Peaked density profiles and MHD activity on FTU in lithium dominated discharges

Tudisco, O.; Mazzotta, C.; Botrugno, A.; Mazzitelli, G.; Apicella, M. L.; Apruzzese, G.; Frigione, D.; Gabellieri, L.; Романо, А.

doi:10.1016/j.fusengdes.2010.08.037

Cited by 20 publications

(30 citation statements)

References 19 publications

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“…The threshold 0.4nG is consistent with the Lispschultz scaling of the MARFE critical density [39]. It was also shown that such toroidal MARFE is caused by a reduction of parallel electron conductivity, preventing energy redistribution along the flux surface, which induces the formation of local cold regions with strong line emission, on the high field side, where the core-originated heat flux is lower [40]. The scaling found in FTU is consistent with the nice, simple model proposed by Tokar [41] for the TEXTOR tokamak of this phenomenon, interpreted as a local imbalance between parallel heat diffusion and ionization/radial particle transport.…”

Section: Role Of the Magnetic Topology On The Density Limitsupporting

confidence: 76%

Overview of the RFX-mod fusion science activity

Zuin¹,

Bello²,

Marrelli³

et al. 2017

Nucl. Fusion

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This paper reports the main recent results of the RFX-mod fusion science activity. The RFX-mod device is characterized by a unique flexibility in terms of accessible magnetic configurations. Axisymmetric and helically shaped Reversed-field pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic configurations in between the two, the so-called ultra low-q ones, has been explored, with the aim of studying specific physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed to extend the range of experimental parameters, as well as to induce specific magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effect in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The first transitions to an improved confinement scenario in circular and D-shaped tokamak plasmas have been obtained thanks to an active modification of the edge electric field through a polarized electrode. The experiments are supported by intense modelling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modifications to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also briefly presented.

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Section: Role Of the Magnetic Topology On The Density Limitsupporting

confidence: 76%

Overview of the RFX-mod fusion science activity

Zuin¹,

Bello²,

Marrelli³

et al. 2017

Nucl. Fusion

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“…Previously, such structures in the FTU tokamak were observed only at high plasma den sities (~10 14 cm -3 and higher). In this case, the fre quencies of such pulsations in the interferometer sig nal also were about 200 Hz [11], which convincingly demonstrates that the MARFE layer is the source of these oscillations in the refractometer signals.…”

Section: Experiments Carried Out On the Ftu Tokamaksupporting

confidence: 64%

First results from plasma density measurements in the FTU tokamak by means of a two-frequency pulsed time-of-flight refractometer

et al. 2012

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A pulsed time of flight refractometer was developed and tested to determine the mean plasma density in the T 11M tokamak by measuring the propagation time of nanosecond microwave pulses in plasma. Later, it was also proposed to use such an instrument to measure and control the mean plasma density in the ITER tokamak by probing the plasma with an extraordinary wave, the electric field of which is perpen dicular to the magnetic field in plasma, in the transparency window at frequencies of 50-100 GHz. To avoid the effect of the density profile shape on the measurement results in the nonlinear mode of refractometer operation (near the cutoff), a system operating at two different probing frequencies was developed and tested. Such a system provides two values of the time delay, which can be used to estimate the peaking factor of the density distribution α and correctly determine the linear density 〈Nl〉, regardless of the density profile (assum ing a smooth density profile of the form of N(ρ) = N(0)(1 -ρ 2 ) α , where N(0) is the central plasma density and ρ = r/a is the normalized plasma radius). The first experiments on density measurements in the FTU tokamak performed with this refractometer are described, and results from these experiments are presented. The formation of a thin dense plasma layer in the zone of a strong magnetic field (the so called MARFE layer) at a relatively low (for FTU) plasma density of ~6 × 10 19 m -3 was detected. The thickness of this layer, deter mined from the refractometry data, agrees well with the data obtained using a digital camera.

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“…This latter observation is also confirmed by the Langmuir probe measurements that display very similar scrape-off layer (SOL) electron density radial profiles for both shots, as shown in figure 5. To better assess the lithium effects, a comparison between boronized and lithizated discharges with I p = 0.5-0.8 MA, B T = 6.0-7.1 T and increasing electron density is performed with the high-resolution FTU scanning interferometer [8]. Particular care is taken in the reconstruction of the density profile in the presence of a MARFE.…”

Section: Peaked Density Profilesmentioning

confidence: 99%

“…Only Li emission lines are visible on UV spectra, while those of Mo, O and other impurities are below the instrument's sensitivity. Another effect of lithization is to reduce the plasma instabilities at the plasma edge as evidenced by a strong decrease in the m = 2, n = 1 tearing mode on FTU [8] and by the quasi-suppression of ELM's activity on NSTX [6]. In all cases, the plasma performance is enhanced in terms of energy confinement time and stored energy.…”

Section: Introductionmentioning

confidence: 99%

FTU results with a liquid lithium limiter

et al. 2011

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Since the end of 2005 most of the plasma-wall interaction experiments on FTU have been focused on the possible use of liquid lithium as the plasma facing material. Liquid lithium limiter is an active method to deposit, during the plasma discharge, a lithium film on the walls with prolonged beneficial effects. Reliable operation with very clean plasmas, very low wall particle recycling, spontaneous peaking of the density profile for line-averaged density values n e > 1.0 × 10 20 m −3 have been obtained. These results have allowed us to extend the density limit to the highest value so far obtained (n e = 4.0 × 10 20 m −3 at I p = 0.7 MA and B T = 7.1 T, q a = 5.0, by gas puffing only) and to increase the energy confinement time by almost 50% with respect to the average value of 50 ms of the old ohmic FTU database. An accurate analysis of these plasmas has been carried out by means of a gyrokinetic code to establish the role of collisionality and density gradients on the observed phenomenology.

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Peaked density profiles and MHD activity on FTU in lithium dominated discharges

Cited by 20 publications

References 19 publications

Overview of the RFX-mod fusion science activity

Overview of the RFX-mod fusion science activity

First results from plasma density measurements in the FTU tokamak by means of a two-frequency pulsed time-of-flight refractometer

FTU results with a liquid lithium limiter

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