Bihe Deng

Microwave imaging reflectometer for TEXTOR (invited)

Understanding the behavior of fluctuations in magnetically confined plasmas is essential to the advancement of turbulence-based transport physics. Though microwave reflectometry has proven to be an extremely useful and sensitive tool for measuring small density fluctuations in some circumstances, this technique has been shown to have limited viability for large amplitude, high kθ fluctuations and/or core measurements. To this end, a new instrument based on two-dimensional imaging reflectometry has been developed to measure density fluctuations over an extended plasma region in the TEXTOR tokamak. This technique is made possible by collecting an extended spectrum of reflected waves with large-aperture imaging optics. Details of the imaging reflectometry concept, as well as technical details of the TEXTOR instrument, are presented. Data from proof-of-principle experiments on TEXTOR using a prototype system is presented, as well as results from a systematic off-line study of the advantages and limitations of...

Fluctuation measurements in tokamaks with microwave imaging reflectometry

To study the mechanism of anomalous transport in tokamaks requires the use of sophisticated diagnostic tools for the measurement of short-scale turbulent fluctuations. In this article, we describe an attempt at developing a technique capable of providing a comprehensive description of plasma fluctuations with k⊥ρi<1, such as those driven by the ion temperature gradient mode in tokamaks. The proposed method is based on microwave reflectometry, and stems from a series of numerical calculations showing that the spatial structure of fluctuations near the cutoff could be obtained from the phase of reflected waves when these are collected with a wide aperture optical system forming an image of the cutoff onto an array of phase sensitive detectors. Preliminary measurements with a prototype apparatus on the Torus Experiment for Technology Oriented Research 94 (TEXTOR-94) [U. Samm, Proceedings of the 16th IEEE Symposium on Fusion Engineering, 1995 (IEEE, Piscataway, NJ, 1995), p. 470] confirm the validity of these...

Electron cyclotron emission imaging diagnostic system for Rijnhuizen Tokamak Project

A 16-channel electron cyclotron emission (ECE) imaging diagnostic system has been developed and installed on the Rijnhuizen Tokamak Project for measuring plasma electron cyclotron emission with a temporal resolution of 2 μs. The high spatial resolution of the system is achieved by utilizing a low cost linear mixer/receiver array. Unlike conventional ECE diagnostics, the sample volumes of the ECE imaging system are aligned vertically, and can be shifted across the plasma cross-section by varying the local oscillator frequency, making possible 2D measurements of electron temperature profiles and fluctuations. The poloidal/radial wavenumber spectra and correlation lengths of Te fluctuations in the plasma core can also be obtained by properly positioning the focal plane of the imaging system. Due to these unique features, ECE imaging is an ideal tool for plasma transport study. Technical details of the system are described, together with preliminary experimental results.

ECE imaging of electron temperature and electron temperature fluctuations (invited)

Electron cyclotron emission imaging (ECE imaging or ECEI) is a novel plasma diagnostic technique for the study of electron temperature profiles and fluctuations in magnetic fusion plasma devices. Instead of a single receiver located in the tokamak midplane as in conventional ECE radiometers, ECEI systems utilize large diameter imaging optics coupled with planar millimeter-wave imaging arrays to form multichannel ECE diagnostics with excellent spatial resolution. Combined with specially designed imaging optics, the use of these compact, low cost arrays has resulted in the excellent spatial resolution of the ECEI systems, the unique capability of two-dimensional measurements, and flexibility in the measurement of plasma fluctuations. Technical details and principles of this emerging diagnostic technique are described in this article. Illustrative experimental results are presented, together with a discussion of the further development of the diagnostic.

