22F13
Development of Ion Beam Prob for the LATE Plasma
LATEプラズマのためのイオンビームプローブの開発
OMI Shota1, TANAKA Hitoshi1, KATSUMA Jun1, KANEMISU Toshiyuki1, HAYASHI Ryota1,
FUKUNAGA Tadahiko1, MIZOGAMI Hikaru1, KURODA Kengoh1, NOGUCHI Yuto1,
WATANABE Fumitake1, UCHIDA Masaki1, MAEKAWA Takashi1, IGUCHI Harukazu2
Kyoto Univ. 1, NIFS2
逢見翔太1, 田中仁1，勝間淳1，金光俊之1，林良太1，福永忠彦1，溝上晃1，黒田賢剛1，
野口悠人1，渡辺文武1，打田正樹1，前川孝1，井口春和2
京大エネ科1，核融合研2
Graduate School of Energy Science, Kyoto University
Kitashirakawaoiwakechou, Sakyo-ku, Kyoto-city 606-8502, Japan
京都大学 〒606-8502 京都市左京区北白川追分町
An Ion Beam Probe is being developed to measure space potential distributions of the LATE(Low Aspectratio Torus Experiment) plasmas which are formed and maintained solely by electron Bernstein wave
heating and current drive. Ion sources of Na+ , K+ and Rb+ have been made. Performance tests of the ion
gun in the test bench and injection tests of ion beams in the LATE device have been carried out. Observed
center positions of beams are almost consistent with calculated ones. But beams spread in the toroidal
direction. It is a conspicuous problem in a low aspect ratio device. We are going to focus beams by using
electrostatic quadrupole lens electrode. Energy analyzer is being designed.

1. Introduction
An ion beam probe (IBP) is a useful tool forthe measurement of space potential in hightemperature plasmas where insertion of Langmuirprobes is impossible. It can measure not only
space potential, but also other plasma quantities
such as electron density and temperature,magnetic field and their fluctuations.
2. The LATE plasmas
In the LATE device, the spherical tokamak
equilibrium configuration is formed solely byelectron Bernstein wave heating and current drive
and the plasma current is carried by a high energy
electron tail[1].
Plasma parameters of typical discharges in theLATE device are following: plasma current Ip ≲10kA, electron density ne = 1011 ~ 1012cm-3, andelectron temperature Te ~ 70eV when toroidal field
Bt = 480 ~ 720 Gauss and vertical field Bz = 15 ~125 Gauss. Plasma major radius is 22.5 cm, plasma
minor radius is 15.5 cm, and aspect ratio is 1.45.
3. Overall IBP System design
To investigate confinement and transportcharacteristics in such wave driven sphericaltokamak plasmas, we are developing an IBP systemfor potential measurement. We use alkali metal ions
(Na+, K+, Rb+) with energy ~ 20 keV so that theLarmor radius of the secondary ion is large enoughto escape from the plasma. Calculations based on

the data of ionization cross section[2] show that thesecondary beam currents of 22nA Na2+, 130nA K2+
and 380nA Rb2+ may be detected respectively when
100 μA beams are injected and the length ofsample volume along the primary beam is 1cm.
These currents are sufficient for measurement.
Overall IBP system is shown in Fig.1
schematically. The injection beam line is set at thetop port of the LATE device and the detection beamline is set at the bottom. The injection beam line
consists of an ion gun, a beam profile monitor, atotal beam current monitor, a toroidal sweeper and apoloidal sweeper. The ion gun consists of an ion
source, a Pierce extraction electrode, and cylindricallens electrodes. The beam ions are accelerated up to~ 20 keV, here. The poloidal and toroidal sweeperchange the injection angle on the poloidal plane andin the toroidal direction respectively. The detection
beam line consists of a poloidal deflector, a toroidal
deflector and an energy analyzer. The poloidal
deflector and the toroidal deflector guide asecondary beam to the entrance aperture of theenergy analyzer, and adjust the incident angle of thebeam to the energy analyzer. The energy analyzer
consists of a pair of parallel electrodes and split
plates detector. The incident angle of the beam tothe energy analyzer is 30 degree. This is theProca-Green type energy analyzer[3,4] and it has
second order focusing property.

consistent with each other when the beam energy is~ 14 keV. But when the beam energy becomes
lower, the difference appears. It becomes ~ 2 cm at
the entrance of the poloidal deflector when thebeam energy is ~ 8 keV.
Thirdly, the detection beam line and the matrix
plates detector(8 × 3 = 24ch) instead of theenergy analyzer are installed into the LATE device,and following tests are carried out. Injected primarybeams are guided by the poloidal deflector to thematrix plates detector and the cross section ofbeams is examined. The observed positions of beam
center changes by the applied poloidal deflector
voltage, which is consistent with the calculation.
These test results show that the beam controllingsystem meets the minimal demand level for themeasurement.
However, the beam spreading in the toroidal
direction is observed. While the beam diameter inthe poloidal direction is ~ 1.5 cm, the beam
diameter in the toroidal direction is more than 6 cm.
It is a conspicuous problem in a low aspect ratio
device. So, we are going to focus beams by using
electrostatic quadrupole lens electrode.
5. Summary
The IBP system to measure space potential
distributions in the LATE plasma has been designed.
The injection beam line and the detection beamline with the matrix plates detector have been
installed into the LATE device and have been tested.
Results show that observed center positions ofbeams are almost consistent with calculated ones.
But beams spread in the toroidal direction due to a
low aspect ratio. We are going to focus beams byusing electrostatic quadrupole lens electrode.
Fig. 1: Overall IBP System
4. Components developments, tests and present
statusFirstly, we have made ion sources of Na+, K+ andRb+, and have tested the ion gun's performance inthe test bench. Ion current more than 100 μA isextracted. Beam diameter is less than 3 cm at 206
cm away from the ion source. This placecorresponds to z ~ 10 cm in the LATE device.
Secondly, the injection beam line is installed into
the LATE device, and following tests are carriedout. The cross section of beams at z = 1.5 cm isexamined by the linear movable detector, and thecross section of beams at the entrance of thepoloidal deflector is examined by the matrix plates
detector (4×6 = 24ch). The observed positions ofbeam center are compared with calculation ones bysweeping the poloidal sweeper voltage. Both areReferences[1] M.Uchida et al., Phys. Rev. Lett. 104, 065001(2010).
[2] T. Kato, K. Masai and M. Arnaud, Research Report,
NIFS-DATA-14 (1991).
[3] T. S. Green and G. A. Proca, Rev. Sci. Instrum. 41,1409 (1970).
[4] G. A. Proca and T. S. Green. Rev. Sci. Instrum. 41,
1778 (1970).