H-mode achievement and edge features in RFX-mod tokamak operation

Abstract

The RFX-Mod experiment is a fusion device designed to operate as a Reversed Field Pinch (RFP), with a major radius R " 2 m and a minor radius a " 0.459 m. Its high versatility recently allowed operating also as an ohmic tokamak allowing comparative studies between the two configurations in the same device. The device is equipped with a state of the art MHD mode feedback control system, providing a magnetic boundary effective control and modulation, by applying resonant or nonresonant magnetic perturbations (MP) both in RFP and in tokamak configurations. In the fusion community the application of MPs is widely studied as a promising tool in particular to limit the impact of plasma filaments and ELMs, in particular on plasma facing components. An important issue is envisaged in the exploitation of the RFX-Mod active control system for ELM mitigation studies.As a first step in this direction, this paper will focus on the most recent achievements in term of RFX-Mod tokamak explored scenarios, which allowed the first investigation of the ohmic and edge biasing induced H-mode. In particular the realization of D-shaped tokamak discharges and the design and deployment of an insertable polarized electrode were accomplished. Both operations took advantage of the implementation of a real-time feedback control system of the electron density and of a multivariable shape feedback control system designed on a full modelbased approach. Indications of Ohmic H-mode were transiently observed during Single Null shaped plasmas. Stationary H-mode phases were instead obtained with insertable electrode biasing stimulation. The edge biasing is applied in new operation scenarios relatively unexplored with this technique in particular in plasma shaped Single Null discharges and in the very low-q circular plasma discharges, qpaq ă 2, achievable in RFX-Mod. Important modification of the edge plasma density and flow properties are observed as well as, in some cases, a complex "dithering" phenomenology, characterized by very fast L-H and H-L transitions. These strong fluctuations appear associated to the onset of MHD activity, whose behaviour will be analyzed and discussed in the paper along with the filamentary structures observed in the fast turbulence electromagnetic fluctuations.Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/Abstract The RFX-mod experiment is a fusion device designed to operate as a Reversed Field Pinch (RFP), with a major radius R= 2 m and a minor radius a = 0.459 m. Its high versatility recently allowed operating also as an ohmic tokamak allowing comparative studies between the two configurations in the same device. The device is equipped with a state of the art MHD mode feedback control system, providing a magnetic boundary effective control, by applying resonant or non-resonant magnetic perturbations (MP) both in RFP and in tokamak configurations. In the fusion community the application of MPs is widely studied as a promising tool to limit the impact of plasma filaments and ELMs on p...

Overview of the RFX-mod fusion science activity

“…On the other hand, no low-frequency coherent magnetic fluctuations are measured, which are characteristic features of the so-called M-mode, as recently described [68]. It is interesting to note that, instead, various higher frequency (above 100kHz) quasi-coherent magnetic fluctuations have been identified, with balloning and antiballoning mode structures (for more details see [69]), which closely resemble those recently described in the COMPASS tokamak in both neutral beam injectorassisted and Ohmic H-mode phases [70]. It is worth to add that, at the moment, the limitation of the experimental capabilities during tokamak operations does not allow to make closer comparison between the long lasting improved confinement regime with no bursty D  behaviour of RFX-mod and the quiescent H-mode (QH mode) obtained in large tokamaks as a result of increased edge particle transport due to an edge harmonic oscillation [71].…”

“…14, the floating potential as a function of the distance from LCFS (indication of the radial electric field behavior) varies according to the polarization of the electrode and a strong electric field builds up in the few cm region within the LCFS [63,64]. The radial profile of ion saturation current, linearly depending on the electron density, steepens during transitions indicating that a local transport barrier is established, sustained by the polarized electrode [64]. Preliminary analysis of experimental data does not show clear evidence of the formation of electron temperature pedestal, so that, the steepened ion saturation profile seems to be interpreted as an indication of the formation of an electron density pedestal.…”

Three-dimensional simulations of plasma turbulence in the RFX-mod scrape-off layer and comparison with experimental measurements

“…25 While RFX-mod plasma discharges have been performed mainly in the reversed field pinch (RFP) configuration, recent developments allow now operating the device also with magnetic geometries that feature inner-wall limited and diverted ohmic tokamaks. 26,28,29 Using a toroidal magnetic field on axis B u ' 0:6 T and a plasma current up to I p ' 150 kA, it is possible to perform plasma pulses longer than 1 s with integrated plasma densities n e ! 10 19 m À3 and core electron temperatures T e !…”

H-mode access and the role of spectral shift with electrode biasing in the TCABR tokamak

“…In fact, the L-H transition and the role of the shear in biasing experiments have already been confirmed. [17][18][19][20][21][22][23][24][25] Furthermore, since electrode biasing experiments are difficult to be performed in larger machines due to obvious limitations related to the insertion of probes and electrodes inside the plasma, such experiments are carried out on small machines with relative simplicity, which stage them in the forefront of current investigation of turbulence suppression mechanisms.…”