Progress of Negative Ions in Fusion Research
L. R. Grisham,* N. Umeda, M. Kawai, M. Kuriyama, T. Ohga, T. Yamamoto
Japan Atomic Energy Research Institute, Naka-Machi, Ibaraki 311-0193 Japan
*Princeton Univ. Plasma Physics Lab., P. O. Box 451, Princeton, N. J. USA 08543
Abstract. Negative ion beams are assuming increasing significance in the field of controlled fusion research. The first
generation of large high current negative ion sources has now operated successfully on the JT-60U tokamak for some
years, producing negative ions of hydrogen or deuterium at energies up to 400 keV for conversion to several megawatts
of neutrals which are used for plasma heating and current drive. We report an unexpected intensification phenomenon,
and some of the improvements made or planned. In the realm of inertial confinement fusion, interest has grown in the
use of heavy negative ion driver beams formed from halogens such as bromine, iodine, or chlorine. We discuss the
advantages of negative ions as heavy ion fusion drivers, and assess the technical feasibility of a halogen negative beam
system. We also discuss the potential for photodetaching the negative ions to produce atomically neutral beams and
reduce beam space charge effects within the target chamber
5.8 megawatts at 400 keV for 0.9 seconds using
deuterium, and 2.6 megawatts at 355 keV for 10
seconds using hydrogen.
INTRODUCTION
Recent years have seen the deployment of the first
generation of large high power negative ion sources to
produce beams of energetic neutral atoms to heat and
drive current in magnetically confined plasmas within
the discipline of magnetic fusion energy (MFE)
research. The operation of these systems has served
both as proof-of-principle of the technology, and as a
guide to future improvements which will be required
to produce a robust technology.
The initial operations of these negative ion sources,
which represented a very large step in the state of the
art with regards to total beam current (from milliamps
to tens of amps), were complicated by secular and
spatial non-uniformities in the source plasma, and by
excessive stripping of the negative ions in the
accelerator. The stripping and secular non-uniformity
problems were solved [2] fairly quickly, but the spatial
non-uniformity has proven less tractable. Although it
has been partially addressed with adjustable ballast
resistors which redistribute the arc current among
different cathode groups within the source, a more
general solution appears to be to alter the source
symmetry, either by adopting a circular source, or by
using only the central 60% of the extraction field of
the existing rectangular sources.
The other major branch of fusion research, inertial
confinement, has recently developed a focused interest
in the possibility of using heavy ion negative halogen
beams as the driver for a heavy ion fusion (HIF)
reactor. While HIF beams will differ in many respects
from MFE beams, they should be able to benefit from
much of the negative ion source experience of the
MFE program.
Another persistent issue in this first generation of
sources has been the difficulty of conditioning the
acceleration grids to high voltages and long pulse
lengths. While this arises in part from the spatial nonuniformity of the negative ion current feeding the
extraction array, it is also a consequence of the source
operating sequence. Due to the slow speed of the high
voltage switching system employed, it is necessary to
MFE NEGATIVE ION BEAMS
The first application of negative ions in MFE was
on the JT-60U tokamak [1] at Naka, Japan. This
system has now injected neutral beam power levels of
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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density beamlets.
The dotted lines depict the
boundaries of the grid sectors, with the areas between
masked from beam extraction.
extinguish the ion source arc for 0.13 sec whenever a
high voltage breakdown occurs in the accelerating
grids.
Because the arc current accounts for
approximately a third of the power heating the
thermionic cathode filaments, the cathodes cool
rapidly through radiation during the arc suppression
period. When the arc restarts after the accelerating
voltage is reestablished, the cooler filaments emit
fewer electrons, resulting in a higher arc impedance.
This results in a different, and less perveance-matched,
negative ion current density illuminating the extraction
plane, which increases the beam divergence and the
chance of further high voltage breakdowns.
Three trends are apparent. There is a long-scale
non-uniformity, with the beam intensity reduced for
the top and bottom grid sectors, especially for the
higher extractor voltages, which focus higher current
densities. Secondly, there are clear dips, as expected,
corresponding to the masked areas between grid
sectors. The third trend is the most interesting. The
boundary beamlets on each side of the masked areas at
the sector boundaries have enhanced current density,
which becomes most apparent at the highest extractor
voltage, which best-focuses the higher current
densities.
