Energetic ions in ITER plasmas

Abstract

This paper discusses the behaviour and consequences of the expected populations of energetic ions in ITER plasmas. It begins with a careful analytic and numerical consideration of the stability of Alfvén Eigenmodes in the ITER 15 MA baseline scenario. The stability threshold is determined by balancing the energetic ion drive against the dominant damping mechanisms and it is found that only in the outer half of the plasma (r/a>0.5) can the fast ions overcome the thermal ion Landau damping. This is in spite of the reduced numbers of alpha-particles and beam ions in this region but means that any Alfvén Eigenmode-induced redistribution is not expected to influence the fusion burn process. The influence of energetic ions upon the main global MHD phenomena expected in ITER's primary operating scenarios, including sawteeth, neoclassical tearing modes and Resistive Wall Modes, is also reviewed. Fast ion losses due to the non-axisymmetric fields arising from the finite number of toroidal field coils, the inclusion of ferromagnetic inserts, the presence of test blanket modules containing ferromagnetic material, and the fields created by the Edge Localised Mode (ELM) control coils in ITER are discussed. The greatest losses and associated heat loads onto the plasma facing components arise due to the use of the ELM control coils and come from neutral beam ions that are ionised in the plasma edge.

Comprehensive evaluation of the linear stability of Alfvén eigenmodes driven by alpha particles in an ITER baseline scenario

“…Two main branches are evident inside the TAE gap: an upper branch made of modes that rise in frequency as n increases, and a lower branch with decreasing frequency modes. The lower-frequency branch is made of even, symmetric modes, while the modes in the upper branch are odd, anti-symmetric modes [6,7]. Furthermore, it has been verified that for a given n the frequency of anti-symmetric modes rises with the number of peaks in their poloidal harmonics, while symmetric modes have progressively lower frequencies as their number of peaks increases.…”

“…Nevertheless, in both scenario variants the n α gradient remains close to its maximum value in the mid-radius region 0.25 s 0.55 which encloses all unstable AEs. At this point it is interesting to verify a well-known estimate for the toroidal mode number of the most driven AE [6,8]. The estimate is based on matching the width of passing alpha-particle orbits and the TAE width, which leads to n ≈ s/q 2 × a/R m × Ω α /ω A , where the cyclotron frequency of the alpha particles is Ω α ≈ 2.5 × 10 8 rad/s and q(s) is evaluated at the location s of the AE with the highest drive.…”

“…Red curves represent the MHD damping-rate calculated by CASTOR as a function of the wave dissipation-rate δ(ν e ) [26,28], whereas blue curves represent the frequency of the non-ideal eigenmode. For the analysis of the TAE on the left, which is the most unstable in HiPed, the values T e = 14.7 keV and T i = 13.0 keV are used in equation 6. The constant slope of the red curve indicates a negligible value of γ rad , as expected since this is an anti-symmetric mode [18].…”

“…With the introduction of finite gyroradius effects TAEs become coupled with Kinetic Alfvén Waves (KAWs), which carry energy in the radial direction away from the eigenmode in an intrinsic TAE damping process known as radiative damping [24,25,26,18,6]. In the case of LSTAEs only the symmetric modes suffer significant radiative damping, as the KAWs excited by the antisymmetric LSTAEs interfere within the mode localization region resulting in a negligible net energy flux [25,18].…”

“…In this article the stability of AEs is systematically addressed for the 15 MA ELMy H-mode ITER baseline scenario [5,6,7,8] making use of a recently introduced framework [8] that is based on the hybrid MHD drift-kinetic code CASTOR-K [9,10], which is the key element in our suite of numerical codes. Growth rates are computed systematically for all possible eigenmodes taking into account the alpha-particle drive, the Landau damping due to the DT …”