Publications by Markus Siegrist

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On the Tokamak a Configuration Variable (TCV), electron internal transport barriers (eITBs) can be formed during a gradual evolution from a centrally peaked to a hollow current profile while all external actuators are held constant. The formation occurs ra ...

Collective Thomson scattering, using a high-power pulsed D2O laser at 385-mu-m and a heterodyne receiver system, has provided local ion temperature (T(i)) measurements of the plasma in the tokamak chauffage Alfven (TCA) tokamak. Recent improvements in the noise-equivalent power of the Schottky barrier diode mixers permitted us to achieve a typical precision of +/- 12% for a single shot measurement at densities above 10(20) m-3. Even at densities of standard TCA discharges (5 x 10(19) m-3) the uncertainty is

Third harmonic generation (THG) efficiency is shown to be a greatly enhanced at the onset of inelastic scattering of electrons on optic phonons. Scaling experiments are performed on n-type InP at the pump wave frequency of 9.43 GHz at 80 K. Monte Carlo modeling is employed for scaling the effect to the 3(rd) harmonic frequency of 1 THz. The THG efficiency in n-type GaAs and InP as well as in the wurtzite phase of n-type InN and GaN compound crystals is compared to that in n-type Si. The efficiency maximum i

Scientific and technical activities on JET focus on the issues likely to affect the ITER design and operation. Our understanding of the ITER reference mode of operation, the ELMy H-mode, has progressed significantly. The extrapolation of ELM size to ITER has been re-evaluated. Neoclassical tearing modes have been shown to be meta-stable in JET, and their beta limits can be raised by destabilization (modification) of sawteeth by ion cyclotron radio frequency heating (ICRH). Alpha simulation experiments with

In future fusion-oriented plasma devices alpha-particle diagnostics will be of great importance. It is generally recognized that collective Thomson scattering techniques show great potential in this field. For ion temperature measurements as well as for alpha-particle diagnostics a radiation source emitting in the submillimetre range is required. However, powerful radiation sources which meet the requirement of pulse durations in the millisecond range are not easily available at these wavelengths. A high fr

The parallel velocity distribution function of the weakly relativistic electron beam of a quasi-optical gyrotron has been determined by measuring the Doppler-shifted Electron Cyclotron Emission (ECE) at an angle theta=15 degrees with respect to the external magnetic de-field. Due to the Doppler shift, the frequency of the spontaneous cyclotron emission at the fundamental (nu(0) = 100 GHz) is upshifted to 140 GHz. A broadening of the spectrum up to 10 GHz [full width at half maximium (FWHM)] was measured. Th

We investigate the nonlinear drift response of electrons in Si, GaAs, and InP crystals to high-power electromagnetic waves by means of a Monte Carlo technique, with the aim of developing an efficient frequency converter for 1 THz output radiation. Drift velocity amplitudes and phases determining the conversion efficiency are calculated for the first, third, and fifth harmonics in the pumping wave amplitude range of 10 < E-1 < 100 kV/cm, for frequencies between 30 and 500 GHz, and at the lattice temperatures

We performed Monte Carlo simulation of electron response to a strong pumping wave (443 GHz) and weak signal harmonic waves (886 and 1329 GHz) in silicon. A negative dynamic conductivity is found to arise for 1329 GHz above a threshold value of the pumping wave amplitude and below a maximum value of the signal wave amplitude in a limited range of phase difference between the waves.

A general method is described for selecting and optimizing a gas mixt. which isolates the amplifier stages of a CO2 laser chain. The isolation increases the useful energy extd. by eliminating self-oscillation. The performance of an optimized nonsatg. gas mixt. in a laser working on the 9P(32) line is given.