This surprising phenomenon was
consistently observed across all operating conditions,
both for high current density beams with cesiated
discharges, and for low current density beams with no
cesium seeding. It was evident for different values of
the electron filter magnetic field. In every case the
peaking of the boundary beamlets was most evident
for the highest extraction voltages, confirming that
these contained higher current densities.
We are presently implementing a change to the
filament control system which allows the voltage of
the filament heating supplies to be increased during
the period of arc suppression. If we can increase the
filament heating power sufficiently to offset the loss of
arc power, then the resulting constant-temperature
cathode should allow the arc to restart at the same
impedance at which it was extinguished, resulting in
similar negative ion current densities, and easier
conditioning.
Preliminary experiments with this
system show improved impedance matching, but some
control problems remain before full implementation.
The only seemingly consistent explanation for this
phenomenon is that the boundary beamlets are
collecting negative ions from the adjacent masked
plasma regions, and thus achieving higher current
densities. This would be unlikely to occur when
extracting positive ions, since any net drift of the
positive ions would immediately engender a retarding
electric field. However, in a three component plasma
consisting of negative ions, positive ions, and
electrons, it appears that the ordinary ambipolar drift
constraint is relaxed for the negative ions. This is
because the electrons, which have the same charge as
the negative ions, are much more mobile, and rapidly
fill any depression in the local electric potential caused
by the net drift of the negative ions.
Negative Ion Diffusion Effects
The advent of high spatial resolution beam profiles
clarified a phenomenon which had only been
suggested by earlier lower resolution measurements.
The extraction and acceleration grids consist of five
grid segments in a vertical array 1.1 x 0.45 m. Each
grid segment contains 9 rows of 24 apertures each.
Within a segment, these apertures are uniformly
spaced. However, the spacing between the rows on
each side of a sector boundary is about 2.6 times as
great as the spacing between rows within a sector.
One might also expect this phenomenon to be
manifest along the outer periphery of the beamlet
array, and, indeed, as shown in figure 2, the two
dimensional image reveals enhanced beamlets along
the longitudinal periphery, as well as showing the
individual beamlets along the sector boundaries. The
enhanced beamlets along the longitudinal periphery
correspond to alternating beamlet rows, with the
enhanced end alternating from one row to the next.
This alternation is probably due to some effect arising
from the magnetic dipoles embedded in the extraction
grid.
The spatial profile is obtained using an IR camera
viewing a molybdenum plate located 3.5 m
downstream of the exit grid of the accelerator. This is
within the near field of the beam optics, so the profile
is similar to that emerging from the accelerator.
Power handling considerations restricted the beam
pulse length to 0.2 sec.
Figure 1 shows the temperature difference profile
(between 125 and 58 msec into the shot) along a line
1.5 cm from the middle of the profile running the
length of the beam footprint. The curves are for three
values of the extraction voltage. The extractor lens is
by far the most sensitive in the optical chain, with
higher voltages being required to focus higher current
This intensification phenomenon, as it is presently
occurring, is an inconvenience, inasmuch as it means
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that, even if the plasma illuminating the extraction
plane were uniform, the extracted current density
would still be non-uniform, with the boundary
beamlets having significantly higher current density
than the central beamlets. This means it is impossible
to simultaneously focus all of the beamlets, which in
turn increases beam interception by the grids, and
increases the likelihood of high voltage breakdowns
between the accelerating grids.
HEAVY ION FUSION BEAMS
Interest has recently grown in the possibility of
using heavy negative ions formed from halogens as the
driver beams for HIF [3]. This would avoid the
potentially challenging problem that positive ion
beams may draw electrons from bordering surfaces
into their deep potential wells while they are being
accelerated and compressed. Contamination with
electrons would adversely alter the focussing
properties of the beam. In addition, negative ion
beams would not have the low energy charge
exchange tails which may accompany heavy positive
ions from the source. Moreover, because HIF beam
pulses will be short and at very high power density, it
should be feasible to convert them to atomic neutrals
by photodetachment after they have been accelerated.
Having an initially-neutral beam entering the HIF
target chamber could substantially reduce beam
expansion due to space charge even if the pressure is
high enough to ionize much of the beam in transit.
