First Intrashot Observation of Runaway-Electron-Driven Instabilities at the Lower-Hybrid Frequency Range under ITER-Relevant Plasma-Wave Dispersion Conditions

Kinetic instabilities driven by runaway electrons (REs) have recently received attention in the fusion community as a means to control and diagnose REs in a tokamak. Experiments aimed at studying such kinetic instabilities have been performed at the Frascati Tokamak Upgrade (FTU), where different families of waves have been identified, from wide-band bursting emissions to quasi-monochromatic waves and sharp lines, in the presence of REs with energies from a few to tens of MeV. A specific family of waves with intense kinetic drive was directly observed for the first time, during both the early Ohmic plasma start-up and the current ramp-up. A clear wave frequency scaling with respect to the electron density was demonstrated. This scaling, with the complementary analysis of signals observed at different magnetic fields, allowed the identification of these instabilities as lower-hybrid waves. The relevant analysis shown in this Letter is based on a continuous intrashot detection of the RE-driven wave, which is reported for the first time for this kind of instability. We demonstrated that unstable waves are excited already at the very beginning of a tokamak discharge, opening the way to new possible research on the exploitation of this kind of measurement for monitoring seed REs formation at the early plasma stage, while most diagnostics still have limited capabilities. The conditions for plasma wave dispersion at the early phase of the FTU discharge are very similar to the ones expected during the ITER start-up, when analogous instabilities might, hence, come to light, in case of formation of suprathermal populations.

Figure 1

Instability waves during ramp-up phases of shots No. 43452 (4.0 T), No. 43438 (5.3 T), No. 43520 (5.5 T), and No. 43550 (6.0 T). Assuming the definitions of ${\zeta }_{n_0}\simeq 0.0073$ and ${\zeta }_{\mathrm{LD}}\simeq 0.0153$ given in the text, yellow dash-dotted lines show the lower-hybrid dispersion branches calculated taking the core density $n_0$ and ${\zeta }_{n_0}$, while red dashed lines are for central line-averaged density $n_{\mathrm{LD}}$ and ${\zeta }_{\mathrm{LD}}$. The color scale of spectrograms is in dB.

Figure 2

Main plasma signals of shot No. 43438. From top: spectrogram; plasma current (red); loop voltage (blue); central line-averaged density (orange); electron temperature by Thomson scattering, with 30 Hz resolution (green); HXR counts, in the range 20–200 keV, on a line-of-sight across the magnetic field, with 200 Hz resolution. The light yellow regions highlight the times of kinetic instabilities, which also correspond to an increase of HXRs.

Figure 3

Top: wave frequencies detected at maximum power during different phases of a set of shots versus the corresponding central line-averaged density $n_{\mathrm{LD}}$. The solid line shows the LH dispersion relation evaluated for ${\zeta }_{\mathrm{LD}}\approx 0.0153$, which is the optimal value allowing the best fit of data. Bottom: multishot analysis of the frequency dependence with the magnetic field, performed as described in the text. Purple square dots are the variations, with respect to the 4.0 T case, expected for whistler waves; blue triangle dots the variations for LHWs. Error bars are calculated by propagation of the uncertainties on the measurements of density and magnetic field.

Figure 4

Spectral patterns emitted during three pairs of shots, compared as described in the text. Yellow arrows indicate the emission frequencies at the selected times. The line-averaged density profiles measured at those times are shown in the bottom.

Figure 5

(a) Cherenkov resonance curves for electrons with different energies and negligible pitch angle. Solid and dashed lines refer, respectively, to slow and fast branches. The dash-dotted line shows the Allis resonance condition. (b) Drive per unit slope along the parallel momentum, considering $n_b/n_e={10}^{-3}$, $n_e$ being a uniform plasma density. Vertical dashed lines correspond to the frequencies obtained from Eq. (1) for ${\zeta }_{n_0}$ and ${\zeta }_{\mathrm{LD}}$.