Speaker
Description
Studies of the effects of hydrogen (H) isotopes are an important issue on achievement of high
performance in the next Deuteriun–Tritium (DT) and Tritium-Tritium (TT) high beta
operations at JET with ITER-like Wall. Experiments with mixed H-D plasmas have been
recently performed at JET to understand the dependence of the confinement on these isotopes
by varying the gas, beam [1] and pellet fuelling [2]. Since strong dependence on confinement
has been found in mixed plasmas close to pure values of H or D and no dependence for other
values of the isotope mixture, while opposite behavior is expected for any DT composition,
magnetohydrodynamic (MHD) instabilities could still play a key role in the confinement loss.
Particularly, the Neoclassical Tearing Modes (NTMs), observed in Baseline and Hybrid
scenarios, are responsible for a decrease of performances leading in some cases to disruptions.
In JET plasmas most detections of NTMs at low poloidal (m≤5) and toroidal (n≤4) mode
numbers have been observed for betaN< 2.2 in Baseline and for betaN>2 in Hybrid [3].
Predictions of effects of different isotope mixtures on NTMs onset and dynamics can provide
indications for the plasma stability, with modes appearance depending on the isotope ion mass
mi . In fact the modelling of the width evolution, integrated in the European Transport
Simulator (ETS) [4], is described by a a generalized Rutherford equation –GRE– [5] where the
bootstrap and the ion polarization terms contain the mi dependence through the ion collision
frequency and the ion Larmor radius. Simulations of HD and DT plasmas, extrapolated from
high power and neutron rate discharges, have been provided taking into account physics aspect
as transport, turbulence and energetic particles [6].
Isotope action on MHD instability is now considered as well. First predictions of the favourable
NTMs onset allowed by high bootstrap current (Jbs) have been provided considering the JET
baseline pulse #92436 (2.8T, 3MA, 28 MW NBI, 6 MM ICRH). The aim was the evaluation
of Jbs considering different fraction of the H-D-T isotopes in the same scenario neglecting other
isotopes effects in the plasma core and edge. In this case, considering the total pressure as the
sum of different fraction of isotopes with same gradients for the ion density profiles, the
bootstrap current changes are negligible (£1%) [7] for all H-D-T ratio as shown in figure 1,
where Jbs from the transport models NCLASS [8] and NEOS [7], integrated in ETS, were
compared. The goal of this paper is to predict the tearing onset and evolution in DT and TT
baseline and hybrid scenarios provided by ETS where transport simulations take into account different physics aspects and also the change of electron, ion and
heavy impurities perpendicular diffusion coefficients due to the presence of NTM [9].
As many discharges with and without NTM
and with the same betaN are in the JET
database, a plasma stability range will be
given in this paper as function of poloidal
beta in order to determine the pressure and
shear conditions for the mode avoidance in
DD and DT plasmas.
Isotopes affect also the duration of the
sawtooth (ST) period very important for the
NTMs destabilization.
These modes can be triggered by a crash of a ST period when the magnetic shear becomes
larger than a crtical one (s1,cr) proportional to the ion mass normalized to the proton mass as
(mi/mp)1/3. It means that for the same q profiles the ST periods in H are smaller than the ones
in D, because s1,cr(H) < s1,cr(D) , as observed in isotope identity experiments [10] and shown in
figure 2. Here, the sawtooth periods in H are about 80-90 ms, while in D about 140-150 ms.
As the mode onset is usually observed after a crash of a long ST period, the condition of the
NTMs destabilization should be more favourable in D plasmas than in H and more in T than
in D. This isotope dependence is investigated in this paper using the sawtooth model integrated
in ETS [11].
Results of the role of NTMs on plasma stability in baseline and hybrid DT and TT scenarios
will be compared with JETTO-JINTRAC [12] simulations obtained in the same plasma
conditions.
Acknowledgements
This work has been carried out within the framework of the EUROfusion Consortium and has received
funding from the EURATOM research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the
European Commission.
References
[1] M. Maslov et al., This Conference, IAEA FEC 2020
[2] M. Valovic et al., This Conference, IAEA FEC 2020
[3] E. Alessi et al., EPS 2018, P2-1044
[4] D. Kalupin et al., Nucl. Fusion 53, 123007 (2013), P. Strand et al., IAEA FEC 2018, TH/P6-
14, IAEA CN-258
[5] V. Basiuk et al Plasma Phys. Control. Fusion 59 (2017) 125012
[6] J. Garcia et al., Nucl. Fusion 59, 086047 (2019)
[7] O. Sauter et al., Physics of Plasmas 6, 2834 (1999)
[8] W. A. Houlberg et al., 1997 Phys. Plasmas 4 3230
[9] S. Nowak et al., IAEA FEC 2018, IAEA-CN-TH/P6-26
[10] C. Maggi et al., Nucl. Fusion 59 076028 (2019)
[11] O. Sauter et al., 1999 Theory of Fusion Plasmas (Proc. Joint Varenna–Lausanne Int. Workshop)
[12] M. Romanelli et al., Plasma Fusion Res. 9 3403023 (2014)
| Affiliation | Istituto per la Scienza e Tecnologia dei Plasmi , CNR, Milano, Italy |
|---|---|
| Country or International Organization | Italy |
