1 MeV triton which produced from the d-d fusion reaction has similar orbit and thermalization rate with those of 3.5 MeV fusion alpha. Thus the triton behavior under certain plasma condition is useful to study a single particle behavior of the alpha under that condition. Combined information of the triton thermalization and confinement can be obtained from the triton burnup measurement. During the slowing down of the triton inside the background deuterium plasma, secondary d-t fusion reaction may occur and this reaction is called triton burnup. Time-resolved triton burnup has been successfully measured in KSTAR.1, 2
To analyze measured triton burnup, expected triton burnup in the measured plasma condition is calculated based on simple triton burnup model. Two fundamental parameters for triton burnup calculation, prompt loss and Coulomb drag are considered in the calculation.2, 3 The calculated value provides a maximum achievable triton burnup in that plasma condition, since it only considers fundamental loss and slowing down mechanism.
Most of high performance plasma discharges in KSTAR such as hybrid scenario discharges have shown beam-driven Alfvén instabilities.4 Since kinetic property of 1 MeV triton is different with that of deuterium beam ion, its interaction with an instability might be different with the beam ion. Based on the established triton burnup diagnostics and analysis routine (Fig. 1.), the interaction between the instability and 1 MeV triton can be quantified and categorized. Therefore, these will be useful to identify the crucial condition for the triton confinement which requires further detailed analysis by using numerical codes.
For the triton burnup measurement in KSTAR, three kinds of diagnostics are prepared. The secondary d-t fusion neutron yield during the plasma discharge is measured by neutron activation system (NAS) with silicon sample.2 For the time-resolved measurements of triton burnup, organic scintillation detectors and scintillating fiber detectors are used.1, 2 Measured triton burnup in scintillator based neutron detectors are cross-calibrated by using NAS measurement.
Prompt loss of 1 MeV triton is calculated statistically based on the plasma equilibrium and full gyro orbit following code.2, 5 Guiding center orbit following code is also prepared and its applicable range is identified.
For the slowing down and burnup calculation, simple code has been developed.2 This code assumes that the triton slows down on the birth position. Due to triton drift orbit width size, the assumption can cause certain amount of systematic error depends on the plasma parameters and triton birth characteristics. The systematic error in the simple code is estimated by comparing it with realistic slowing down calculation using orbit following code.
Analyses of measured triton burnup
Measured triton burnup in Alfvén instability mitigation experiment (KSTAR plasma discharge #21695) is analyzed by using triton burnup calculation tools. In this discharge, beam driven Alfvén instability is mitigated by the appropriate electron cyclotron current drive.6 When the instability is mitigated (after 5.5 s in Fig. 2.) total neutron rate becomes about double compare with the instability active phase (before 5 s in Fig. 2.). The triton burnup ratio as well as secondary d-t neuron emission rate also increases about three times higher when the instability is mitigated. In order to identify additional effects other than fundamental loss and slowing down mechanism, measured and calculated triton burnups are compared. On the calculation, the d-d fusion reaction profile is derived from the TRANSP calculation. In the Alfvén instability active phase, by using TRANSP with kick model7, interaction between the instability and beam ion considered d-d fusion reaction profile is derived. For the prompt loss fraction profile calculation, the full gyro orbit following code and kinetic EFIT are utilized. For the slowing down calculation, it is assumed that tritons are slowed down on the birth position.
In the Alfvén instability mitigated phase, the measured and calculated triton burnup matched within the experimental error (7.4 s magenta star in Fig. 2.). In the Alfvén instability active phase, difference between measurement and calculation becomes larger (4.45 s magenta star in Fig. 2.) compared with mitigated phase, but the difference is smaller than difference in measured values in the two regions. The lower expected triton burnup ratio in the active phase is caused by higher prompt loss rate and flattened triton birth profile. From the interaction between the instability and beam ion, current density profile becomes slightly reversed shear shape8 which can enhance the prompt loss of energetic triton. At the same time, d-d fusion reaction profile becomes flattened compare with the mitigated phase. These two changes could be main reasons for the decreases in the measured triton burnup in the Alfvén instability active phase.
1 Jungmin Jo et al., Rev. Sci. Instrum. 89, 10I118 (2018)
2 Jungmin Jo, Ph.D. dissertation, Seoul National University (2019)
3 P. Batistoni and C. W. Barnes, Plasma Phys. Control. Fusion 33, 1735 (1991)
4 Junghee Kim et al., 26th IAEA Fusion Energy Conference, EX/P4-26, Kyoto, Japan (2016)
5 M. Isobe et al., J. Plasma Fusion Res. SERIES 8, 330 (2009)
6 Junghee Kim et al., 16th IAEA Technical Meeting on Energetic Particle Physics, P1-2, Shizuoka, Japan (2019)
7 M.Podestà et al., Plasma Phys. Control. Fusion 56, 055003 (2014)
8 Jisung Kang et al., 16th IAEA Technical Meeting on Energetic Particle Physics, P1-17, Shizuoka, Japan (2019)
|Affiliation||National Fusion Research Institute|
|Country or International Organization||Korea, Republic of|