Proceedings of ITC/ISHW2007

Design Study of Lost Fast-Ion Probe on the Large Helical Device
K. OGAWA, 1M. ISOBE, 1K. TOI
Department of Energy Science and Engineering, Nagoya University, Nagoya 464-8603, Japan
1National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan(Received ****** / Accepted ******)
A lost fast-ion probe (LIP) with a scintillator screen was designed for measurements of energetic ion loss flux
caused by energetic-ion-driven MHD instabilities as well as magnetic field ripple in the Large Helical Device(LHD). The optimum position for the installation of the probe was determined by searching for energetic ion
trajectories along which energetic ions launched from the probe can reach the region inside the last closed flux
surface, without intersecting with any in-vessel components of the LHD vacuum chamber. The LIP is installed at
the outboard side of a horizontally elongated poloidal cross section. It can be moved in the horizontal direction by
750 mm per 1.5 minutes with a pneumatic motor. The aperture of LIP was arranged to measure losses of co-goingenergetic ions, whose detectable ranges of the pitch angle and gyroradius are 35-50 degrees and 1.5-15 cm,respectively. The scintillator P46 was adopted, being taken into account its high sensitivity and short decay timeeven at high temperature. The light emitted from the scintillator by energetic ion bombardment is detected by aphotomultiplier array with high-time-resolution (up to 200 kHz) and an image intensified CCD camera (up to 2kHz). Accordingly, thus designed LIP is expected to detect fast loss events of energetic ions induced by MHD
instabilities such as Alfvén eigenmodes.
Keywords: lost-ion probe, energetic ion, scintillator, orbit calclation, Alfven eigenmodes

1. Introduction
Good confinement of energetic particles such asalpha particles is crucial for realization of a nuclear fusion
reactor. Large amount of loss of energetic ions caused byenergetic-ion-driven magneto hydrodynamic (MHD)
instabilities should be avoided, because it would quench
fusion burn, leading to localized serious damages of thefirst wall. In the Large Helical Device (LHD),
energetic-ion-driven MHD instabilities such as
toroidicity-induced Alfvén eigenmodes (TAE) andenergetic-particle mode (EPM) are excited in neutral beam(NB)-heated plasmas [1, 2], and induce anomaloustransport of energetic ions [3]. We should measure
distribution of pitch angle (χ=arccos(v///v)) and energy oflost-fast ions due to energetic-ion-driven MHD instabilities,
in order to understand detailed loss mechanisms andminimize it. A most powerful diagnostic tool to detect
fast-ion loss flux is a lost fast-ion probe (LIP) with
scintillator screen [4]. The LIP has been successfully
applied to helical/stellarator plasmas [5, 6] as well as
tokamaks [7-9]. For this reason, we have designed the LIP
which will be installed at the outboard side of ahorizontally elongated poloidal cross section in LHD.
The LHD is a helical device with toroidal field
Author’s e-mail: ogawa.kunihiro@LHD.nifs.ac.jp

periods of 10 and poloidal periods of 2. It has a plasma
major radius R is ~3.6 m and an average minor radius<a> is ~0.6m. The toroidal magnetic field strength Bt canbe increased up to 2.9 T.
Figure 1 shows a top view of the LHD, NBIinjectors, and location of the head part of LIP. The LHD
has four NB injectors, three of which (BL-1~3) injecthydrogen neutral beams tangentially (co- orcounter-direction) into the plasma, having energies up to180 keV, and another one (BL-4) injects beam
perpendicularly at energy of 40 keV.

