Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

Abstract

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2 MW, 2 s, 140 GHz system to an 8 MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10 MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q 95 > 5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported.

Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

“…Specifically, the observations were made using the CTS system [46], which is a heterodyne radiometer utilizing two receivers (referred to as Receiver A and Receiver B [47]), each consisting of a filter bank system sampling at 100 kSamples/s [48] and a fast-acquisition system sampling a single channel at 6.25 GSamples/s [47], connected to the ECRH waveguide and steerable mirror systems [7]. Rather than the CTS system operating near 105 GHz described in [47,48], the measurements are made by an upgraded version of the system operating near 140 GHz [20,49], which is the ECRH frequency used in standard secondharmonic X-mode ECRH at ASDEX Upgrade [7,8]. In addition to the channels sampling the signal near 140 GHz, the upgraded system has channels operating near 70 GHz to detect signatures of the waves at around half the ECRH frequency expected in connection with PDIs at the second-harmonic UHR [13].…”

“…Due to energy and momentum conservation, the frequencies and wave vectors of the daughter waves should sum to those of the pump wave. The wave vector selection rule particularly means that PDIs will only occur in a narrow region in inhomogeneous plasmas [1][2][3][4][5][6], ordinarily increasing the PDI power thresholds of electron cyclotron resonance heating (ECRH) beams in fusion-relevant plasmas to ≳10 MW [4,5], which is an order of magnitude above the power available from the gyrotron sources (~1 MW) typically used for ECRH in such plasmas [7,8]. However, the PDI power thresholds may be reduced to a level accessible with ECRH when the daughter waves excited by the PDIs are trapped, since the amplification region is traversed multiple times in such cases [9].…”

Validation of quasi-linear turbulent transport models against plasmas with dominant electron heating for the prediction of ITER PFPO-1 plasmas

“…Integrated modelling results presented in [25] confirm this low density at the minimum P L-H . Nevertheless, the plasmas in the database cover a wide range of different plasma conditions with the temperature ratio in the core (around ρ tor = 0.3) in the range of T e /T i = 1.3-5.5 and T e /T i = 1.1-3.5 around mid-radius, high T e /T i enabled by the comparably large available ECRH power of ASDEX Upgrade, which can exceed 5 MW [26]. As it is described in section 5, variation in ECRH power and density allowed us to explore conditions with locally dominant ion temperature gradient (ITG), electron temperature gradient (ETG) and trapped electron modes (TEM).…”

H-mode power threshold studies in mixed ion species plasmas at ASDEX Upgrade

“…Two different heating schemes were employed to influence the ratio of direct electron to ion heating: electron cyclotron resonance wave heating (ECRH), which exclusively heats the electrons, and neutral beam injection (NBI), which, in these discharges, preferentially heated the ions. The ECRH power ramp was made of 200 kW steps central X2 heating (see figure 1 (a)) [6]. NBI blips of 2 MW and a duration of 12 ms were programmed at the very end of each heating step for charge exchange recombination spectroscopy (CXRS) measurements [7].…”