C. Castaldo

Current drive at plasma densities required for thermonuclear reactors

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

Electron fishbones: theory and experimental evidence

We discuss the processes underlying the excitation of fishbone-like internal kink instabilities driven by supra-thermal electrons generated experimentally by different means: electron cyclotron resonance heating (ECRH) and by lower hybrid (LH) power injection. The peculiarity and interest of exciting these electron fishbones by ECRH only or by LH only is also analysed. Not only is the mode stability explained, but also the transition between steady state nonlinear oscillations to bursting (almost regular) pulsations, as observed in FTU, is interpreted in terms of the LH power input. These results are directly relevant to the investigation of trapped alpha particle interactions with low-frequency MHD modes in burning plasmas: in fact, alpha particles in reactor relevant conditions are characterized by small dimensionless orbits, similarly to electrons; the trapped particle bounce averaged dynamics, meanwhile, depends on energy and not mass.

Spectral broadening of lower hybrid waves produced by parametric instability in current drive experiments of tokamak plasmas

In order to explain the results of the non-inductive current produced in the lower hybrid current drive (LHCD) experiments, a broadening of the radiofrequency (RF) power spectrum coupled to tokamak plasma needs to occur. The presented modelling, supported by diagnostic measurements, shows that the parametric instability (PI) driven by ion sound quasimodes, which occur in the scrape-off plasma layer located near the antenna mouth, produces a significant broadening of the launched LH spectrum. Considering the parameters of LHCD experiments of JET (Joint European Torus), and other machines as well, the PI growth rate is high enough for producing the compensation of the convective losses and, consequently, the broadening of a small fraction (of the order of 10%) of the launched power spectrum. Such a phenomenon is identified to be intrinsic to the RF power coupling in the LHCD experiments. As the principal implication of considering such spectral broadening in modelling the LH deposition profile, experiments of LHCD-sustained internal transport barriers in JET were successfully interpreted, which evidenced the effects of a well-defined LH deposition profile. The present work is important for addressing the long-lasting debate on the problem of the so-called spectral gap in LHCD. The design of LHCD scenarios relevant to the modern fusion research programme, an important requirement of which is the control of the plasma current profile in the outer half of plasma, can be properly achieved by considering PI-induced spectral broadening.

Diagnostics of mobile dust in scrape-off layer plasmas

Dust production and accumulation pose serious safety and operational implications for the next generation fusion devices. Mobile dust particles can result in core plasma contamination with impuriti ...

Progress in internal transport barrier plasmas with lower hybrid current drive and heating in JET (Joint European Torus)

In optimized shear plasmas in the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)], safety factor (q) profiles with negative magnetic shear are produced by applying lower hybrid (LH) waves during the plasma current ramp-up phase. These plasmas produce a barrier to the electron energy transport. The radius at which the barrier is located increases with the LH wave power. When heated with high power from ion cyclotron resonance heating and neutral beam injection, they can additionally produce transient internal transport barriers (ITBs) seen on the ion temperature, electron density, and toroidal rotation velocity profiles. Due to recent improvements in coupling, q profile control with LH current drive in ITB plasmas with strong combined heating can be explored. These new experiments have led to ITBs sustained for several seconds by the LH wave. Simulations show that the current driven by the LH waves peaks at the ITB location, indicating that it can act in the region of low m...

Spectral broadening of parametric instability in lower hybrid current drive at a high density

The important goal of adding to the bootstrap current a more flexible tool, capable of producing and controlling steady-state profiles with a high fraction of non-inductive plasma current, could be reached using the lower hybrid current drive (LHCD) effect. Experiments performed on FTU (Frascati Tokamak Upgrade) demonstrated that LHCD can occur at reactor-graded high plasma density, provided that the parametric instability (PI)-produced broadening of the spectrum launched by the antenna is reduced under proper operating conditions, capable of producing relatively high temperature in the outer region of plasma column. This condition was produced by operations that reduce particle recycling from the vessel walls, and enhance the gas fuelling in the core by means of fast pellet.New results of FTU experiments are presented documenting that the useful effect of temperature at the periphery, which reduces the LH spectral broadening and enhances the LH-induced hard-x ray emission level, occurs in a broader range of plasma parameters than in previous work. Modelling results show that a further tool for helping LHCD at a high density would be provided by electron cyclotron resonant heating of plasma periphery. New information is provided on the modelling, able determining frequencies, growth rates and LH spectral broadening produced by PI, which allowed assessing the new method for enabling LHCD at high densities. Further robustness is provided to theoretical and experimental fundaments of the method for LHCD at a high density.

