Nuclear Fusion Research and Development

4-11 Fast Ion Transport Study Using HELIOS－ Fast Ion’s Dance with Alfvén Waves in ITER Plasma －(ｂ)(ｃ)
Neutron emission rate (1015 s -1)

Fast−ion−driven
Alfven mode

80

Vertical distance (m)Frequency ν (kHz)

1.0

60
40
200

Alfven spectrum

0.0

0.2

0.4

0.6

0.8

1.0

Plasma radius (× 0.95 m)

0.5

0.0

3.0

3.5

4.0

Horizontal distance (m)

(ｄ)

2.0

1.5

１．
０

Experimental
value

beforeafterrelaxation

0.50123

Fast ion beta (%)

Normalized
neutron emission rate / Vol.

(a)
100

Fast ion beta = 1.3%
1.0
0.8
0.6
0.4
0.2
0.0
0.00 0.25

0.50

0.75 1.00

Plasma radius (× 0.95 m)

Fig.4-26 Properties of the fast-ion-driven Alfvén mode(a) Radial profile and frequency. The radius is normalized by
the volume-averaged plasma radius of 0.95 m. The mode
frequency of 45 kHz is consistent with experimental data, inwhich mode activity in the 30 − 70 kHz range is observed,and lies precisely on the continuous Alfvén spectrum (whitedots).
(b) Mode structure in the poloidal cross section of the torus.
The mode has toroidal mode number 1, consistent withexperimental results.

Fig.4-27 Effect on the neutron emission rate(c) Dependence of the neutron emission rate on the fast ion
beta. The box indicates experimental values, which arereproduced with beta values around 1% − 1.5%.
(d) Radial profile of the neutron emission rate per unit volume,
normalized by the central value of the initial profile Blue:initial, red: after relaxation. As in the experiment, the central
value decreases by 25% when the initial fast ion beta usedin the simulation is 1.3%. This is half the beta predicted byclassical calculations without Alfvén waves.

ITER is an experimental reactor intended to create theconditions required to sustain nuclear fusion reactions that canserve as a source of energy. It will confine deuterium-tritium
plasmas at 100 million degrees Celsius and produce neutrons
and helium ions via fusion reactions. The helium ions areborn with a kinetic energy 100 times larger than that of thebulk plasma and are used to heat it via collisions. However,the fast helium ions move with a rhythm that is similar to thatof plasma waves called Alfvén waves, so they dance together
and exchange energy. If the waves reach large amplitudes, thefast ions are pumped out of the confinement region. Thus, it isimportant to evaluate the fast ion transport.
Together with the National Institute for Fusion Science, weconducted simulations of fast ion transport using HELIOS, asupercomputer operated by the International Fusion Energy
Research Center. The simulation model was validated bysimulating scenarios from the JT-60U fusion experimental
device and by comparing the results with experimental data.
To mimic the ITER conditions in JT-60U, energetic deuterium
ions were introduced by powerful beams. The fast ions wereobserved to drive intensive cyclic bursts of Alfvén wave
activity. Because the bursting phenomena depend on the formof the fast ion distribution in phase space, a novel method wasdeveloped that allows us to initialize simulations with fast ion
distributions computed by an orbit-following Monte Carlocode for realistic beam geometry and collisions.
The simulation results show that the frequency and modenumber of the instability, which triggers profile relaxationaccompanied by the burst, are consistent with experimental
observation (Fig.4-26). By varying the fast ion beta value, weobtained a value that reproduced the fast ion redistribution
and the resulting changes in the neutron emission rate seen
in experiments (Fig.4-27). In classical calculations, the total
neutron emission rate is overestimated because Alfvén wave
activity is not taken into account. Our simulations indicate thatthe true value of the confined fast ion beta is about a factor of
two lower.
This work was partially supported by a Japan Society for thePromotion of Science (JSPS) KAKENHI Grants-in-Aid for
Scientific Research (No.22860081, No.23360416, No.25820443).

Reference
Bierwage, A. et al., Role of Convective Amplification of n =1 Energetic Particle Modes for N-NB Ion Dynamics in JT-60U, Nuclear Fusion, vol.53, no.7,
2013, p.073007-1-073007-12.

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JAEA R&D Review 2013