S-P4-2
Energetic Particle Confinement in Burning Plasmas
核燃焼プラズマにおける高エネルギー粒子閉じ込め
Takeo Nishitani, Kenji Tobita, Masao Ishikawa and Kouji Shinohara
西谷健夫, 飛田健次，石川正男，篠原孝司
Japan Atomic Energy Agency
2-166 Omotedate, Obuchi, Rokkasho-mura, Aomori-ken, 039-3212 Japan
日本原子力研究開発機構 〒039-3212 青森県上北郡六ヶ所村大字尾駮字表舘2番166
Energetic particle confinement in burning plasmas done at mainly JT-60U is reviewed, where neutron
diagnostics is an effective tool for this study. The slowing down process of NB-injected fast ions wasstudied by the NB blip experiments at JT-60U, where toroidal ripple effect on the energetic ion
confinement was confirmed. Triton burn-up measurement was carried out with the 14-MeV neutron
measurement in the DD plasma, which showed that the behavior of 3.5-MeV tritons was almost classical.
Also collective effects of the energetic particles such as Alfven Eigenmode were investigated in the D-D
plasma with 400-keV neutral beam injection.

1. Introduction
Energetic particle confinement especially 3.5
MeV alpha particle produced in D-T burning
plasma is very important in account of the alpha–heating of the plasma. In order to understandthe energetic particle behaviors in burning
plasmas, many studies have been carried out in
D-D plasma. Single particle behaviors of neutralbeam (NB)-injected energetic ions and 1 MeV
tritons produced by d(d,p)T reactions wereinvestigated by NB–blip and triton burn-up
experiments, respectively. Also collective effectsof the energetic particles such as Alfven
Eigenmode were investigated in the D-D plasma
with negative-ion-based 400-keV NB injection.
2. Toroidal Ripple Effect on the Energetic Ion
Confinement
The transport of energetic particles in toroidal
field (TF) ripple in a tokamak is one of the mostimportant issues as single particle behavior of alphaparticles in burning plasmas. In burning plasmas,
ripple loss of alphas not only will reduce the alphaheating power but also can cause serious localizedheat deposition on the first wall.
Therefore, TF ripple effect on the NB injectedenergetic particles was investigated in D-D plasmaof the JT-60U. Cross-section of the D-D reaction isa strong function of the deuteron energy, so thetime history of the neutron intensity after shot-pile
injection of D NB injection into the D plasma isdetermined by the slowing down and theconfinement of the NB injected energetic ions,which is called “NB-blip” method [1].
Figure1 shows neutron decay following anNB-blip injection for a 0.7 MA, 65m3 plasmacompared with the classical sowing down modelwith out loss and the Monte Carlo calculation(OFMC)[2] taking account of the TF ripple. The
measured neutron decay was faster than theclassical slowing down (solid line), and agreed withthe OFMC calculation. The discrepancy between
the measurement and the classical sowing down
model is considered due to the effect of a TF ripple.

Fig.1. Neutron decay following an NB-blip injection
compared with the classical sowing down model with outloss and the OFMC calculation.

In case that the energetic D ion loss is negligible,
neutron intensity is proportional to exp(-2.3/τs),
where τs is a classical slowing down time. Figure 2
shows the decay constant of the neuron intensity τnafter NB-blip injection plotted against the τs/2.3 forthe plasma volume 40 m3, 55 m3 and 80 m3, whichcorresponds to low, medium and high TF ripple
plasma. It was found that decay constant of theneuron intensity decreased with increase in the TF
ripple.
In ITER, TF ripple effect on alphas and NB
injected energetic ions caused by ferritic materialsof the test blanket modules is under discussion [3,4].

Recent calculation result of the effect will be shownin the presentation.

Fig.2. Neutron decay time plotted against the slowing
down time in different volume plasmas.

3. Triton Burn-up in D-D plasmaCollective effects of the energetic particles suchas Alfven Eigenmode driven by 400-keV neutralbeam injection were investigated. Tritons of 1.0
MeV are produced in the d(d,p)t reaction at
approximately the same rate as the 2.5 MeV
neutrons from the d(d,n)3He reaction. The behaviorof 1 MeV tritons is important to predict theproperties of D-T produced 3.5 MeV alphas
because 1 MeV tritons and 3.5 MeV alphas have
similar kinematics properties, such as Larmor
radius and precession frequency. The 1 MeV tritons
slow down and may undergo a D-T fusion reaction,
emitting 14 MeV neutron. The confinement andslowing down of the fast tritons can be investigatedby measuring the 14 MeV and the 2.5 MeV neutron
production rates. The time resolved triton burn-up
measurements have been performed using a new
type 14 MeV neutron detector based on scintillatingfibers with high time response at JT-60U. Figure 3
shows the decay curve of the 14 MeV neutron
emission rate measured by the scintillating fiber
detector after NB injection, where a diffusivity offast tritons was evaluated to be ~0.1 m2/s [5].

Fig.3. Experimental and calculated 14 MeV neutron
emissions in the NB heated JT-60U Plasma.

4. Collective Effect on the Energetic Particle
Confinement
The confinement degradation and transport ofenergetic ions due to Alfven eigenmodes (AEs)destabilized by negative-ion-based NB injection arequantitatively evaluated. AEs were observed in JT60U weak shear plasmas with frequencies sweeping
up to 30–50 kHz on a time scale of a few hundred
milliseconds as the safety factor at the plasma
center, q (0), decreases from 1.75 to 1.5. Then as q(0) decreases from 1.5 to 1.25 in about 600 ms, theAE frequencies are saturated and the AEs areidentifiied as the TAEs. The measured total neutronemission rate (Sn) in the presence of these AEs iscompared with that predicted by classical theory. As
a result, the confinement degradation of energetic
ions is confirmed and the reduction rate in Sn islargest when AEs are destabilized and are located at
the large fast ion pressure gradient region in theplasma core. In order to investigate the energetic
ion transport due to these AEs, line-integrated
neutron emission profiles measured with a large
neutron collimator array are compared with theclassical confinement calculations. It is found thatenergetic ions are transported from the plasma coreregion to the outer region due to these AEs.
Furthermore, changes in the energy distribution of
charge exchange energetic neutral particle fluxes
suggest that the radial energetic ion transport is dueto the resonance interaction between the energetic
ions and the AEs [6].
References[1] K. Tobita, et al.: Nucl. Fusion 34 (1994) 1097.
[2] K. Tani, et al.: J. Phys. Soc. Jpn. 50 (1981) 1726.
[3] K. Shinohara, et al.: Fusion Eng. Des. 84 (2009) 24.
[4] K. Shinohara, et al.: Nucl. Fusion 51 (2011) 063028.
[5] T. Nishitani, et al.: Plasma Phys. Control. Fusion 38(1996) 355.
[6] M. Ishikawa, et al.: Nucl. Fusion 47 (2007) 849.