D. P. Hutchinson

Measurement of CO2 laser small angle Thomson scattering on a magnetically confined plasma

We report the first successful small‐angle (less than 1°) Thomson scattering measurement of 10 μm radiation from a magnetically confined toroidal plasma. This represents a proof‐of‐principle demonstration of a new diagnostic technique for confined deuterium‐tritium fusion‐product alpha particles in future fusion reactors. This result was achieved by detecting scattered CO2 laser light from the plasma of the ATF torsatron at an angle of 0.86° using a novel heterodyne receiver scheme. A predicted resonance in the scattered power as a function of plasma electron density is clearly resolved in the measurements.

Collective Thomson scattering based on CO2 laser for ion energy spectrum measurements in JT-60U

A collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser has been developed and brought into operation to establish a measurement technique for ion temperature and the energy spectrum of fast ions. The pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) and a heterodyne receiver were installed on the JT-60U tokamak. Calculation of the scattered power spectrum from high temperature plasma in JT-60U shows that a good signal-to-noise value is expected for the bulk-ion temperature measurement. Calibration of the heterodyne receiver system has been carried out using a large area (12×12 cm2) blackbody radiation source. Commissioning of the CTS system by injecting the CO2 laser into the plasmas has been accomplished. However, a scattered signal has not yet been detected due to electrical noise originating from the pulsed lasers discharge and stray signal coming from mode impurities in the pulsed laser. After the electrical and stray light reduction, ion temperatur...

CO2 laser polarimeter for measurement of plasma current profile in Alcator C‐Mod

A multichannel infrared polarimeter system for measurement of the plasma current profile in Alcator C‐Mod has been designed, constructed, and tested. The system utilizes a cw CO2 laser at a wavelength of 10.6 μm. An electro‐optic polarization‐modulation technique has been used to achieve the high sensitivity required for the measurement. The recent results of the measurements as well as the feasibility of its application on international thermonuclear experimental reactor are presented.

Collective Thomson scattering using a pulsed CO2 laser in JT-60U

A collective Thomson scattering (CTS) diagnostic system using a carbon dioxide (CO2) laser has been developed for the purpose of establishing measurement technique of ion temperature and fast alpha particle in a fusion reaction plasma. A pulsed CO2 laser (15 J, 1 μs at 10.6 μm) and a heterodyne receiver with a stray light notch filter has been successfully installed. The noise equivalent power of the heterodyne receiver is below 9×10−19 W/Hz up to a frequency of 8 GHz. A six-channel filter bank analyzes the spectrum of the scattered light in the frequency range from 0.4 to 4.5 GHz to measure ion temperature and to detect fast ions generated by negative-ion source neutral beam injection of the JT-60U (Japan Atomic Energy Research Institute Tokamak 60-Upgrade) plasmas [S. Ishida, JT-60U Team, Nucl. Fusion 39, 1211 (1999)]. Test of the CTS system by injecting the CO2 laser into the vacuum vessel of JT-60U has started. Stray light signal around the JT-60U vacuum vessel was detected and optimization of the opt...

Tritium to deuterium ratio measurement by collective Thomson scattering

A technique is proposed for the determination of the tritium to deuterium density ratio in ITER using collective Thomson scattering at a wavelength of 10 μm. The measurement is made by viewing the component of scattered laser power nearly perpendicular to the magnetic field giving rise to ion cyclotron modulation of the scattered spectrum. The tritium/deuterium ratio may be inferred by a deconvolution of the scattered laser power. Due to the narrow bandwidth of the scattered signal and the spatially varying magnetic field, a narrowband heterodyne imaging array receiver will be required to observe the scattered spectrum.

Drift‐wave‐like density fluctuations in the Advanced Toroidal Facility (ATF) torsatron

Density fluctuations in low‐collisionality, low‐beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0...

Absolute broadband calibration procedure for infrared heterodyne receivers

A dual-channel low-noise heterodyne receiver is presented as part of a development effort to build a carbon dioxide laser-based Thomson scattering alpha particle diagnostic for a burning plasma experiment./aip/

Position-sensitive scintillation neutron detectors using a crossed-fiber optic readout array

The Spallation Neutron Source (SNS) under construction at the oak Ridge National Laboratory will be the most important new neutron scattering facility in the United States. Neutron scattering instruments for the SNS will require large area detectors with fast response (< 1 microsecond), high efficiency over a wide range of neutron energies (0.1 to 10 eV), and low gamma ray sensitivity. We are currently developing area neutron detectors based on a combination of 6LiF/ZnS scintillator screens coupled to a wavelength- shifting fiber optic readout array. A 25 X 25-cm prototype detector is currently under development. Initial tests at the High Flux Isotope Reactor have demonstrated good imaging properties coupled with very low gamma ray sensitivity. In addition, we have developed a multi-layer scintillator/fiber detector to replace existing He-3 gas detector tubes for higher speed operation. This detector has demonstrated a neutron detection efficiency of over 75% at a neutron energy of 0.056 eV or about twice thermal. The response time of this detector is approximately 1 microsecond. Details of the design and test results of both detectors will be presented.

Applying the CO2 laser collective Thomson scattering results from JT-60U to other machines

A collective Thomson scattering (CTS) system based on a pulsed CO2 laser is being developed to demonstrate the feasibility of alpha-particle diagnostics. Tests on this system are being conducted on the JT-60U tokamak. The system consists of a pulsed laser (15 J in 1:s at 10.6:m) and a wide band (∼8 GHz) heterodyne receiver with a quantum-well infrared photodetector [Liu et al., IEEE Electron Device Lett. 16, 253 (1995)]. Stray light is reduced by a notch filter containing hot CO2 gas. The heterodyne receiver is absolutely calibrated using a large area blackbody radiation source [Bennett et al., Appl. Opt. 27, 3324 (1988)]. Results from the tests on JT-60U indicate a larger than expected noise background, especially for the lower velocity measurements determined by the smaller frequency shifts from the laser line. These results are used to estimate the signal-to-noise ratio that would be expected for a CTS diagnostic installed on other devices; ITER (“the way” in Latin), Joint European Torus-Enhanced Perfo...

A CMOS integrated circuit for pulse-shape discrimination

A CMOS integrated circuit (IC) for pulse-shape discrimination (PSD) has been developed. The IC performs discrimination of gamma-rays and neutrons as part of a monitoring system for stored nuclear materials. The method extracts the pulse tail decay time constant using a leading edge trigger for identifying the start of the pulse and a zero-crossing discriminator to determine the zero crossing of the bipolar shaped signal. The circuit is designed to interface with two photomultiplier tubes-one for pulse processing and one for coincidence detection. Two outputs from the IC, a start and stop, can be used with a high speed timing system for pulse characterization with minimal external control. The circuit was fabricated in Orbit 1.2 /spl mu/m CMOS and operates from a 5-V supply. Specifics of the design including overall topology, charge-sensitive preamplifier and discriminator characteristics, shaping method and time constant selections, system timing, and implementation are discussed. Circuit performance is presented including timing walk, system dead time, and power consumption.