T. Geist

Heterodyne methods in millimetre wave plasma diagnostics with applications to ECE, interferometry and reflectometry

Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed.

Ion temperature and beam-driven plasma waves from collective scattering of gyrotron radiation in W7-AS

Collective scattering of gyrotron radiation and its potential for plasma diagnostics was investigated using the W7-AS stellarator. The ion temperature was determined from thermal spectra in neutral-beam-heated plasmas. The excitation of a narrow band plasma wave by a non-thermal ion component was observed and the wave characteristics were investigated experimentally. The results constitute an important step toward alpha -particle diagnostic via scattering of powerful microwave radiation.

Collective Thomson scattering at W7-AS

Collective Thomson scattering (CTS) of electromagnetic radiation from thermal plasma fluctuations in principle allows the velocity distribution of plasma ions and its composition in the plasma to be measured. The use of powerful microwave radiation from gyrotrons opens new perspectives for the application of CTS, which is considered to be a promising candidate for alpha-particle diagnostics in reactor-size tokamaks with D/T operation. We have performed the first experiments at W7-AS with different scattering geometries to prove the applicability of gyrotrons for CTS. The experiments were performed with a 140 GHz gyrotron which is routinely used for ECRH, delivering a power of 0.45 MW. The receiver antenna and detection system for the registration of CTS spectra were especially designed for the scattering experiment. In backscattering experiments, which have inherently no spatial resolution, we have measured a transversely propagating, non-thermal lower-hybrid turbulence, which is driven by perpendicularly injected fast particles from a diagnostic neutral beam. The instability is excited by the beam ions under double-resonance conditions, where the LH frequency coincides with some harmonic of the beam ion gyrofrequency. For scattering geometries with the scattering wavevector not perpendicular to the magnetic field, thermal density fluctuations in the plasma were experimentally detected. The ion temperatures derived from these thermal spectra agree well with other diagnostics. A modified scattering geometry ( scattering) allows local measurements of the ion temperature and is considered a prototype for the design of a routine diagnostic for ion-temperature measurements.

Amplitude modulated heterodyne reflectometer for density profile and density fluctuation profile measurements at W7-AS

A broadband heterodyne reflectometer operating in the frequency range 75–110 GHz in extraordinary mode polarization is used at the W7‐AS stellarator for both fast density profile determination and density fluctuation studies. The probing signal is amplitude modulated at a frequency 133 MHz using the envelope phase for profile evaluation and the carrier phase to determine the fluctuation information simultaneously. Separate Gaussian beam optics for final signal launch and detection permits a beam waist of about 2 cm at the reflecting layer in the plasma. Amplitude modulated detection is accomplished in the intermediate frequency part by synchronous detection after recovery of the carrier by narrow‐band filtering. Voltage controlled solid state oscillators followed by active frequency multiplication allow to scan the full frequency band within less than 1 ms. For typical W7‐AS operation the accessible density range is 1×1019 to 6×1019 m−3 for on axis magnetic field of 2.5 T and 4.5×1019 to 10×1019 m−3 for 1.25 T, respectively. The probed radial positions range between 0.2

Diagnostic strategy of the W7-X stellarator

The Wendelstein 7-X experiment is a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features include a modular superconducting coil assembly, a fivefold toroidal symmetry, and a helical magnetic axis. Due to the optimization process, W7-X is characterized by a vacuum magnetic field configuration with smooth magnetic surfaces, improved equilibrium properties with a weak dependence of rotational transform and shear on the plasma pressure β, good magneto-hydrodynamic stability properties due to magnetic well stabilization, reduced neoclassical transport losses and negligible bootstrap current in the long mean-free-path regime, good collisionless α-particle confinement in an equivalent reactor, and, as a technical aspect, good feasibility of the superconducting modular coils. W7-X will be heated by continuous electron cyclotron resonance heating and pulsed neutral beam injection and ion cyclotron resonance heating. The envisaged parameters are Te⩽10 keV, Ti⩽6 keV ce...