2. Positioning Study
In order to detect lost energetic ions effectively,
the probe should be placed close to the plasma. On theother hand, the damages of the probe such as melting by
lost energetic ion flux should be avoided. From this pointof view, a scanning range of the probe in the horizontal
direction was carefully determined as explained below.
The procedure of positioning study is asfollows : 1) An energetic ion is launched from the possible
candidate position of LIP head, having various initial
velocities and pitch angles expected in LHD plasmaProceedings of ITC/ISHW2007
conditions, 2) The Lorentz force equation for charged
dv
particle's motion ( m= q(v ×b ) ) is solved backwardly
dtin time in the LHD magnetic field in the Cartesian
coordinates, 3) If the fast ion launched from the LIP head
enters inside the last closed flux surface of an LHD plasma,
we judge that a trajectory of lost ion is found.
Figure 2 show typical orbits of energetic ionsreaching the LIP. One has initial energy Ei of 150 keV andpitch angle χ of 108.7 degrees (Fig.2a) and the other has Eiof 150 keV and χ of 41.8 degrees (Fig.2b). Bt and themagnetic axis position of the vacuum field Rax are 0.75 T
and 3.6 m, respectively in this case. In this calculation, thetrajectories of energetic ions are followed in the static
magnetic field without any field perturbations and theradial electric field is assumed to be zero (E=0). In the caseshown in Fig.2(a), a launched ion doesn’t go back into theplasma region and hits the first wall surface immediately. Itmeans that such ion does not reach the probe from theplasma confinement region, that is, will not be detected by
the probe actually. On the other hand, in the case shown inFig. 2 (b), the ion goes back into the plasma and it meansthat it is detectable by the probe. As can be seen, this isco-going transit energetic ion whose orbit deviates
substantially from magnetic flux surfaces.
We have tried to find energetic ions whichhave small χ, because such ions would easily interact
with Alfvén eigenmodes (AE). We have evaluated thenumbers of such lost trajectories, by changing the radial
position of the probe head in the horizontal direction.
Figure 3 shows the counts of lost ions as a function of theradial position in the horizontal direction, where 750
particles are launched from the LIP head, having various
energies and pitch angles. We evaluated the counts of lostions found using above calculation, retracting the positionof the LIP head outward in the accessible range of LIP(from R=4.46 m to R=4.6 m) by every 20 mm step. Thiscalculation was carried out for the reference configurationof Rax=3.6 m in two different Bt of 0.75 T and 1 T.

3. Diagnostic Setup
Figure 4 shows a schematic view of thedesigned LIP installed in the horizontally elongatedpoloidal cross section of LHD. The probe head is attached
to a stainless steel shaft of 3 m long. The LIP head can beinserted in the horizontal plane shifted upward by 80 mm
for the equatorial plane of the LHD device, to the innermost position (R=4.46 m) by a pneumatic motor, as shownin Fig. 4a. The LIP passes through a diverter leg to theinnermost position. A stainless steel shaft is partiallycovered with a graphite sleeve of 380 mm long and 5 mm
thick, to protect the LIP against high heat flux flowing

from a main plasma along divertor leg. The scintillator
box of LIP is made of stainless steel and covered withmolybdenum plates. Lost energetic ions pass thorough afront- and rear- aperture, and then hit a scintillator surfaceof which strike point gives information of both gyroradiusand pitch angle of the lost ion. (Fig.5) Once a size ofaperture, scintillator position and magnetic field strength at
the scintillator position are specified, a pattern ofscintillator light is uniquely decided.
Two dimensional image of scintillation light due
to impact of lost-fast ions has to be transferred outside of
the vacuum vessel. In our probe, because the plane of
scintillator surface is parallel to the axis of the probe shaft,the scintillation pattern appeared on the screen is firstreflected 90 degrees by means of a polished stainless steel
mirror mounted inside the probe head box. After thisreflection, two dimensional light image is transferred by aseries of relay lens mounted inside the probe shaft and ismeasured with an image-intensified CCD camera and/or
photomultiplier tubes. (Fig4b) Form the analysis of lightpattern appeared on the scintillator surface, gyroradius andpitch angle of lost-ion flux can be derived.

4. Details of the LIP HeadThe position that the LIP is installed to LHD isshown in Fig.1. When the direction of Bt is clock wise(CW) as seen from the top, the ion grad-B and curvaturedrifts are upward and ions tend to drift toward the probe.
The LIP designed in this study can work only in thisoperation. The LIP will not be able to detect any lost-ion
flux when Bt is directed to be counter-clock-wise becauseof following primary two reasons. One is that ions tend togo away from the LIP because the ion grad-B drift isdownward. Another is that gyromotion of energetic ionsbecomes opposite compared with the CW-Bt case. Even if
lost-energetic ions reach the LIP and enter the detector box,
they can not hit the scintillator surface. Detail of the LIP
head is shown in Fig.6. The size of LIP head is 58 mm(width) × 52 mm (length) × 66 (height), but has defective
part near the aperture. The angle between the side surfaceof the LIP and the aperture is selected to be 21 degrees, sothat lost ions having wide range of gyroradius and pitch
angle would pass through the aperture effectively. Thefront aperture has 3 mm wide and 1 mm high whereas therear aperture has 24.1 mm wide and 1 mm high. The sizeof aperture was determined so as to obtain optimum energy
and pitch angle resolution and signal intensity fromcalculation. The distance between the two apertures is 10mm. The normal vector of the aperture is 113 degrees withrespect to the local magnetic field direction, to guideenergetic ions with 35 to 50 degrees (co-going particle).

