A highly efficient quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK). The converter consists of a dimpled-wall waveguide launcher, one quasi-elliptical mirror and two toroidal mirrors. The coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher; the radiated fields from the cut of the launcher have been calculated by the scalar diffraction integral. Simulation results show that the advanced dimpled-wall launcher generates a well-focused Gaussian radiation pattern with low diffraction losses. In this case, toroidal mirrors are sufficient to obtain a desired output beam pattern. An efficiency of more than 98% has been achieved to convert the rotating TE/sub 28,8/ cavity mode at 140 GHz into a fundamental Gaussian beam. Experimental measurements show close agreement with theoretical predictions.

A high-efficiency quasi-optical mode converter for a 140-GHz 1-MW CW gyrotron

The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is optimized following a set of criteria including a rotational transform profile with low shear and minimized bootstrap current that must be controlled for proper functioning of the island divertor. This paper studies the compensation of residual bootstrap current by using electron cyclotron current drive (ECCD). The modeling shows that the loop voltage induced by ECCD leads to a redistribution of the current density with a diffusion time of ∼2 s. The relaxation time of the total current is much longer, however-for W7-X plasma parameters the total toroidal current reaches steady state after several L/R times requiring hundreds of seconds. In order to keep the toroidal current and its profile in the acceptable range, a feed-forward or predictive control method using ECCD as actuator is proposed. The main steps are as follows: (a) calculate the bootstrap current distribution using plasma parameters measured in the online transport analysis and (b) determine and apply ECCD as needed. For the current control to work properly and to avoid long relaxation times, the reaction time of the control loop must be less than the current skin time.

Current control by ECCD for W7-X

This report gives an update of the worldwide experimental achievements in the development of long-pulse gyrotron oscillators for fusion plasma heating, plasma diagnostics and materials processing. In addition, this work gives a short overview of the present development status of coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, quasi-optical gyrotrons, cyclotron autoresonance masers, gyro-amplifiers, gyro-BWO's, free electron masers and of vacuum windows for such high-power mm-wave sources.

/pdf/state-of-the-art-of-high-power-gyro-devices-and-free-1mmeiw5vph.pdf

State-of-the-Art of High Power Gyro-Devices and Free Electron Masers. Update 2011. (KIT Scientific Reports ; 7606)

Problems related to the development of diagnostics for steady state fusion plasma experiments are discussed. The paper concentrates on those necessities already appearing in current non-burning plasma fusion experiments when extending pulse lengths beyond 10 s, i.e. thermal load, erosion, deposition and long-time signal integration in magnetic diagnostics. Problems arising from high power ECRH under conditions of incomplete absorption are outlined. Individual standard diagnostic systems are discussed to identify their specific problems as well as the opportunities connected with long pulse operation. Burning plasma experiments characterized by intense n- and γ-radiation are briefly reviewed for reasons of completeness, dealing with radiation induced processes in windows, fibres, cables and mirrors. Methods of data handling, real time monitoring and plasma control are outlined.

/pdf/diagnostics-for-steady-state-plasmas-3yh19dtxc0.pdf

Diagnostics for steady state plasmas

This paper is a summary of some of the major results from the Wendelstein 7-AS stellarator (W7-AS). W7-AS [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code [Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 (2002)]. It has extended the operational range to high density (4×1020m−3 at 2.5T) and high ⟨β⟩ (3.4% at 0.9T); it has demonstrated successfully the application of electron cyclotron resonance heating (ECRH) beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X [J. Nuhrenberg et al., Trans. Fusion Technol. 27, 71 (1995)], aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a “non-to...

/pdf/w7-as-one-step-of-the-wendelstein-stellarator-line-2jwppyap9s.pdf

W7-AS: One step of the Wendelstein stellarator line

The Wendelstein 7-X (W7-X) stellarator is the largest fusion experiment presently under construction. The main objective of W7-X is to prove the reactor relevance of a stellarator, based on the HELIAS principle, as an alternative to the tokamak. Details of the optimization criteria and the scientific and technical objectives can be found in several publications. At present, the construction of W7-X is close to the start of the assembly. The first superconducting nonplanar coil is undergoing acceptance testing, the first sector of the plasma vessel has been leak tested, the main parts for the outer vessel have been fabricated, the first ports are close to delivery, and many rigs for the assembly are already installed. The assembly started at the end of 2003 with the attachment of saddle coils for magnetic diagnostics on the plasma vessel and will last until 2009 when the torus will be closed. Start of plasma operation is scheduled for the middle of 2010.

Status of Wendelstein 7-X construction

/pdf/status-of-wendelstein-7-x-construction-1o6bapn4zn.pdf

W X Construction Team

Papers

Status of Wendelstein 7-X construction

Status of Wendelstein 7-X Construction