Konstantinos A. Avramides

Experimental study from linear to chaotic regimes on a terahertz-frequency gyrotron oscillator

Basic wave-particle interaction dynamics from linear to chaotic regimes is experimentally studied on a frequency tunable gyrotron generating THz radiation in continuous mode (200W) at 263GHz 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 TEm;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. VC

A New Concept for the Collection of an Electron Beam Configured by an Externally Applied Axial Magnetic Field

A new concept for the collection of an electron beam, which is configured by an externally applied axial magnetic field, is presented. The two major advantages of this new idea are the significant increase of the collector efficiency and the reduction of the power absorption on the collector wall. To demonstrate the concept, an indicative collector design has been prepared for the hollow electron beam of the European 170-GHz 2-MW coaxial gyrotron for the international thermonuclear experimental reactor. The simulation shows that the efficiency of this advanced collector is more than 90%.

Simulation and experimental investigations on dynamic after cavity interaction (ACI)

In recent years, the so called after cavity interaction (ACI) in high power gyrotrons operating in the 100–200 GHz range gained attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that an undesired interaction in the uptaper region can also result in additional parasitic oscillations. In this paper, such non-stationary, dynamic processes are investigated in first simulations and experiments.

Design of a frequency-tunable gyrotron for DNP-enhanced NMR spectroscopy

We report on the design of a modular low-power (10–50W) high-frequency gyrotron (265–530GHz) for DNP enhanced Solid-State NMR spectroscopy. With the view of covering a wide range of frequencies, a 9.7T helium-free superconducting magnet (SCM) is planned for the gyrotron operation on either the fundamental or second harmonic of the electron cyclotron frequency. The gyrotron design is based on a triode electron gun (Vk=15kV, Ib=100mA, Va= 6–8kV) which is very flexible for adapting the electron beam properties to a wide variety of cavities operating at the fundamental or at the second harmonic. The gyrotron is designed for a lateral output with an internal Vlasov-type converter. The reference parameters for application of DNP-enhanced NMR spectroscopy on a 400MHz (1H) spectrometer are optimized with a RF frequency tunability corresponding to twice the proton NMR frequency. The modularity of the construction of the gyrotron allows for the possibility of changing only some elements like the cavity-uptaper system in order to adapt to the wide range of NMR spectrometers existing at EPFL.

Canonical perturbation theory for complex electron dynamics in gyrotron resonators

Complex electron dynamics in gyrotron resonators are analyzed in the context of the Hamiltonian formalism. Application of the canonical perturbation theory provides analytical approximate invariants of the electron motion. The latter are used for describing the resonant structure of the electron phase space and the electron rest energies at the output of the cavity. Hysteresis effects are also described through analytic expressions and approximate electron distribution functions are provided. The general case of resonant interaction at an arbitrary harmonic of the electron cyclotron frequency is considered and the effect of a varying frequency mismatch is studied. Also, the case of electron interaction with multiple rf modes is investigated.

Hamiltonian map description of electron dynamics in gyrotrons

Electron dynamics in gyrotron resonators are described in terms of a Hamiltonian map. This map incorporates the dependency of electron dynamics on the parameters of the interacting radio-frequency (RF) field and it can be used for trajectory calculations through successive iteration, resulting in a symplectic integration scheme. The direct relation of the map to the physics of the model, along with its canonical form (phase space volume preserving) and the significant reduction of the number of iteration steps required for acceptable accuracy, are the main advantages of this method in comparison with standard methods such as Runge-Kutta. The general form of the Hamiltonian map allows for wide applications as a part of several numerical algorithms which incorporate CPU-consuming electron trajectories calculations

Chaotic electron dynamics in gyrotron resonators

Phase-space analysis of electron dynamics is used in combination with the canonical perturbation method and the KAM theory in order to study the dependence of the efficient gyrotron operation on the RF field profile and frequency mismatch. In order to analyze the phase space of the system, we use appropriate Poincare surfaces of section. The flow can be considered in the three-dimensional subspace.

EU gyrotron development for ITER: Recent achievements and experimental results of the coaxial 2 MW gyrotron

Summary form only given. The European Gyrotron Consortium (EGYC) is responsible for developing one set of 170 GHz mm-wave sources, in support of Europes contribution to ITER1. The original plan of targeting a 2 MW coaxial gyrotron is currently under discussion, in view of essential delays and damages. This paper reports on the latest results and plans with regard to the 2 MW gyrotron prototypes.

Design of a low-power high-frequency gyrotron for DNP-enhanced NMR spectroscopy

Summary form only given as follows. We report on a design of a modular low-power (10-50 W) high-frequency gyrotron (200-530 GHz) for DNP-enhanced NMR spectroscopy. For covering this wide range of frequencies a 9.5 T helium-free superconducting magnet (SCM) with a warm bore diameter of 75 mm is foreseen. Considering the wide frequency range, the SCM field level is compatible with an operation of the gyrotron at both the fundamental or second harmonic of the electron cyclotron frequency. The gyrotron design is based on a triode electron gun (Vk=15 kV, Ib=100 mA, Va= plusmn5 kV) which is very flexible for adapting the electron beam properties to a wide variety of cavities operating at the fundamental or at the second harmonic. The gyrotron is designed for an axial output and a quasi-optical mode-converter to a Gaussian mode will be placed after the gyrotron window. The reference parameters for the first application of DNP-enhanced NMR spectroscopy on a 400 MHz (1H) spectrometer are: frequency = 263.5 GHz, frequency tunability = 0.12%, RF-power = 10-50 W. The modularity of the gyrotron refers to the possibility to change only some elements like the cavity-uptaper system to be adapted to the wide range of NMR spectrometers existing at EPFL.

Mode competition in the 170 GHz coaxial gyrotron cavity for ITER

170 GHz coaxial cavity gyrotrons with 2 MW output power in continuous wave (CW) operation are regarded as potential ECRH sources in ITER. A first industrial prototype of such a gyrotron has already been fabricated and delivered to CRPP Lausanne, where a suitable test facility has been constructed. Experimental tests are expected for the second half of this year. In view of the further development work, it is important to improve our understanding of the complicated processes of mode competition occurring in oversized coaxial cavities. An example of the comparison between the results obtained by means of four independent codes is presented and discrepancies between the theoretical end experimental results are discussed.