Multichannel millimeter wave interferometer for W7-AS

A newly designed ten channel microwave interferometer for line integrated electron density measurements at the experiment W7-AS stellarator is described. The probing signals in the frequency range of 160–162 GHz are generated by Gunn oscillators with subsequent frequency doublers. Double conversion heterodyne receivers with 2 MHz bandwidth are used for detection. All oscillators involved are phase locked. Use of each Gunn oscillator is twofold: as a source for the probing signal and as a local oscillator for heterodyne detection. The dynamic range of the millimeter wave part is about 70 dB. The instrument operates up to a peak density of 7×1019 m−3 with a temporal resolution of 5 μs. This article gives a detailed technical description of the system and demonstrates its usefulness by means of an experimental example.

BROADBAND HETERODYNE REFLECTOMETER AT THE W7-AS STELLARATOR

A broadband heterodyne reflectometer, operating in the frequency range 75–110 GHz in extraordinary mode polarization, has been installed and operated at the Wendelstein 7‐AS stellarator for the study of density fluctuations and density profile determination. At 2.5 T it covers the density range between 5×1018 and 5×1019 m−3. The system differs from the usual setup as it combines the advantages of heterodyne detection with broadband capability avoiding the limitations of phase‐locked sources and permits the unambiguous determination of the phase delay independent of amplitude fluctuations in the reflected beam. After a first downconversion to an intermediate frequency of about 6 GHz, two additional mixing steps lead to an intermediate frequency of 60 MHz, which carries the phase delay from the plasma as a phase modulation. The phase information is yielded numerically from sin/cos detection at 60 MHz. Due to the balanced detection scheme, the drift as well as the broadband noise of the signal and local osci...

Lower hybrid turbulence excited by a fast transverse ion beam in a magnetized plasma

Experimental and theoretical investigations of lower hybrid (LH) turbulence in the W7-AS stellarator are presented. The turbulence is excited by an ion beam, which is generated by a weak neutral hydrogen beam injected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radiation. From the measured density dependence of the frequency it was identified as an LH type of instability. The spectrum is characterized by a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instability driven by a transverse fast ion component under the double resonance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate is derived. The stabilizing effect of high bulk ion temperatures was observed experimentally, in accordance with theoretical modelling. An instability saturation mechanism similar to the well known stochastic ion heating is proposed.

Heterodyne reflectometer for phase and time delay measurements in W7‐AS

A broadband heterodyne reflectometer operating in the frequency range 75–110 GHz in extraordinary mode polarization, is used at the W7‐AS stellarator for density profile and density fluctuation studies. The system allows for broadband operation and permits unambiguous determination of the phase delay independent of the amplitude fluctuations in the reflected beam. The system uses two different gauss beam optics for both signal launch and detection with a slim beam waist of about 2 cm at the relevant plasma location. An amplitude modulation system has been incorporated operating at 133 MHz modulation frequency sharing with the phase sensitive system the main oscillators, antennas, and the receiver front end. AM detection is accomplished in the IF part by synchronous detection after recovery of the carrier by narrowband filtering. Time delay measurements can be done simultaneously with the carrier phase determination. The main purpose of the diagnostic in AM mode is fast edge density profile monitoring, con...

Diagnostic strategy of the W7-X stellarator (abstract)

The Wendelstein 7-X experiment is a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features include a modular superconducting coil assembly, a five-fold toroidal symmetry, and a helical magnetic axis. Due to the optimization process, W7-X is characterized by a vacuum magnetic field configuration with smooth magnetic surfaces, improved magnetic field configuration with smooth magnetic surfaces, improved equilibrium properties with a weak dependence of rotational transform and shear on the plasma pressure β, good magneto-hydrodynamic stability properties due to magnetic well stabilization, reduced neoclassical transport losses and negligible bootstrap current in the long mean-free-path regime, good collisionless α-particle confinement in an equivalent reactor, and, as a technical aspect, good feasibility of the superconducting modular coils. W7-X will be heated by continuous electron cyclotron resonance heating and pulsed neutral beam injection and ion cyclotron ...