Reduction of the electron thermal conductivity produced by ion Bernstein waves on the Frascati Tokamak Upgrade tokamak

Operating with a high frequency and a wave guide antenna, the ion Bernstein wave (IBW) experiment on the Frascati Tokamak Upgrade is not dominated, as expected, by nonlinear plasma edge phenomena. By coupling IBW power, a simultaneous increase of plasma density and central electron temperature (⩾2 keV) is produced when the confinement magnetic field is adjusted to set an ion cyclotron resonant layer in the plasma bulk. Transport analysis indicates a reduction of the electron thermal transport inside the internal resonant layer larger than a factor of 2.

Lower hybrid wave produced supra-thermal electrons and fishbone-like instability in FTU

The fishbone-like internal kink instability driven by supra-thermal electrons generated by lower hybrid current drive is an important issue of burning plasma research. Indeed, the trapped particle averaged bounce characterizing the interaction of trapped alpha particles with low-frequency MHD modes in burning plasmas depends on energy, not on mass, hence the charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low-frequency MHD modes. Fishbone-like internal kink instabilities driven by electrons were observed during experiments on Frascati Tokamak Upgrade (FTU) and interpreted in terms of an oscillating ‘fixed point’ activity followed by one of ‘limit cycle’, produced by suprathermal electrons in the presence of a q-profile with qmin ≈ 1. As an interesting behaviour of the fast electron population produced by the LH power when the q-profile meets the condition qmin ≈ 1, the fast electronbremsstrahlungdataofFTUshowamarkedredistributioninspaceacrosslayerscentredat r/a ≈ 0.20‐0.33. This redistribution occurs during and in phase with the fishbone oscillation observed by the MHD diagnostic, with the same time scale and at the same radial position of the peak emission seen by x-ray tomography.

Effect of low magnetic shear induced by lower hybrid current drive on high performance internal transport barriers in the Joint European Torus (JET)

~Received 18 February 2002; accepted 12 April 2002!A low/negative magnetic shear profile is maintained in the Joint European Torus in a plasma targetwith plasma current of 2.4 MA using 2.2 MW of lower hybrid ~LH! power combined with neutralbeam injection and ion cyclotron radiofrequency heating. In this scenario, an internal transportbarrier ~ITB! with time duration up to about4sisproduced. The fraction of LH driven current isabout 25% the total plasma current. During LH power application, the layer with reversed shearq-profile can be maintained in a suitable radial position to inhibit the onset of turbulence, whichmight otherwise drive the ITB to collapse. LH power could be used as a tool to drive moderateamounts of noninductive off-axis current and sustain high performance ITBs at high plasma current.© 2002 American Institute of Physics. @DOI: 10.1063/1.1488951#Internal transport barriers ~ITBs! are produced in manytokamaks as a result of auxiliary heating power injection inconditions of reversed shear q profile.

Analysis of the heating scenarios of the ion Bernstein wave (IBW) experiment in Frascati Tokamak Upgrade

Ion Bernstein waves (IBWs) have been proposed as useful for heating and improving transport in tokamak plasmas. The experiments carried out so far have not provided a sufficient assessment of this concept, due to the occurrence of parasitic phenomena that caused poor reproducibility of IBW heating and confinement effects. An experiment on FTU has given the first test of IBW coupling to a tokamak plasma by a waveguide antenna utilized in place of the standard dipole antenna. In addition, the experiment operates at a high frequency that is expected to reduce the non-linear wave-plasma interaction at the edge. These phenomena were considered to be the cause of the lack of RF power penetration in the plasma bulk and of the production of impurity influx. The possible IBW experimental scenarios expected for the FTU plasma are described. To perform this analysis, quasi-linear and non-linear models are utilized for the wave propagation, damping and turbulence suppression suitable for transport barrier formation. As a result of this study, FTU is expected to be a useful facility for testing the IBW concept for advanced tokamaks.