Quasi-Single-Helicity Reversed-Field-Pinch Plasmas

Escande, Dominique; Martin, P.; Ortolani, S.; Buffa, A.; Franz, P.; Marrelli, L.; Martines, E.; Spizzo, G.; Cappello, S.; Murari, A.; Pasqualotto, R.; Zanca, P.

doi:10.1103/physrevlett.85.1662

Cited by 156 publications

(193 citation statements)

References 23 publications

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“…To now, the best approximation of single helicity were quasi-singlehelicity (QSH) states: here, the innermost resonant m = 1 mode dominates over the others, the so-called secondary modes, which maintain a finite amplitude. QSH states have been observed in several RFP devices 5,16,17 and predicted in numerical simulations 18 . They are an intermediate state between the multiple-helicity and the theoretical single-helicity ones, because the secondary modes are still present.…”

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confidence: 84%

Self-organized helical equilibria as a new paradigm for ohmically heated fusion plasmas

Lorenzini¹,

Martines²,

Piovesan³

et al. 2009

Nature Phys

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In the quest for new energy sources, the research on controlled thermonuclear fusion 1 has been boosted by the start of the construction phase of the International Thermonuclear Experimental Reactor (ITER). ITER is based on the tokamak magnetic configuration 3, which is the best performing one in terms of energy confinement. Alternative concepts are however actively researched, which in the long term could be considered for a second generation of reactors. Here, we show results concerning one of these configurations, the reversed-field pinch 4,5 (RFP). By increasing the plasma current, a spontaneous transition to a helical equilibrium occurs, with a change of magnetic topology. Partially conserved magnetic flux surfaces emerge within residual magnetic chaos, resulting in the onset of a transport barrier. This is a structural change and sheds new light on the potential of the RFP as the basis for a low-magnetic-field ohmic fusion reactor.The main magnetic field configurations studied for the confinement of toroidal fusion-relevant plasmas are the tokamak 3 , the stellarator 6 and the reversed-field pinch 4,5 (RFP). In the tokamak, a strong magnetic field is produced in the toroidal direction by a set of coils approximating a toroidal solenoid, and the poloidal field generated by a toroidal current flowing into the plasma gives the field lines a weak helical twist. This is the configuration that has been most studied and has achieved the best levels of energy confinement time. Thus, it is the natural choice for the International Thermonuclear Experimental Reactor, which has the mission of demonstrating the scientific and technical feasibility of controlled fusion with magnetic confinement.The RFP, like the tokamak, is axisymmetric and exploits the pinch effect due to a current flowing in a plasma embedded in a toroidal magnetic field. The main difference is that, for a given plasma current, the toroidal magnetic field in a RFP is one order of magnitude smaller than in a tokamak, and is mainly generated by currents flowing in the plasma itself. This feature is underlying the main potential advantage of the RFP as a reactor concept, namely the capability of achieving fusion conditions with ohmic heating only in a much simpler and compact device. In the past, this positive feature was overcome by the poorer stability properties, which led to the growth and saturation of several magnetohydrodynamic (MHD) instabilities, eventually downgrading the confinement performance. These instabilities, represented by Fourier modes in the poloidal and toroidal angles θ and φ as exp [i(mθ − nφ) were considered as an unavoidable ingredient of the dynamo self-organization process 4,8,9 , necessary for the sustainment of the configuration in time. The occurrence of several MHD modes resonating on different plasma layers gives rise to overlapping magnetic islands, which result in a chaotic region, extending over most of the plasma volume 10 , where the magnetic surfaces are destroyed and the confinement level is modest. This conditi...

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confidence: 84%

Self-organized helical equilibria as a new paradigm for ohmically heated fusion plasmas

Lorenzini¹,

Martines²,

Piovesan³

et al. 2009

Nature Phys

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304

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“…Recent progress in confinement improvement in RFP results from realizing the tearing-mode-stable current profile with pulsed poloidal current drive (PPCD) [1,2]. The quasisingle helicity (QSH) RFP state, where the internally resonant single tearing mode grows significantly larger than the others, is another candidate for confinement improvement [3].The aspect ratio A is an important parameters for characterizing the RFP properties. Recent analysis has shown that when A is lowered to 2, a neoclassical equilibrium with a bootstrap current fraction higher than 90% exists with reactor regime plasma parameters [4].…”

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Quasi-Periodic Growth of a Single Helical Instability in a Low-Aspect Ratio RFP

Ikezoe

Onchi

Oki

et al. 2008

Plasma and Fusion Research

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Quasi-periodic growth of a single core-resonant instability has been observed in a low-aspect ratio (low-A) reversed field pinch experiment in RELAX. In round-topped current discharges, the innermost core resonant m = 1/n = 4 mode grows with suppression of the neighboring m = 1/n = 5, 6 modes. The resultant toroidal mode spectrum of the m = 1 modes is similar to that of the quasi-single helicity (QSH) state with higher amplitudes. These phenomena are discussed along with the low-A characteristic of RELAX, indicating simpler MHD mode dynamics and easier access to the QSH state. The reversed field pinch (RFP) is a magnetic confinement concept for compact high-beta plasmas. Recent progress in confinement improvement in RFP results from realizing the tearing-mode-stable current profile with pulsed poloidal current drive (PPCD) [1,2]. The quasisingle helicity (QSH) RFP state, where the internally resonant single tearing mode grows significantly larger than the others, is another candidate for confinement improvement [3].The aspect ratio A is an important parameters for characterizing the RFP properties. Recent analysis has shown that when A is lowered to 2, a neoclassical equilibrium with a bootstrap current fraction higher than 90% exists with reactor regime plasma parameters [4]. An additional advantage of a low-A RFP configuration is the possibility of simpler MHD mode dynamics because the radial separation of the mode rational surfaces increases in the core region.We have started experiments to study low-A RFP properties in Reversed field pinch of Low-Aspect ratio eXperiment (RELAX) machine, which has the world's lowest aspect ratio of 2 (R = 0.5 m/a = 0.25 m) [5]. RELAX uses a 4-mm thick vacuum vessel with a field penetration time of ∼1.5 ms. It is operated without any other shell structure. The edge magnetic fluctuations are measured with two toroidal arrays, each consisting of 14 pick-up coils set at the top and bottom ports, which are equally spaced in the toroidal direction except at the two poloidal gaps.Signals from these coils are sampled with a 2 MHz sampling frequency. After integrating numerically, we use a high-pass filter to obtain a fluctuating component with f > 0.5 kHz. The difference of the top and bottom signals author's e-mail: ikezoe1t@nuclear.dj.kit.ac.jp provides the odd (mostly m = 1) component, while the sum, the even (both m = 0 and m = 2) components. The signals are then Fourier transformed spatially to obtain a time evolution of the amplitude and phase. Figure 1 shows the time evolution of the plasma current I p , loop voltage V loop , edge toroidal field B tw and average toroidal field B t of a round-topped discharge. The Fig. 1 Time evolution, from top to bottom, of the plasma current I p , toroidal loop voltage V loop , edge toroidal field B tw , and average toroidal field B t in a round-topped discharge of RELAX.

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“…Quasi-single-helicity (QSH) states, in which there is a single m = 1 mode which dominates other existing modes, have been observed in RFPs. [10] However, these QSH states are not the same as the Taylor double helix and do not have ka = 1.25.…”

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confidence: 99%