Significant progress has been made in the area of advanced modes of operation that are candidates for achieving steady state conditions in a fusion reactor. The corresponding parameters, domain of operation, scenarios and integration issues of advanced scenarios are discussed in this chapter. A review of the presently developed scenarios, including discussions on operational space, is given. Significant progress has been made in the domain of heating and current drive in recent years, especially in the domain of off-axis current drive, which is essential for the achievement of the required current profile. The actuators for heating and current drive that are necessary to produce and control the advanced tokamak discharges are discussed, including modelling and predictions for ITER. The specific control issues for steady state operation are discussed, including the already existing experimental results as well as the various strategies and needs (qψ profile control and temperature gradients). Achievable parameters for the ITER steady state and hybrid scenarios with foreseen heating and current drive systems are discussed using modelling including actuators, allowing an assessment of achievable current profiles. Finally, a summary is given in the last section including outstanding issues and recommendations for further research and development.

Chapter 6: Steady state operation

This review discusses the present state-of-the-art of passive high-power microwave components for applications in microwave systems for RF plasma generation and heating, plasma diagnostics, plasma and microwave materials processing, spectroscopy, communication, radar ranging and imaging, and for drivers of next generation high-field-gradient electron-positron linear colliders. The paper reports on high-power components for overmoded high-power transmission systems such as smooth-wall waveguides, HE/sub 11/ hybrid mode waveguides and quasi-optical TEM/sub 00/ beam waveguides. These include various types of mode converters, polarizers, cross-section tapers, bends, mode selective filters, pulse compressors, DC-breaks, directional couplers, beam combiners and dividers, vacuum windows, and instruments for mode analysis. Problems of ohmic attenuation and unwanted conversion to parasitic modes are discussed in detail and rules for alignment requirements are given. In the case of waveguide transmission, this review mainly concentrates on circular waveguide components but also deals with rectangular waveguide.

Passive high-power microwave components

A 250-GHz corrugated transmission line with a directional coupler for forward and backward power monitoring has been constructed and tested for use with a 25-W continuous-wave gyrotron for dynamic nuclear polarization (DNP) experiments. The main corrugated line (22-mm internal diameter, 2.4-m long) connects the gyrotron output to the DNP probe input. The directional coupler, inserted approximately midway, is a four-port crossed waveguide beamsplitter design. Two beamsplitters, a quartz plate and ten-wire array, were tested with output coupling of 2.5% (-16dB) at 250.6 GHz and 1.6% (-18dB), respectively. A pair of mirrors in the DNP probe transferred the gyrotron beam from the 22-mm waveguide to an 8-mm helically corrugated waveguide for transmission through the final 0.58-m distance inside the NMR magnet to the sample. The transmission-line components were all cold tested with a 248/spl plusmn/4-GHz radiometer. A total insertion loss of 0.8 dB was achieved for HE/sub 11/-mode propagation from the gyrotron to the sample with only 1% insertion loss for the 22-mm-diameter waveguide. A clean Gaussian gyrotron beam at the waveguide output and reliable forward power monitoring were achieved for many hours of continuous operation.

/pdf/corrugated-waveguide-and-directional-coupler-for-cw-250-ghz-17hxrwc30p.pdf

Corrugated waveguide and directional coupler for CW 250-GHz gyrotron DNP experiments

High-power millimeter wave sources are a key enabling technology in fusion energy research. The present state of the art of application of these sources to the areas of heating, current generation, and scattering for diagnostic purposes in fusion plasmas is reviewed. The extrapolation of these applications to future devices and the requirements which they place on sources and transmission lines are also discussed.

Applications of high-power millimeter waves in fusion energy research

Applications of high-power Terahertz (THz) sources require low-loss transmission lines to minimize loss, prevent overheating and preserve the purity of the transmission mode. Concepts for THz transmission lines are reviewed with special emphasis on overmoded, metallic, corrugated transmission lines. Using the fundamental HE11 mode, these transmission lines have been successfully implemented with very low-loss at high average power levels on plasma heating experiments and THz dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments. Loss in these lines occurs directly, due to ohmic loss in the fundamental mode, and indirectly, due to mode conversion into high order modes whose ohmic loss increases as the square of the mode index. An analytic expression is derived for ohmic loss in the modes of a corrugated, metallic waveguide, including loss on both the waveguide inner surfaces and grooves. Simulations of loss with the numerical code HFSS are in good agreement with the analytic expression. Experimental tests were conducted to determine the loss of the HE11 mode in a 19 mm diameter, helically-tapped, three meter long brass waveguide with a design frequency of 330 GHz. The measured loss at 250 GHz was 0.029 ± 0.009 dB/m using a vector network analyzer approach and 0.047 ± 0.01 dB/m using a radiometer. The experimental results are in reasonable agreement with theory. These values of loss, amounting to about 1% or less per meter, are acceptable for the DNP NMR application. Loss in a practical transmission line may be much higher than the loss calculated for the HE11 mode due to mode conversion to higher order modes caused by waveguide imperfections or miter bends.

Low-loss Transmission Lines for High-power Terahertz Radiation

Experience with the ECRH System of ASDEX Upgrade

The ECRH system of ASDEX Upgrade is now completed. Four gyrotrons generate a total Gaussian beam power of 2 MW/2 s at 140 GHz. The power is transmitted partly quasioptically and partly via corrugated HE-11 waveguides. The transmission line losses, determined calorimetrically, are about 12%. The four focused beams lead to a very localised power deposition in the plasma and can be steered in both poloidal and toroidal directions.

The ECRH system of ASDEX Upgrade

Electron cyclotron current drive at ω≈ωc with X-mode launched from the low field side

In ASDEX Upgrade ECRH and ECCD with a power of up to 2 MW has been used. With counter ECCD in discharges with an internal transport barrier we achieved an electron temperature of ≈13 keV. In low density ohmic plasmas we obtained an rf driven current of ≈80% both in co- and counter directions. Neoclassical tearing modes have been completly stabilised by driving a current on the resonant q-surface. Electron heat transport has been studied in standard L- and H-mode plasmas and can be described by a dependence on a critical electron temperature gradient.

ECRH Experiments in ASDEX Upgrade

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Experience with the ECRH System of ASDEX Upgrade

The ECRH system of ASDEX Upgrade

Electron cyclotron current drive at ω≈ωc with X-mode launched from the low field side

ECRH Experiments in ASDEX Upgrade