However, if we redesign the grids to take
advantage of this intensification phenomenon, we can
use a lower transparency plasma grid with more
uniform aperture spacing to extract higher current
density beamlets over the whole array. Using a lower
transparency (smaller apertures) plasma grid will
reduce gas efflux and beam stripping, while improving
the optics because the beamlet current densities will be
more uniform, and only the central portion of each
accelerator grid aperture will be used, reducing
aberrations. We have built new plasma grid segments
to test this concept in the near future.
The halogens fluorine, chlorine, bromine, and
iodine are all strongly electronegative, with electron
affinities of 3.06 to 3.63 eV. All of them exist as
dimers at appropriate temperatures, and readily form
negative ions through dissociative attachment in
moderate power density plasma discharges.
At
pressure of 10 –20 mtorr negative ions are the
dominant species in such discharges [4]. Accordingly,
RF plasma discharge ion sources should be suitable for
producing current densities of halogens at similar
levels to those that could be generated with positive
ions of similar masses. In such a case, the extractable
current density will depend upon the strength of the
extractor field which can be applied, which is a
function of extractor design, rather than ion polarity.
The various techniques developed for MFE negative
ion sources using magnetic fields and electric potential
biases to suppress co-extraction of electrons will need
to be applied to halogen sources. An experiment is
planned at Lawrence Berkeley National Laboratory [5]
to test halogen beam properties using chlorine. Since
all the halogens have similar electron affinities, this
should serve as a proof of principle test for bromine or
iodine, which are more appropriate masses, but which
require elevated source temperatures.
DT [deg]
150
100
50
0
- 450
Top
- 300
- 150
0
Y [mm]
150
300
450
Bottom
FIGURE 1. Longitudinal profile for extractor voltages: 4.9
kV (lower), 5.8 kV (middle), and 6.9 kV (upper)
Although we presently lack photodetachment cross
sections for the beams we would most like to use,
calculations and data for variety of other negative ions
in Massey [6] show cross sections in the range of 1 x
10-17 cm2 to 2.4 x 10-16 cm2. If we use the lower end of
this range, and take as an example a 4 GeV beam of
negative iodine, then photodetaching 99% of the beam
using 4.7 eV photons from a Kr-F laser requires a line
FIGURE 2. Beamlet images early in beam pulse.
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density of 2.77 x 109 watts per cm of beam width [3].
Using a 20 nanosecond laser pulse to neutralize a 10
nanosecond beam pulse of 3 cm width would require
166.2 joules of laser energy per pulse. Lining the
neutralizer cell with lasers allowing 100 low-loss
reflections, which should be readily available, the laser
energy requirement drops to 1.7 joules at a laser power
of 8.31 x 107 watts. This is about a factor of 100 less
than the state of the art for Kr-F lasers in 1996. With a
laser efficiency of 8%, the input power requirement
would be about 21 joules.
We can, moreover, allow much more beam
ionization and still appreciably improve beam
dynamics in the target chamber. The self-field
perveance scales as the square of the average charge,
and the effect upon the beam spot size at target
depends upon the distance at the which the beam
becomes ionized; ionization close to the target is
much less deleterious than ionization far away. If the
atomic beam became 50% ionized while traversing a 3
m radius target chamber at 1.3 x 10-4 torr, then the
average ionization would be 25%, and the average
self-field perveance, which determines target spot size
growth, would be about 5% of what it would have
been if the beam had been singly ionized across the
entire path. This is qualitative evaluation; a full
comparison will need to include partial space charge
neutralization by electrons and ionization of beam by
x-rays close to the target.
Negative ions are more fragile than positive ions at
low energies of a few tens of kev/amu, but this is
routinely accomodated in MFE D- beams, which are
much less tightly bound than halogen negative ions.
However, once beams reach energies of hundreds
of keV/amu to tens of MeV/amu, the vacuum
requirements for transporting negative ions are similar
to those for positive ions. The positive ions are subject
to ionization to higher charge states with total cross
sections not much less than for the negative ions. One
can easily see this from the fact that the translational
kinetic energy of the electrons exceeds their binding
energies for most of the electrons in the projectile’s
electron cloud, not just the extra electron of the
negative ion. At just 1.4 MeV/amu, for example, the
translational kinetic energy of the bound electrons is
760 eV.
ACKNOWLEDGMENTS
This work was supported by U.S. DOE contract
DEAC0276CHO3073.
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