This article presents a feasibility study for a motional Stark effect diagnostic on the TCV tokamak. A numerical simulation code has been used to identify the optimal port arrangement and geometrical layout. It predicts the expected measurement accuracy for a range of typical plasma scenarios. With the existing neutral particle injector and a detection system based on current day technology, it should be possible to determine the safety factor with an accuracy of the order of 5%. A vertically injected beam

Monte Carlo simulations of the electron drift response to an ac-electric field are used to calculate the power dependent third harmonic generation efficiency for far-infrared radiation. The results are compared with far-infrared frequency tripling experiments. It is shown that the nonlinear optical properties are much more sensitive to parameter changes in the Monte Carlo simulation than conventional drift velocity results. Hence, this is a sensitive method to test Monte Carlo transport simulations. (C) 199

This paper presents a feasibility study for a motional Stark effect (MSE) [F. M. Levinton , Phys. Rev. Lett. 63, 2060 (1989)] diagnostic on the TCV tokamak. A numerical simulation code has been used to identify the optimal port arrangement and geometrical layout. It predicts the expected measurement accuracy for a range of typical plasma scenarios. With the existing neutral beam injector (NBI) and a detection system based on current day technology, it should be possible to determine the safety factor with a

A high power radiation source in the THz range with long pulse and narrow line width is required for diagnosing fusion type plasmas by collective Thomson scattering. Gyrotrons currently meet the requirements concerning power, pulse length and line width when operating in the 50-150 GHz range. They may have the potential also to do this in the THz range. In this paper two methods based on harmonic operation are investigated. Both look promising, if high conversion efficiency can be attained. This, however, s

The feasibility of Thomson scattering to determine the beam velocity in a gyrotron has been analyzed and preliminary experiments to implement such a system on our 100 GHz quasi-optical gyrotron are reported. Although the project had to be abandoned due to technical problems, the conclusions are that for the 90-degrees scattering arrangement discussed it should be possible to determine at least one velocity component with an acceptable signal-to-noise ratio.

A new laser line of CH3F at 676 mum has been found. The gas was pumped by a powerful pulsed CO2 laser at 9P20. The line could be observed simultaneously with the 496 mum line and can be assigned to a rotational transition in the 2nu3 band which is stepwise pumped from the vibrational ground state via the nu3 = 1 state.

We investigate experimental dependence of the third harmonic generation efficiency in the n-type Si crystals on the geometrical dimensions of the sample, polarization and power of the fundamental wave. The efficiency increases monotonically with the rise o ...

Most ionic crystals have strong reflection bands in the far infrared. Many such crystals are also transmissive to infrared radiation. An unstable resonator that exploits these properties is described. Preliminary results from a device 134 cm long and of 6 cm diameter are presented. For a pump pulse of 10 J and of duration 75 ns output powers of 0.7 MW have been measured. A correlation between the mode beating of the pump wave and the FIR wave it generates is demonstrated.

Markus Siegrist

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Safety factor profile requirements for electron ITB formation in TCV

Ion Temperature-Measurements in a Tokamak by Collective Thomson Scattering

Enhanced third-order nonlinearity in semiconductors giving rise to 1 THz radiation

Overview of JET results

Far-Infrared Wave Generation by Frequency Tripling

Measurement of the parallel velocity distribution function of the electron beam in a quasi-optical gyrotron by Electron Cyclotron Emission

Suitability of drift nonlinearity in Si, GaAs, and InP for high-power frequency converters with a 1 THz radiation output

Negative dynamic mobility of electrons in silicon in the far-infrared range

Isolation between Amplifiers in a TEA CO2-Laser Using Absorbing Gases

Feasibility of a motional Stark effect system on the TCV tokamak

A precise new method to evaluate Monte Carlo simulations of electron transport in semiconductors

Feasibility of a motional Stark effect system on the TCV tokamak

Development of a high-power THz radiation source for plasma diagnostics

Experimental Considerations Concerning the Velocity-Measurement of the Relativistic Electron-Beam in a Gyrotron by Means of Thomson Scattering

A New Pulsed Fir Laser Line in Ch3f

Third-Order Susceptibility of Silicon Crystals Measured with Millimeter-Wave Gyrotron

A 66-Mu-Md2o Laser Having an Unstable Reststrahlen Resonator

The Heavy Ion Beam Diagnostic Project for the TCV Tokamak

Parametric Study of an Optically Pumped Far Infrared Oscillator

Measurement of the parallel velocity distribution function of the electron beam in a quasi-optical gyrotron by electron cyclotron emission