Initial physics results from the National Spherical Torus Experiment

The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a approximately equal to 1.25) in low collisionality regimes. NSTX is designed to operate with up to 6 MW of High Harmonic Fast Wave (HHFW) heating and current drive, 5 MW of Neutral Beam Injection (NBI) and Co-Axial Helicity Injection (CHI) for non-inductive startup. Initial experiments focused on establishing conditions that will allow NSTX to achieve its aims of simultaneous high-bt and high-bootstrap current fraction, and to develop methods for non-inductive operation, which will be necessary for Spherical Torus power plants. Ohmic discharges with plasma currents up to 1 MA and with a range of shapes and configurations were produced. Density limits in deuterium and helium reached 80% and 120% of the Greenwald limit respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal current for up to 200 msec was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and bt =21% were produced.

Mode structure of turbulent electron temperature fluctuations in the Texas Experimental Tokamak Upgrade

High spatial resolution electron cyclotron emission imaging (ECEI) has been employed on TEXT-U [Texas Experimental Tokamak Upgrade, G. Cima et al., Phys. Plasmas 2, 720 (1995)] to measure turbulent electron temperature fluctuations using an intensity interferometric technique. With the first dispersion relation measurements in the plasma confinement region, a broadband spectral feature is identified at poloidal wave numbers consistent with expectations for electron drift waves.

Electron cyclotron emission imaging diagnostic of Te profiles and fluctuations

A novel high resolution, two-dimensional (2-D) plasma diagnostic technique, electron cyclotron emission imaging, is reviewed. Its high spatial and temporal resolution is illustrated from the measurement of small-scale structure in Te profiles. Its 2-D measurement capability has been utilized to “visualize” plasma turbulence structures in the Rijnhuizen Tokamak Project (RTP) [Hogeweij et al., Phys. Scr. 51, 627 (1995)]. Comprehensive experimental results of Te fluctuations in RTP are presented. Possible correlation between the characteristics of the measured Te fluctuations and ηi mode theory predictions are indicated, which calls for further theoretical and experimental investigation. Future diagnostic developments are also discussed.

Hybrid electron cyclotron emission imaging array system for Texas experimental tokamak upgrade

A novel wide bandwidth, low-cost, 20 channel hybrid Schottky diode mixer array-based imaging system has been developed, tested and implemented for electron cyclotron emission diagnostics on the Texas experimental tokamak upgrade (TEXT-U). The array has been successfully utilized to measure 1D and 2D electron temperature profiles, to study sawteeth and magnetohydrodynamic phenomena, and to make measurements of the poloidal/radial correlation lengths and wave-number spectra of electron temperature fluctuations. Fabrication and laboratory characterization results are presented, together with details and test results from the actual implementation on TEXT-U.

Poloidal asymmetry and gradient drive in core electron density and temperature fluctuations on the Texas Experimental Tokamak-Upgrade

Electron temperature and density fluctuations are measured in the core of the Texas Experimental Tokamak‐Upgrade (TEXT‐U) [P. H. Edmonds, E. R. Solano, and A. J. Wootton, in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier Science, Amsterdam, 1989), Vol. 1, p. 342] plasma across the poloidal cross section. The high spatial resolution of the heavy‐ion beam probe (HIBP) and correlation radiometry of electron cyclotron emission (CRECE) reveal that both the density and temperature fluctuations are strongly poloidally asymmetric. Temperature fluctuation measurements indicate a broadband drift wave feature localized near the plasma equatorial plane on both the high‐ and low‐field sides, which is consistent with density fluctuation measurements by far infrared (FIR) scattering. In contrast, the HIBP observes this feature localized only to the low‐field side. Excellent spatial resolution allows us to investigate whether changes in the gradient affect the fluctuation amplitudes. We find th...

Electron cyclotron emission imaging diagnostic on TEXTOR

A 16-channel electron cyclotron emission imaging diagnostic system, developed by UC Davis in collaboration with the FOM-Instituut voor Plasmafysica Rijnhuizen, has been installed on the Torus Experiment for Technology Oriented Research tokamak. The system is designed to collect multichannel electron temperature profile and fluctuation data along a vertical chord of constant magnetic field, by collecting second harmonic X-mode emission. Technical details of the system are presented together with preliminary experimental data.