Chaos often represents a severe obstacle for the set-up of many-body experiments, e.g., in fusion plasmas or turbulent flows. We propose a strategy to control chaotic diffusion in conservative systems. The core of our approach is a small apt modification of the system which channels chaos by building barriers to diffusion. It leads to practical prescriptions for an experimental apparatus to operate in a regular regime (drastic enhancement of confinement). The experimental realization of this control on a Travelling Wave Tube opens the possibility to practically achieve the control of a wide range of systems at a low additional cost of energy.

A test electron beam is propagated in a specially designed traveling wave tube. It interacts with a nonresonant wave, and its energy distribution is recorded at the tube output. We report the direct experimental observation of the spatially periodic electron velocity bunching, and of a nonlinear effect on the electron velocity modulation: the synchronization of the particles with the wave responsible for Landau damping in plasma physics. The results are explained by second order perturbation theory in the wave amplitude.

Modes oscillating at the acoustic frequency and identified as beta Alfvén eigenmodes (BAEs) have been observed in Tore-Supra under ion cyclotron resonant heating. In this paper, the linear excitation threshold of these modes, thought to be driven by suprathermal ions, is calculated and compared with Tore-Supra observations. Similar studies of the linear excitation threshold of energetic particles driven modes were carried out previously for toroidal Alfvén eigenmodes or fishbones. In the case of BAEs, the main point is to understand whether the energetic particle drive is able to exceed ion Landau damping, which is expected to be important in the acoustic frequency range. For this, the BAE dispersion relation is computed and simplified in order to derive a tractable excitation criterion suitable for comparison with experiments. The observation of BAEs in Tore-Supra is found to be in agreement with the calculated criterion and confirms the possibility to trigger these modes in the presence of ion Landau damping. Moreover, the conducted analysis clearly puts forward the role of the global tunable parameters which play a role in the BAE excitation (the magnetic field, the density etc), as well as the role of some plasma profiles. In particular, the outcome of a modification of the shear or of the heating localization is found to be non-negligible and it is discussed in the paper.

Abstract-A stereolithography-based manufacturing process for monolithic high aspect ratio components for mm-wave and sub-mm-wave applications is demonstrated. A 25 mm long straight waveguide and a diagonal horn antenna, both for the WR-3.4 band (220-330 GHz), are manufactured and characterized. The waveguide is found to exhibit transmission losses close to the theoretical minimum for Cu, and the performance of the diagonal antenna in terms of cross-polarization and directivity matches closely a metallic split-block reference antenna. These results confirm the high surface quality and mechanical accuracy of the employed 3D printing and plating techniques and thus validate the process for rapid manufacturing of monolithic components up to 330 GHz.

MHD instabilities driven by fast electrons identified as fishbonelike modes have been detected on Tore Supra during lower hybrid current drive discharges. Direct experimental evidence is reported of a novel feature: the regular redistribution of suprathermal electrons toward external tokamak regions which are correlated to periodic mode frequency jumps. Sharp drops of the electron temperature time trace are factually linked to the cyclical deterioration of the fast electron confinement.

Basic wave-particle interaction dynamics from linear to chaotic regimes is experimentally studied on a frequency tunable gyrotron generating THz radiation in continuous mode (200 W) at 263 GHz which will be used for dynamic nuclear polarization nuclear magnetic resonance spectroscopy applications. In the studied system, the nonlinear dynamics associated to the waveparticle interaction is dominated by longitudinal mode competition of a given transverse TE m;p cavity-mode. This study covers a wide range of control parameter from gyro-traveling wave tube (gyro-TWT) to gyro-backward wave oscillator (gyro-BWO) like interactions for which extensive theoretical studies have been performed in the past on a simplified system. Besides the common route to chaos characterized by period doubling, other routes have been identified among which some are characterized by line-width frequency-broadening on the side-bands. The complex nonlinear dynamics is in good agreement with the theory and the experimental results are discussed on the basis of the prediction obtained with the nonlinear time-dependent selfconsistent codes TWANG and EURIDICE both based on a slow-time scale formulation of the self-consistent equations governing the wave-particle dynamics. V C 2012 American Institute of Physics. [http://dx

Energetic particles driven modes are one of the concerns for burning plasmas. On Tore-Supra, fast ions and electrons are generated by the RF heating systems and fast particles driven modes are detected with ECE and reflectometry diagnostics. In ICRH heated plasmas, modes are observed in the acoustic frequency range 30–70 kHz. The observed frequency agrees with the frequency predicted for both geodesic acoustic modes and beta Alfvén eigenmodes (BAE), but their structure and their excitation by fast ions advocate for an identification as BAE. Experimental analysis displays the existence of an excitation threshold depending on various parameters such as the ICRH power, the minority fraction and the density, in agreement with the theoretical prediction resulting from a balance between the fast ion drive and Landau damping by thermal ions. In lower hybrid current drive plasmas, electron fishbones have been detected below 20 kHz. Evolution of the fishbone modes with LH power was studied. Precession fishbones are observed at moderate power to be replaced by a mode at diamagnetic frequency at higher power. Precession fishbone frequency exhibits periodical jumps (0.1 s). These jumps are linked to spontaneous transitions between modes at different wavenumbers and a redistribution of the fast electrons in resonance with these modes. The number of excited modes and their wave number change with the LH power.

A test electron beam is propagated in a specially designed traveling wave tube. The beam intensity is low enough to ensure that beam-plasma instabilities are ruled out. By recording the beam energy distribution at the output of the tube, we report the experimental observation of the resonant domain of a single wave and of the overlap of the resonance domains of two waves associated to the destruction of Kolmogorov–Arnold–Moser tori constituting barriers in phase space. This overlap mechanism is responsible for the transition to large scale chaos common to a large class of Hamiltonian systems.