Observations of core ion cyclotron emission on ASDEX Upgrade tokamak

Abstract

(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX) (Dates appearing here are provided by the Editorial Office) The B-dot probe diagnostic suite on the ASDEX Upgrade tokamak has recently been upgraded with a new 125 MHz, 14 Bit resolution digitizer to study ion cyclotron emission (ICE). While classic edge emission from the low field side plasma is often observed, we also measure waves originating from the core with fast fusion protons or beam injected deuterons being possible emission driver. Comparing the measured frequency values with ion cyclotron harmonics present in the plasma places the origin of this emission on the magnetic axis, with the fundamental hydrogen/second deuterium cyclotron harmonic matching the observed values. The actual values range from ~27 MHz at on-axis toroidal field B T = -1.79 T to ~40 MHz at B T = -2.62 T. When the magnetic axis position evolves during this emission, the measured frequency values track the changes in the estimated on-axis cyclotron frequency values. Core ICE is usually a transient event lasting ~100 ms during the neutral beam startup phase. However, in some cases core emission occurs in steady-state plasmas and lasts for longer than 1 s. These observations suggest an attractive possibility of using a non-perturbing ICE-based diagnostic to passively monitor fusion alpha particles at the location of their birth in the plasma core, in deuterium-tritium burning devices such as ITER and DEMO.

Core plasma ion cyclotron emission driven by fusion-born ions

“…A core ICE signal with frequency matching CH (and degenerate with 2×CD) is often observed in ASDEX Upgrade deuterium plasmas heated with deuterium NBI [1]. An example is provided in figure 1, where a train of NBI pulses (figure 1 (c)) triggers an instability pulse train at 39-40 MHz (figure 1 (a)).…”

“…For the case of ASDEX Upgrade, ICE is detected directly using magnetic induction coils (or B-dot probes) placed in the far periphery of the machine, essentially in the vacuum region of the tokamak [9]. The radio frequency (RF) magnetic field component sampled by the B-dot probes is then processed via so-called fast and slow data acquisition systems [1]. The fast system digitizes the signal directly at 125 MHz over a 1 ms (rectangular) time window, every 10 ms, for a duration of 8 seconds, which is long enough for most ASDEX Upgrade plasmas [1].…”

“…The radio frequency (RF) magnetic field component sampled by the B-dot probes is then processed via so-called fast and slow data acquisition systems [1]. The fast system digitizes the signal directly at 125 MHz over a 1 ms (rectangular) time window, every 10 ms, for a duration of 8 seconds, which is long enough for most ASDEX Upgrade plasmas [1]. The slow system first rectifies the RF signal from the B-dot probes and then digitizes the resulting DC output continuously at 200 kHz [1].…”

“…The fast system digitizes the signal directly at 125 MHz over a 1 ms (rectangular) time window, every 10 ms, for a duration of 8 seconds, which is long enough for most ASDEX Upgrade plasmas [1]. The slow system first rectifies the RF signal from the B-dot probes and then digitizes the resulting DC output continuously at 200 kHz [1]. The slow system can accept two RF signals simultaneously to measure the phase difference  between them [1].…”

“…The slow system first rectifies the RF signal from the B-dot probes and then digitizes the resulting DC output continuously at 200 kHz [1]. The slow system can accept two RF signals simultaneously to measure the phase difference  between them [1]. To measure the phase (and, hence, the parallel wavenumber k//), a pair of B-dot probes is positioned in the periphery of the ASDEX Upgrade torus radially 10 cm behind the limiter surface, on the low field side [1].…”