Advancements in electron cyclotron emission imaging demonstrated by the TEXTOR ECEI diagnostic upgrade.

A new TEXTOR electron cyclotron emission imaging system has been developed and employed, providing a diagnostic with new features and enhanced capabilities when compared to the legacy system it replaces. Optical coupling to the plasma has been completely redesigned, making use of new minilens arrays for reduced optical aberration and providing the new feature of vertical zoom, whereby the vertical coverage is now remotely adjustable on a shot-by-shot basis from 20-35 cm. Other innovations, such as the implementation of stacked quasioptical planar notch filters, allow for the diagnostic to be operated without interruption or degradation in performance during electron cyclotron resonance heating. Successful commissioning of the new diagnostic and a demonstration of the improved capabilities are presented in this paper, along with a discussion of the new technologies employed.

The next generation of electron cyclotron emission imaging diagnostics (invited).

A 128 channel two-dimensional electron cyclotron emission imaging system collects time-resolved 16x8 images of T(e) profiles and fluctuations on the TEXTOR tokamak. Electron cyclotron emission imaging (ECEI) is undergoing significant changes which promise to revolutionize and extend its capabilities far beyond what has been achieved to date. These include the development of a minilens array configuration with increased sensitivity antennas, a new local oscillator pumping scheme, enhanced electron cyclotron resonance heating shielding, and a highly flexible optical design with vertical zoom capability. Horizontal zoom and spot size (rf bandwidth) capabilities are also being developed with new ECEI electronics. An interface module is under development to remotely control all key features of the new ECEI instrument, many of which can be changed during a plasma discharge for maximum flexibility.

High sensitivity imaging Thomson scattering for low temperature plasma.

A highly sensitive imaging Thomson scattering system was developed for low temperature (0.1-10 eV) plasma applications at the Pilot-PSI linear plasma generator. The essential parts of the diagnostic are a neodymium doped yttrium aluminum garnet laser operating at the second harmonic (532 nm), a laser beam line with a unique stray light suppression system and a detection branch consisting of a Littrow spectrometer equipped with an efficient detector based on a "Generation III" image intensifier combined with an intensified charged coupled device camera. The system is capable of measuring electron density and temperature profiles of a plasma column of 30 mm in diameter with a spatial resolution of 0.6 mm and an observational error of 3% in the electron density (n(e)) and 6% in the electron temperature (T(e)) at n(e) = 4 x 10(19) m(-3). This is achievable at an accumulated laser input energy of 11 J (from 30 laser pulses at 10 Hz repetition frequency). The stray light contribution is below 9 x 10(17) m(-3) in electron density equivalents by the application of a unique stray light suppression system. The amount of laser energy that is required for a n(e) and T(e) measurement is 7 x 10(20)n(e) J, which means that single shot measurements are possible for n(e)>2 x 10(21) m(-3).

Effect of heating on the suppression of tearing modes in tokamaks.

The suppression of (neoclassical) tearing modes is of great importance for the success of future fusion reactors like ITER. Electron cyclotron waves can suppress islands, both by driving noninductive current in the island region and by heating the island, causing a perturbation to the Ohmic plasma current. This Letter reports on experiments on the TEXTOR tokamak, investigating the effect of heating, which is usually neglected. The unique set of tools available on TEXTOR, notably the dynamic ergodic divertor to create islands with a fully known driving term, and the electron cyclotron emission imaging diagnostic to provide detailed 2D electron temperature information, enables a detailed study of the suppression process and a comparison with theory.

Observation of high-field-side crash and heat transfer during sawtooth oscillation in magnetically confined plasmas.

High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the "fingering event" of the ballooning mode model with the high- mode only predicted at the low field side.

Comparison study of 2D images of temperature fluctuations during sawtooth oscillation with theoretical models.

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model.

Two-color facility based on a broadly tunable infrared free-electron laser and a subpicosecond-synchronized 10-fs-Ti:sapphire laser.

Subpicosecond synchronization between a mirror-dispersion-controlled 10-fs Ti:sapphire laser and the Free-Electron Laser for Infrared Experiments has been achieved. The measured intensity cross correlation between the two lasers is consistent with a jitter of only 400 fs rms. The wide and continuous tunability of the free-electron laser (FEL; 4.2-300mum) combined with ultrashort pulse duration of six optical cycles and high pulse energy of several tens of microjoules makes a series of two-color experiments possible in a previously inaccessible wavelength range. We demonstrate these capabilities by performing a two-color pump-probe experiment to study carrier cooling in GaAs. A FEL tuned from 8 to 17mum is used as the pump, and a synchronized Ti:sapphire laser pulse serves as the probe.