. Thus e (q (z) - Q i = T"'ln(n(z)/nd, theaxial potential profile qualitatively reflects the density

generates a plasma stieam with a density -10Ia cm-3,
electron and ion temperatures -10 eV. This stream froman annular discharge channel flows along magnetic fieldlines and enters the mirror trap. Since there is a strong

profile, and there is a potential minimum in the entrance
throat, which prevents the electrons captured in the trap

radial electric field E, - 200 + 300 V/cm between thecylindrical electrodes of the plasma source, the plasma
rotates in the azimuthal direction in the crossed radial
electric and axial magnetic fields. This rotation velocity
in our conditions has a strong radial shear, which leads
to the Kelvin-Helmholtz instability, and as a result, thefrom flowing back to the plasma source' Plasma
potential in the center of the trap (Fig. 2) ig -JQ : 100 V
higher than in the entrance throat, thus leading tothermal insulation of the electrons in the trap whichhave a typical temperature -50 eV, from cold electrons
near the plasma source. Another mechanism leading toelectrostatic turbulence develops in the plasma stream'
This turbulence leads to strong ion heating up to 250 eVin the trap and to the radial diffusion of the plasma'

plasma heating is the presence of the axial electric
current of -1.5 kA flowing through the plasma, which
results in the Joule heating of the electrons' Thus, theThus, the radial profile of the plasma density, which has
a shape of a narrow annulus near the plasma source, ishot initial plasma in the trap with the ion temperature -250 eV and electron temperature -50 eV is createdduring the pulse of the plasma source. The ions areheated by the electrostatic turbulence developed in thesmoothed by the diffusion and becomes flat in themirror trap with a typical value of -6'1012 cm-3 in the
midplane.

plasma and sustained by the power of the plasma source'

As the plasma stream moves from the plasma

The electrons obtain energy in electron-ion collisions,and due to the Joule heating by the axial electron current

source to the entrance magnetic throat of the mirror trap,
ions become collisionless and owing to conservation ofin the plasma. The electron temperature remains

the magnetic moment F = vzLlB (where vl is thetransverse ion velocity and B is the magnetic field

relatively high thanks to the thermal insulation of thestrength) a substantial part of the plasma stream reflectsfrom the region of strong magnetic field' leading to atrapped plasma by the electrostatic potential well, whichgoverned by the Boltzmann distribution in the3. Experiments with Hydrogen Puffing into
the Initial Plasma in the TraP

substantially reduces electron thermal conductivity
between the trap and the plasma source.

considerable density decrease towards the trap'
Electrons remain Maxwellian and their density iselectrostatic potential along the magnetic field lines n *exp(eglTr), where T, is the electron temperature, I is

After detailed investigations of the initial plasma in200

Akhmetov T. et al.' lncxease of Hot Initial Plasma Energy Content in the End System of AMBAL-M

the trap attempts were made to increase the plasma

During Hydrogen puffing

or better confinement of the trapped plasma, or both ofthese factors. Therefore, experimental study of thiseffect was made. It was found out that at moderatedensity and temperature by different means. One of thesimplest methods to increase the plasma density is toadd some amount of molecular hydrogen into the hotplasma. The gas is ionized by the electron impact andhydrogen puffing -2Ton.L/s the electron temperature

charge-exchange processes and the total amount ofdensity increase. With increased hydrogen puffing rate

grows from 40-45 eV to 55-60 eV together with theplasma should increase. In order to make this gas supply-10 Torr.L/s the electron temperature in the central
region remained almost the same and fell down to 3035 eV at the periphery. The plasma density and energy
content increased nonlinearly with the value of thehydrogen puffing rate. With small hydrogen supply, themore efficient and spatially uniform we installed a
special gas-box inside the mirror trap. The gas-box is acircular metal tube surrounding the plasma, and it has
niurow diaphragms on its ends. After entering the gasbox the hydrogen molecules reflect from its walls andplasma density and diamagnetism increased sharply andpenetrate into the plasma volume. The gas-box in ourthen the rate of growth decreased. The ion saturation
current profiles for three gas puffing rates are presented
in Fig.4, and one can easily observe that puffing at2Torr.Lls led almost to the same increase in the ion
saturation current as the increase of the puffing rate
ftom 2 to 10 Torr.L/s. It is essential that during thehydrogen puffing the ambipolar potential of the plasma
considerably increased, which was confirmed by theprofiles of the probe floating potential presented inFig. 5. An average ion energy measured by an end-loss
energy analyzer, did not change within the experimental

machine was installed in the mirror trap between themidplane and the exit throat and the hydrogen puffing
was performed at various rates from 2 to lO Torr.L/s.

