Model for humpback relaxation oscillations

Abstract

Abstract. An interpretation of non-standard, central MHD events in the TCV tokamak during localized ECRH is presented. It is shown that the non-standard behaviour is a consequence of specific features in the electron temperature profile produced when ECRH power is deposited close to the q = 1 surface and by the advection and mixing of electron thermal energy resulting from a resistive MHD instability.Experimental research on controlled thermonuclear fusion requires the production of hot plasmas with temperatures in the multi-keV range. One effective scheme providing intense localized heating is ECRH [1]. Resonant electromagnetic waves are launched into a magnetically confined plasma at a frequency matching a low integer multiple of the local EC frequency, ω ce = eB/mc. In recent years, record values of ECRH power density have been attained. This has revealed new and peculiar plasma behaviour. One example is the observation of multipeaked electron temperature profiles and sharp temperature gradients in the RTP [2] and TEXT Upgrade [3] experiments, explained in Ref.[4] as being the consequence of interaction between ECRH and resistive MHD modes with toroidal n = 1 and dominant poloidal m = 1 mode numbers. These modes are known to be responsible for the well known internal plasma sawtooth relaxation oscillations [5]. In TCV experiments with intense ECRH, sawtooth oscillations acquire a non-standard character, as reported in Refs [6,7]. In particular, when the ECRH power is deposited close to the q = 1 surface, the line integrated soft X ray temporal traces exhibit peculiar shapes, which suggested the nickname 'humpback' when they were first observed in T-10 experiments [8]. The purpose of this Letter is to propose a theoretical interpretation of the humpback phenomenon. We argue that specific features in the electron temperature profile are produced by localized ECRH. Then, the humpback behaviour is consistent with the advection and mixing of electron thermal energy in the plasma core associated with the excitation of resistive m/n = 1 internal modes.TCV is a tokamak with major radius R = 0.88 m, minor radius a = 0.25 m, vacuum vessel elongation κ = 3 and central magnetic field B = 1.43 T in vacuum. Up to three 82.7 GHz, 500 kW gyrotrons, each with a pulse length of 2 s, for heating at the cyclotron second harmonic resonance via the extraordinary mode, were used in this study. The vertical microwave beam diameter near the plasma centre is of the order of 5 cm, while the horizontal width of the deposition region is O (1 cm). ECRH power densities in excess of 1 × 10 2 MW/m 3 can be obtained. New insight into the temporal evolution of the different types of MHD modes observed in TCV has become possible because of the recently upgraded, 200 channel soft X ray tomographic system, allowing for a sampling time of 13 µs. The spatial reconstruction of the central emissivity profile is based on a pixel method with characteristic spatial resolution of about 3 cm, to be compared with a typical sawtooth inversion radius of 5-10...

Dynamics of multiple flux tubes in sawtoothing KSTAR plasmas heated by electron cyclotron waves: I. Experimental analysis of the tube structure

“…However, in those experiments, even more complex patterns of sawtooth oscillations (e.g. 'saturated' and 'humpback') were observed in the time traces of soft x-ray or electron temperature (T e ), which suggests that the localized ECH may substantially alter the profiles of T e and current [9] and can induce multiple tearing modes near the q = 1 surface [10].…”

Effects of ECRH power and safety factor on laser blow-off injected impurity confinement in TCV

“…It is important however to note that sawteeth persist throughout the accumulative phase as seen in Fig.5, which shows a raw SXR signal with no filtering applied. Sawtooth crashes have the effect of expelling energy and particles from the core, beyond the inversion radius, up to the mixing radius [24]. It is well known that heat deposited beyond the inversion radius is rapidly transported to the edge, giving rise to the characteristic sawtooth heat pulse (see e.g.…”

Understanding sawtooth activity during intense electron cyclotron heating experiments on TCV

“…Recent attempts to simulate the humpback oscillations [33], using the same model, resulted in a sawtooth shape in which the drop and rise of the central electron temperature occur on the same timescale. This was mainly due to the choice of the displacement function ξ(t) and to the assumption of full magnetic reconnection after the sawtooth crash.…”