High resolution density pedestal measurements during edge localized modes by short-pulse reflectometry in the TCV tokamak

Abstract

This publication presents high spatio-temporal resolution (mm µs −1 ) density profile measurements of the pedestal top during type I, III, and small edge localized mode (ELM) H-mode plasmas in the tokamak à configuration variable (TCV). These measurements were performed using a novel short-pulse reflectometer. Average inter-ELM density profiles are obtained via conditional averaging using the D α trace as ELM indicator. Changes to the pedestal density profile gradients prior to type-III ELMs reveal unique pedestal dynamics leading to the ELM crash which can provide important experimental data for validation of non-linear MHD ELM simulations. The small-ELM scenario is found to feature a ∼25-35 kHz quasi-coherent density fluctuation near the separatrix ρ ψ ∼ 0.993-1.05 not observed during a similar type-I ELM discharge. This oscillation is also found in low-field-side magnetic pick-up probes displaying a ballooning character and n = +1 toroidal mode number. This oscillation could help explain the markedly different pedestal dynamics observed in the small-ELM regime.

Linear scattering theory of short-pulse reflectometry

“…One advantage of this method is that if the optimal conditions are not fulfilled, it would be immediately obvious from the fact that the shape of the measured scattered pulse would coincide with that of the probing pulse. We can use the TCV SPR system [12,13] to give a quantitative example. The possible probing beam parameters for the TCV SPR are f = 50 GHz, ρ ≈ 4 cm and t p =0.7 ns.…”

“…This means that the broadening condition in this case requires us to have L 0 > 39 cm. The L-mode profile given in [13] has values of L 0 varying between 10 and 20 cm, making it not suitable for such measurements. The H-mode profile on the other hand can reach L 0 of 35 cm, which is close to fulfilling the broadening condition.…”

“…Considerable experimental [7,8,9] and theoretical [10,11] work was carried out to develop the diagnostic, most of it focused on density profile measurements rather than turbulence studies. Recently, a short-pulse reflectometry (SPR) system has been developed and employed on the TCV tokamak [12,13]. It is capable of using very short (< ns) microwave probing pulses and digitally recording the envelope of the reflected pulse.…”

Overview of the TCV tokamak experimental programme

“…A comparison of the pedestal dynamics of the small ELM regime with type-I ELMs was performed using a novel shortpulse reflectometer [59]. The reflectometer takes advantage of new advances in hardware, particularly GHz-range arbitrary waveform generators and vector-network-analyser extension modules.…”

Excitation of toroidal Alfvén eigenmodes with counter-current NBI in the TCV tokamak

“…The study on the fast ion population injected by such beams and on the interaction with MHD modes has been explored in [5][6][7]. TCV is also equipped with electron cyclotron emission (ECE) diagnostic and short pulse reflectometry [8], which allow to measure the fine fluctuations of electron temperature and density and thus reconstruct the MHD mode spatial structure. TCV's capability to explore non-conventional plasma shapes (such as negative triangularity [9]) and its high electron cyclotron resonance heating (ECH) power available (⩽3.5 MW) makes it an attractive device where energetic particle modes can be studied and their suppression achieved.…”