From the very beginning of the experiments withthis gas-box we obtained the unexpected result. The
plasma diamagnetism in the mirror trap increased during

the gas puffing. From the oscillogram presented inFig. 3 one can see the increase in the plasma
diamagnetism and also the increase in its growth rate.
The diamagnetism increase points to additional heating

Ua'v

U, a.u.
0.4----*,2f,';*,/./'-""""-!--z'.' 2Ton.l/s

0with gas

LOTon'l/s*- ".t/-100
0.2-zffi-300

Q=00-400

3 t,ms0246r,cm

Fig. 3 Plasma diamagnetism without gas and withhydrogen puffing at 2.5 Torr.Us.

Fig. 5 Probe floating potential with different gas-puffing
rates.

I""1, a.U.

10 Torr.l/s

Ii.oa, &.u.

r-....+---.-+

80.44O]j--*.-^246o.2'+"-..+:---0or,cmOFig. 4 lon saturation current in the trap with different
gas-puffing rates.

2468 e,Torr.l/s

Fig. 6 EndJoss ion current as a function of the hydrogenflow into the gas-box,20r

Akhmetov T. et al., lncrease of Hot Initial Plasma Energy Content in the End System of AMBAL-M During Hydrogenaccuracy and was 400 + 60 eV. The nonlinear
dependence of plasma density on the hydrogen puffing

rate was also substantiated by the end-loss ion current
measurement using a gridded ion-energy analyzer at thePuffing

agreement with the experimental data. The model
requires further improvement in order to account moreaccurately for the influence of the increase in theelectrostatic potential in the trap on electron and ion
capturing from the plasma stream. This numerical model
will be useful for planning experiments with a gas-box,
which will be installed in a long solenoid of a newmodification of the AMBAL-M machine [3]. Similargas puffing is successfully used in a long central cell ofaxis (Fig.6).
The observed improvement of the plasma energy
content can be explained by enhancement of relaxationof the axial electron cunent flowing in the plasma in theaxial direction, and by the increase in the plasma

the ambipolar trap GAMMA l0 [4].

potential in the trap which improved the electron
confinement. The axial current of -l-1.6 kA with anaverage electron energy of -250-300 eV flowed through4. Conclusions

the plasma and the fraction of electrons captured into
the mirror trap plasma due to classical collisions was

the initial plasma created in the end system of AMBAL-

estimated as -0.3 [2]. When the plasma density
increased as a result of ionization of the puffed

M, the plasma density was increased about 3 times. At
the same time, the gas puffing did not cause degradationhydrogen, the electron capturing from the flowof the ion and electron temperatures in the mirror trap.
This result is explained both by the enhanced electronand ion capturing from the hot plasma stream generated

In the experiments with the hydrogen puffing intoimproved. Since with the density increase the ambipolarpotential grew in the mirror trap and practically did notchange outside the entrance throat, an average energy ofby the plasma source, and by continuous ion andelectron heating during the pulse of the plasma source.
The gas-box proved its efficiency as a simple tool forincreasing the plasma density, and a similar gas-box will

the captured electrons also increased'

In order to achieve better understanding of theplasma confinement and influence of gas puffing, wehave developed a numerical model of the plasma

be used in experiments at the new modification ofAMBAL-M with a long solenoid.

creation in the trap by the plasma source and gas puffing
into this plasma. For simplicity we use a zero-ordermodel, i.e. it does not account for spatial variation of theplasma and gas parameters and the plasma isReferencestll

characterized by its typical length and radius. Interaction

G.I. Dimov, In Proc. Int. Conf. on Open Plasma

Confinement Systems for Fusion, 1993,
Novosibirsk, Russia, World Sci. Publ., p.23 (1994).

between the plasma and gas is described by simple
balance equations for the ion and gas densities, ion andt2l T.D. Akhmetov, V.S. Belkin, E.D.Bender et al.,Plasma Physics Reports 23(ll), 9ll (1991)

electron energies. We consider two ion populations:relatively hot ions with a typical energy of 200 eV,
which are generated by the plasma source and captured
into the trap, and relatively cold ions with a typicalltranslated from Russian Fizika Plazmy, 23(ll),988 (1997)1.

t3l T.D. Akhmetov, S.A. Bekher, V'S. Belkin et al., tobe published in Transactions of Fusion Technology,energy of several tens of electronvolts, which areproduced from cold 2 eV Frank-Condon atoms in thecharge-exchange and electron-impact ionization
processes. First numerical results obtained from this(2000).

t4l K.Yatsu, L.G. Bruskin, T. Cho el al., Transactionsof Fusion Technology 35(1T), 52 (1999).

model provide promising qualitative and quantitative202