Energetic ions play a crucial role in fusion plasmas14. Indeed, the success of magnetic fusion relies upon good confinement of fast alpha particles (4He ions with birth energies 3.5 MeV). This is required to sustain high plasma temperatures and for economical operation of a fusion reactor1. However, these energetic 4He ions can also trigger instabilities that degrade the plasma performance. To mimic the behaviour of fusion-born alphas, but without actually using D–T plasmas, ICRH has been extensively used in the past.

For fundamental ion cyclotron absorption the acquired ion energies scale with the absorbed RF power per particle15. Since three-ion scenarios allow minimizing the number of resonant particles down to ‰ levels, ions with rather high energies can be generated. For plasma densities and ICRH power levels available in the JET and C-Mod experiments, self-consistent power deposition computations with the codes AORSA16, PION17 and SCENIC18 predicted acceleration of 3He ions to energies of a few MeV.

Figure 2b shows fast repetitive drops in T e0 (so-called ‘sawtooth’ oscillations) with a period of ∼0.2 s during the NBI-only phase of JET pulses #90753 and #90758 (t = 7–8 s). Extended sawtooth periods up to ∼1.0 s are seen when ICRH is applied on top of NBI. Similarly, in the three-ion Alcator C-Mod discharge in Fig. 2a, the sawtooth period increases from ∼0.13 s during the 2 MW ICRH phase to ∼0.23 s during the 4 MW phase. The observation of long-period sawteeth is a first indication of the creation of energetic ions by ICRH, as the presence of fast ions in a plasma is well known to have a stabilizing effect on sawteeth19,20.

An independent confirmation of accelerating 3He ions to high energies is provided by gamma-ray emission spectroscopy on JET21,22. Figure 3a shows the gamma-ray spectrum for pulse #90753 during t = 8–14 s (P ICRH = 4.4 MW), recorded with the LaBr 3 spectrometer23. The observed lines originate from 9Be(3He, pγ)11B and 9Be(3He, nγ)11C nuclear reactions between fast 3He ions and beryllium (9Be) impurities. These impurities are intrinsically present in JET plasmas with the ITER-like wall. The reported plasmas were contaminated with ∼0.5% 9Be, as estimated by charge exchange measurements.

Figure 3: Gamma-ray emission from 3He + 9Be nuclear reactions, proving the presence of energetic ICRH-accelerated 3He ions. a, Gamma-ray spectra measured in JET pulse #90753 (three-ion scenario, X[3He] ≈ 0.2 –0.4%, red) and in pulse #91323 ((3He)–H scenario, X[3He] ≈ 1 –2%, blue). The error bars represent the square root of the number of counts in each channel of the spectrum and arise from the underlying Poisson statistics of the gamma-ray detection process. b,c, The JET plasma cross-section and 19 lines-of-sight of the neutron/gamma camera. The reconstructed high-energy gamma-ray emission (E γ = 4.5–9.0 MeV) visualizes the population of the confined energetic 3He ions (E[3He] > 1–2 MeV). Pulses #90752 (b) and #90753 (c) had a nearly identical plasma composition (X[H] ≈ 70–75%, X[3He] ≈ 0.2–0.4%) and RF heating power (P ICRH = 4.3–4.4 MW), except for the ICRH antenna phasing. A factor-of-two increase in the γ-ray emissivity was observed in pulse #90753, in which 2 MW of RF power was coupled to the plasma with +π/2 phasing (see text for more details). Full size image

The observation of the E γ ≈ 4.44 MeV line implies immediately the presence of confined fast 3He ions with energies >0.9 MeV (ref. 21). Alpha particles, born in concurrent 3He–D fusion reactions, also contribute to the gamma-emission at this energy through 4He + 9Be reactions. Figure 3a also shows a number of characteristic gamma lines at E γ > 4.44 MeV, originating from transitions between higher excited states of 11B and 11C nuclei (products of 3He + 9Be reactions). The excitation efficiency for such high-energy levels increases by a factor of ten when the energy of the projectile 3He ions increases from 1 MeV to 2 MeV (ref. 24). For comparison, we also display the γ-spectrum recorded in JET pulse #91323, in which 3He ions (≈1–2%) were heated as a minority with up to 7.6 MW of ICRH in an almost pure H plasma (see Supplementary Fig. 3). Figure 3a clearly shows higher gamma-count rates for the three-ion pulse #90753 (X[3He] ≈ 0.2–0.4%), although a factor of two less ICRH power was injected into the plasma.

In JET, we further enhanced the efficiency for fast-ion generation by changing the configuration of ICRH antennas from dipole to +π/2 phasing. The phasing defines the dominant k ∥ and the spectrum of emitted waves, where k ∥ is the wavenumber parallel to B. The +π/2 phasing launches waves predominantly in the direction of the plasma current with typical values |k ∥ (ant)| ≈ 3.4 m−1, which is two times smaller than for dipole phasing (|k ∥ (ant)| ≈ 6.7 m−1). Since the width of the absorption zone scales with |k ∥ |, reducing it has the advantage of increasing the absorbed RF power per ion. Furthermore, the +π/2 phasing allows one to exploit the RF-induced pinch effect, beneficial to localize the energetic ions towards the plasma core25.

The result is clearly visible in Fig. 3b, c, showing the two-dimensional tomographic reconstruction of the E γ = 4.5–9.0 MeV gamma-ray emission21 for two comparable three-ion heating pulses #90752 and #90753. Both had a similar edge H/(H + D) ratio, varying from ∼0.84 at the beginning of the pulse to ∼0.75 at the end (X[H] ≈ 68–76%), and X[3He] ≈ 0.2–0.4%. In pulse #90752 (Fig. 3b), all ICRH power was applied using dipole phasing, while in pulse #90753 (Fig. 3c) about half of the ICRH power (2.1 MW) was launched with +π/2 phasing. Energetic 3He ions are more centrally localized and the number of gamma-ray counts increases by a factor of two in pulse #90753. The period of the sawtooth oscillations also increases from ∼0.54 s to ∼0.78 s.

We also observed excitation of Alfvén eigenmodes (AE) in JET plasmas with frequencies ≈320–340 kHz in pulses, where P ICRH ≥ 2 MW was delivered with +π/2 phasing. These instabilities are excited if a sufficiently large number of energetic ions with velocities comparable to the Alfvén velocity is present in the plasma. Figure 4a shows the AE dynamics for JET pulse #90758 (previously shown in Fig. 2b), with a sequential excitation of modes with mode numbers from n = 8 to n = 5 during a long-period sawtooth. The MHD code MISHKA26 yields eigenfrequencies f AE (0) ≈ 285–295 kHz for n = 5–7 modes in the plasma frame. Even closer correspondence to the observations is obtained when plasma rotation due to NBI (f rot ≈ 5 kHz measured at R ≈ 3.25 m) is taken into account (f AE (lab) = f AE (0) + n f rot ≈ 320 kHz). Further analysis of the conditions for energetic ions to interact with the n = 5 AE mode yields 3He ions with energies ≈1.5–2.5 MeV.

Figure 4: Excitation of Alfvénic eigenmodes in magnetic fluctuation spectrograms, another proof of the presence of ICRH-accelerated fast ions. a, JET pulse #90758. b, Alcator C-Mod pulse #1160901023. The evolution of the central electron temperature T e0 is also plotted in the bottom part of the figures. Full size image