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Improvement of JT-60U Negative Ion Source Performance
Improvement of JT-60U Negative Ion Source Performance
L.R. Grisham 1), M. Kuriyama 2), M. Kawai 2), T. Itoh 2), N. Umeda 2), JT-60U Team 2)
1) Princeton Plasma Physics Laboratory, PO Box 451, Princeton, N. J. USA 085453

2) Japan Atomic Energy Research Institute, Naka Fusion Research Establishment
Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
e-mail contact: lgrisham@pppl.gov
Abstract The negative ion neutral beam system now operating on JT-60U was the first application of negative
ion technology to the production of beams of high current and power for conversion to neutral beams, and has
successfully demonstrated the feasibility of negative ion beam heating systems for ITER and future tokamak
reactors [1, 2]. It also demonstrated significant electron heating[3] and high current drive efficiency in JT-
60U[4]. Because this was such a large advance in the state of the art with respect to all system parameters, many
new physical processes appeared during the earlier phases of the beam injection experiments. We have explored
the physical mechanisms responsible for these processes, and implemented solutions for some of them, in
particular excessive beam stripping, the secular dependence of the arc and beam parameters, and nonuniformity
of the plasma illuminating the beam extraction grid. This has reduced the percentage of beam heat loading on
the downstream grids by roughly a third, and permitted longer beam pulses at higher powers. Progress is being
made in improving the negative ion current density, and in coping with the sensitivity of the cesium in the ion
sources to oxidation by tiny air or water leaks, and the cathode operation is being altered.
1. Introduction
During initial operations of the JT-60U negative ion system, the beam pulse length, power,
and operating voltage were all somewhat less than was desired. While this condition arose
from a number of processes, they all Manifest themselves as excessive heating of the
accelerator and ground grids of the ion sources due to interception of the grids by energetic
divergent beam particles. These grids were receiving roughly three times the fractional heat
load from the beams that their cooling was designed to handle (in the case of the ground grid
a load fraction of as much as 15% of the accelerated power, versus an expectation of less than
5%). This in turn limited the beam parameters which could be achieved without high voltage
breakdowns in the grids. Several of the principal phenomena arise from the characteristics of
large area sources, and thus had been less apparent in smaller ones. This paper discusses
measures taken to address these phenomena.
2. Beam Stripping
One cause of the heavy grid interception was excessive stripping of the negative ions in their
passage through the grid structure. Prematurely neutralized ions did not experience the full
set of electrostatic lenses, leaving some energetic neutrals lodged on very divergent
trajectories. Since beam ions were neutralized after passing through differing lengths of the
accelerating field, some of them had energies lower than the nominal accelerating voltage,
producing a lower energy continuum in the beam. The stripping was found to arise from a
steeply increasing pressure in the source and accelerator as a function of time, which
occurred as a consequence of the long vacuum time constant of the gas feed system. While it
was not immediately feasible to change the geometric characteristics that gave rise to this
behavior, it was possible to change the gas pulse timing to allow the gas to equilibrate before
the arc, which in turn allowed a lower gas throughput to be used. After this improvement, a Doppler shift measurement revealed that essentially all of the beam power transmitted to JT-
60U was at the full acceleration energy.
3. Secular Dependence of Arc and Beam Parameters
After the sharp time dependence of the source and accelerator pressure was corrected, a
strong secular dependence still persisted in nearly all of the source parameters, including the
arc voltage and current, the extraction current, the extraction grid bias current, and the
fraction of co-extracted electrons in the beam. In particular, the arc impedance was declining
significantly, which in turn reflected a change in arc characteristics, and consequently a time
dependence in the extractable negative ion current density. This led to time-varying
divergence in the beam, which increased the average grid interception. It was found that the
time required for the arc to equilibrate in negative ion sources of this sort is very long, and
that the early operation had been done during the long turn-on transient. This problem was
also corrected, so that the source plasma characteristics are equilibrated by the inception of
beam extraction. This was accomplished by increasing the time the arc was on prior to beam
extraction. The period required for equilibration was a decreasing function of arc and
filament power, with 1.5 to 2.0 seconds being sufficient for most conditions, as opposed to
the 0.5 second of arc prior to beam extraction which had been used during early operations.
Recently, further control over the secular dependence of the arc has been implemented with a
filament control system which allows the programming of eight different values of the
filament heating current at different times during the arc and beam pulse. This facilitates
stable operation for longer pulses. It also results in lower average filament temperatures than
was previously the case, reducing the evaporation of tungsten, and also the incidence of
unipolar arcs which erode the filament. It is expected that this will increase filament lifetime,
and reduce cesium burial on the source surfaces.
4. Spatial Non-Uniformity of the Ion Source Plasma
A strong spatial non-uniformity in the source plasma persisted even in the equilibrated arc,
and this in turn led to a non-uniformity in the local negative ion current density extracted
from different areas of the source grids. Since the same voltage gradients are applied over the
whole area of the grids, local variations in current density result in mismatches between the
radially outward force of the beam space charge and the radially inward electrostatic
focussing field, and cause position-dependent divergence growth.
A number of diagnostic techniques revealed a consistent view of the non-uniformity,
including a beam scanning calorimeter, the ratios of the arc currents flowing through the
spatially distributed filament groups, the sharpness of reverse-accelerated beamlet burn
marks on the back wall of the source, the relative temperatures of the vertically arrayed
sectors of the plasma grid, and, more recently, Langmuir probes.
Figure 1 shows the negative ion beam profile measured by a scanning calorimeter located
about three meters downstream of the ground grid of the uppermost of the two negative ion
sources. This distance is still in the near field of these large, 24 meter focal length sources, so
the beam uniformity is a measure of the corresponding extractable negative ion current
density illuminating the source plasma grid. It is apparent that there is some short scale length
non-uniformity, both across the narrow dimension of the source, and also in terms of local clumping which may be related to the discrete array of filament groups. However, the
dominant inhomogeneity lies along the vertical direction, with the density declining by
several tens of percent in the lower part of the source.
In attempting to determine the driving mechanism for this vertical non-uniformity, all
parameters within the sources were scanned, and those which could be reversed (such as the
plasma grid magnetic field) were also reversed. The non-uniformity was largely unchanged
by any of these variations, leading to the supposition that it is probably driven at least in part
by the one parameter that could not be reversed: the direction of arc current flow through the
filaments and into the plasma. This produces a linked magnetic field flowing the length of the
source which is the net field produced by the arc current flowing through the filaments.
Unlike the magnetic field produced by the filament heating current, the magnetic field from
the arc is not self-canceling. The potential for this causing non-uniformity is greater for
sources with large long extraction areas than for this sources smaller predecessors
Figure 2 shows the arc current distribution among the eight filament groups for typical
conditions when the series resistors all have values of 100 milliohms. When the filaments are
run in the space-charge limited mode, which was th