S1-2
NIFS-R&Dイオン源での統合的負イオンプラズマ計測
Comprehensive diagnostic study of negative-ion-rich plasmas in NIFS-R&D
negative ion source
津守 克嘉
K. Tsumori
National Institute for Fusion Science
Hydrogen/deuterium negative ion (H-/D-) source
has been developed and applied for neutral beam
injector (NBI) to obtain high power beam of theenergy range above 100 keV [1]. Negative-ionbased neutral beam injector (NBI) systems arescheduled to be adopted and considered for thereactor level fusion machines such as ITER andDEMO [2,3]. In order to enhance the H-/Dcurrent, tiny amount of caesium (Cs) vapor isseeded in this type of negative ion source. Seeded
Cs lowers the work function on the surface of
so-called plasma grid (PG), which separates the ion
source into negative-ion generator and acceleration
region, and H-/D- is produced on the PG surface.
At the same time, co-extracted electron current
reduces as H-/D- increases.

Driverregionregion has started with use of multiply combined
diagnostics. The alignment of the diagnostic
modules is indicated in Fig. 1. Outer size of themeasured region is 700 (H) x 350 (W) x 35 (D) mm.
Some of the modules are two- dimensionally
movable and can measure the spatial distributionsof densities and potential in the extraction region.
In the negative ion source, tiny amount of
caesium (Cs) is seeded in the plasma chamber toenhance the H- production by lowering the work
function on so-called plasma grid (PG), which
separates the plasma chamber and beam accelerator.
Electron density in the extraction region decreases
by seeding Cs, and the plasma consists of onlyhydrogen positive and negative ions finally [4].
Shielding feature of electrostatic field in this H- richplasma is different from “normal” electron-positiveion plasmas as shown in Fig. 2.
Potential slopes do not change in the “normal”

Extraction
region(b)

Fig. 1. Cross section of NIFS-R&D negative ion source on
short side (upper) and alignment of multiple diagnostic modules(lower) in a view on the top of upper figure. Notice that thescale of lower figure is compressed in lower one.

In order to obtain higher and stable H- current
density, production and extraction mechanisms of
H- was necessary to understand. For this reason,
measurement of the plasmas at beam extraction

Fig. 2. Spatial distributions of plasma potentials obtained withLangmuir single probe (a) in hydrogen plasma without Cs and(b) in Cs-seeded plasma. Plots marks indicates difference ofthe bias voltage and the probe tip scanned in the direction
normal to the plane of PG surface toward driver region.

hydrogen plasma and sheath difference (Δsh), which
is defined as the gap between the bias voltage
applied at PG and extrapolated fitted line of(a)potential distribution [5]. On the other hand, thepotential distribution in Cs-seeded case changes theslopes and the sheath gap is shallower. Similareffect is seen by applying electrostatic field to
extract beams from H- rich plasma. Actually, Hafiltered CCD shows extracted H- ions distributewidely over the extraction region [6]. Charge
neutrality is conserved by positive ions and Hbefore beam extraction, while electrons diffusedfrom driver region to conserve the excess positive
charge during H- extraction [4].
In the case of H- rich plasma, electric field in theplasma at extraction region is sensitive to appliedexternal field. Magnetic field is relatively strong
in this region and drifts of charged particles areexpected. Thus, flows of charged particles aremeasured, and they are compared to theirtemperatures. Temperature of hydrogen atom ismeasured by means of high-resolution opticalemission spectroscopy (HR-OES) [7] and Hα laser
absorption spectroscopy (Hα LAS) [8]. The Hotemperatures in the cases of OES and Hα LAS are
0.5 – 1.0 eV and 0.3 eV, respectively. In the HROES, Hα can emit from neutralized hydrogen
positive ions via following two reactions;

H 2+ + e → H (n = 3) + H (n = 1)
H + + H − → H (n = 3) + H (n = 1)
Here n indicates principal quantum number.
The Doppler shift changes with bias voltage, indeed.
The largest difference of HR-OES and Hα LAS isdirection of the sights, and it is necessary to flip
their sights to confirm the difference.
Temperatures and flows of charged particles aremeasured with a 4-pin Langmuir probe shown inFig. 3(a). The probe is movable in three directionof x, y and z, and rotatable around the probe axis
parallel to the x direction. The probe tips areirradiated with Nd:YAG laser to apply photodetachment technique [9] for the measurement of Hdensity. Flow directions of positive ions areshown in Fig. 3(b) at three different distance of themeasurement. Flow directions of electron arealmost parallel to the ion flows due to ambipolar
diffusion. Those flows of H- are illustrated in (c)of the same figure. The H- flows direct parallel tothe magnetic field lines and to the opposite
directions of the positive ion and electron.
Considering the source regions of positive ion andelectron defused on right hand side in the figure andH- production on the surface of PG, theexperimental result is consistent. The H- flow
velocity and temperatures are listed in Table 1.
According to the experimental results, Htemperature is about 0.1 eV, which is confirmed(c)Fig. 3. (a) Structure of 4-pin Langmuir probe, (b) flow
directions of positive ions at z of 12, 19 and 26 mm, and (c)that of H- flow direction at z of 14, 19 and 24 mm.

with saturated CRD measurement [10], and flow
speed is 0.1 to 0.5 times of thermal velocity of H-.
This shows cold H- rich plasma is produced in theextaction region.
TABLE 1. H- temperature (TH-) and flow velocities in y (vy)and z directions (vz).

1Temperature

z=14 [mm]0.13 eV

z=19 [mm]0.10 eV

z=26 [mm]0.12 eVvy-69 m/sec-308 m/sec-378 m/sec

vz

477 m/sec

152 m/sec-137 m/sec

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