Proceedings of ITC/ISHW2007
Also, in Fig. 5, calculated hit point area of lost ionshaving the range of 1.5 to 15 cm in gyroradius and 35 to 50
degrees in pitch angle is shown on the scintillator screen. Itshould be noted that the gyroradius is defined with ρ=|v|/ωcwhere ρ, v, and ωc indicate gyroradius, velocity of ion, andcyclotron frequency of ion at the position of probe. That is,
the gyroradius and pitch angle are not equal to those in aplasma. As a scintillator material, YAG:Ce (P46) is used,because it has high sensitivity and short decay time even athigh temperature. P46 is deposited on a quartz plate coatedwith aluminum, and emits green light. The light emitted
from the scintillator is detected by an array ofphotomultiplier with high-time-resolution (up to 200 kHz).

designed LIP are arranged. Both BL-1(180keV)
and BL-3(180keV) are in operation to be co or
counter injection simultaneously. On the other
hand, BL-2 (180keV) is in operation to the opposite
direction against BL-1 and BL-3. BL-4 is used for
perpendicular NBI. The position of LIP is marked
near BL-3.

5 Conclusion
A lost fast ion probe with scintillator (LIP) installedon LHD was designed by finding trajectories of energeticions which connect from the probe to the plasma
confinement region. It is concluded that when the LIP isplaced in the range of R=4.46 m to 4.6 m in a planned port
position of LHD, energetic lost ion flux due to AEs would
be detected. This LIP will be a powerful tool to study
interaction between energetic ions and energetic ion driven
instabilities.
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Fig.1 Top view of LHD where NB injectors and a newly

(a)

(b)
Fig.2 Examples of calculated trajectory. Red X markindicates the initial position of orbit (equal toposition of probe head). Blue X mark indicates thehit poison of the ion at the vessel surface.
The~
condition is Bt=0.75 T, Rax = 3.6 m, E=0, B=0. (a)Poloidal projection of a calculated orbit trajectory inLHD, where the initial condition is Ei=150 keV,
χ=108.7 degrees. In this case, the probe can’tdetect this lost energetic ion. (b) Poincare plot of acalculated orbit trajectory in LHD. Initial condition isEi=150 keV, χ=41.8 degrees. In this case, theprobe can detect them.

Proceedings of ITC/ISHW2007

Fig.3 Counts of energetic ions that are lost from theplasma core region and reach to the LIP, where750 particles are launched from the probe, having
various energies and pitch angles. In theconfiguration of Rax=3.6m. The results at Bt = 0.75
T and 1 T are shown by circles and triangles,
respectively.

a)b)

Fig.4 a) Schematic view of LIP placed at the innermost
position in the horizontally elongated section ofLHD. An example of Poincare plots of thetrajectorie of lost ion with 150keV energy and 35.0
degrees pitch angle. It can be moved in thehorizontal direction by 750 mm per 1.5 min with apneumatic motor. A stainless pipe supports probehead is covered by a graphite sleeve of 380 mm
length and 5 mm thickness, to protect from highheat flux along divertor legb) The route of signal transfer system and dataacquisition system of LIP.

Fig.5 The side view of scintillator box. Lost energetic ionspass through apertures, and then hit a scintillator
surface. Strike point gives information of bothgyroradius and pitch angle of the lost ion.

Fig.6 Schematic view of the probe head and expectedarrival area of energetic ion. Expected hit point
area of lost ions having the range of 1.5 to 15 cm for
gyroradius and 35 to 50 degrees. The normal vectorof the aperture is 113 degrees with respect to the
local magnetic field direction. The size of a front
aperture is 3 mm (width) × 1 mm (height) and thesize of a rear aperture is 24 mm (width) × 1 mm(height). The distance between two apertures is 10mm.