John L. Lowrance

Gas puff imaging of edge turbulence (invited)

The gas puff imaging (GPI) diagnostic can be used to study the turbulence present at the edge of magnetically confined plasmas. In this diagnostic the instantaneous two-dimensional (2D) radial vs poloidal structure of the turbulence is measured using fast-gated cameras and discrete fast chords. By imaging a controlled neutral gas puff, of typically helium or deuterium, the brightness and contrast of the turbulent emission fluctuations are increased and the structure can be measured independently of natural gas recycling. In addition, recent advances in ultrafast framing cameras allow the turbulence to be followed in time. The gas puff itself does not perturb the edge turbulence and the neutral gas does not introduce fluctuations in the emission that could possibly arise from a nonsmooth (turbulent) neutral gas puff. Results from neutral transport and atomic physics simulations using the DEGAS 2 code are discussed showing that the observed line emission is sensitive to modulations in both the electron density and the electron temperature. The GPI diagnostic implementation in the National Spherical Torus Experiment (NSTX) and Alcator C-Mod tokamak is presented together with example results from these two experiments.

360/spl times/360 element three-phase very high frame rate burst image sensor: design, operation and performance

Design, fabrication, operation and performance are described for a 360/spl times/360 element very high frame rate (VHFR) image sensor that can capture images at a frame rate of up to 10/sup 6/ frames/s. This VHFR imager has 50/spl times/50 /spl mu/m macropixels, each consisting of a high-speed zero-lag photodetector with a 13.5% fill factor and 30 stages of serial-parallel type buried-channel CCD registers for storage and readout of the last 30 image frames acquired. Readout of this imager is similar to a frame transfer CCD consisting of four quadrants. Each quadrant contains one million pixels, readout at a relatively slow rate compatible with low readout noise, and PC class microcomputer-based data acquisition and storage. This imager is designed using SiO/sub 2//Si/sub 3/N/sub 4/ gate dielectric, four levels of polysilicon, three levels of metal, eight implants, 21 photo masks using 1.5-/spl mu/m design rules, and it has four output ports. The high-speed photodetector is designed in the form of a graded three-potential-level pinned-buried BCCD structure. This 33-/spl mu/m long photodetector can achieve complete readout of photogenerated electrons in less than 0.1 /spl mu/s.

The spectrum of the quasi-stellar object PHL 957.

Spectra of the radio-quiet quasar PHL 957 were obtained at a 200-in. telescope with a Cassegrain image tube, a Cassegrain multichannel spectrometer, and an integrating television camera on the spectrograph. The image-tube spectra revealed 64 absorption lines between 3200 and 6800 A. The absolute energy distribution indicates that this quasar is one of the most luminous known. The absorption line redshifts and line profiles observed are discussed.

ICCD Development at Princeton

Publisher Summary This chapter discusses the intensified charge coupled devices (ICCD) development at Princeton. ICCDs are promising detectors for both laboratory measurements of high temperature plasmas in Tokamak nuclear fusion machines and in astronomy. There has been an interest for some time in image intensifier configurations that allow the use of opaque substrate photocathodes which exhibit higher quantum efficiency than semi-transparent photocathodes, especially in the ultraviolet and near infrared. Because the purpose of oblique magnetic focusing is to obtain a given lateral displacement of the focal plane from the optical axis, one must consider the trade-off of increased focal distance versus larger tilt angles. For a given accelerating voltage the aberrations increase with focal distance.

Development of EBCCD Cameras for the Far Ultraviolet

Publisher Summary This chapter discusses development of electron-bombarded charge-coupled device (EBCCD) cameras for the far ultraviolet. Princeton University and the Naval Research Laboratory have ongoing programmes to develop imaging detectors with high quantum efficiency and photon-counting capability. The electron-bombarded CCD arrays were operated in Schmidt imaging systems with opaque KBr photocathodes. These were compared with a chevron micro channel plate detector (MCP) operated in pulse-counting mode, or a collecting electrode for direct photocurrent measurement, placed interchangeably at the Schmidt camera electron focus, with simultaneous light-source monitoring by calibrated photomultipliers. The EBCCD read-out electronics were operated in analogue read-out mode, with signal levels per pixel digitized and recorded on tape. A liquid nitrogen cooling system maintained the CCDs at –80oC or less during operations. Theoretically, the single-photoelectron responses of the two types of CCD differ in that in the thinned back surface bombarded CCD there is likely to be significant lateral spread of the generated secondary charge before collection at the front-side electrodes.

Echelle spectroscopy with a charge-coupled device /CCD/

Observations made with an RCA thinned back illuminated CCD on the echelle spectrograph of the Kitt Peak 4-meter Mayall telescope are discussed. With emphasis on interstellar line measurements in bright and faint background sources, the system efficiency, spectrum format, achieved signal-to-noise, interference fringes, and overall performance are discussed. A signal-to-noise of > 100/1 has been achieved at 0.2 Å resolution in two hours on a 13.2 mag star. Projected CCD improvements should yield signal-to-noise ratios of 20/1 for 18th magnitude objects (V) in 2 hours. With appropriate gratings, 1500 Å of continuous spectrum can be obtained.

Electron Bombarded Charge-Coupled Device (CCD) Detectors For The Vacuum Ultraviolet

Development work is currently underway at Princeton and NRL on electronic-readout imaging detectors for the vacuum ultraviolet which are based on electron-bombarded CCD arrays and opaque alkali-halide photocathodes. These detectors provide single-photoelectron detection ability and wide dynamic range combined with nearly the maximum possible quantum efficiency (in excess of 50% in the wavelength range below 1300A •). The Princeton work is based on an oblique-focusing electron optical configuration. The NRL program is based on a Schmidt optical system with opaque photocathode as used previously with electrographic image recording. Thinned, back-surface-bombarded CCD arrays manufactured by RCA and by Texas Instruments are being used in the developmental devices. Results of preliminary tests at Princeton, and an overall system concepts for the Princeton and NRL cameras, will be discussed. Applications to possible space astronomy investigations will be described.

Intensified Charge-Coupled Device (ICCD) Single Photoelectron Response

Princeton University Observatory has developed an Intensified Charge Coupled Device (ICCD) image tube with a 200 mm diagonal S-20 photocathode and a 100 x 160 pixel Texas Instruments CCD. Measurements of the CCDs response to accelerated photoelectrons have been made with this tube, as well as other configurations. The paper presents the pulse height distribution and point spread function for single photoelectrons of 20 keV energy. These data are discussed in relation to a photon counting image system.

Virtual Phase Charge Coupled Device Imager Operated In Frontside Electron-Bombarded Mode

The Texas Instruments virtual phase CCD imager has been successfully operated in the frontside electron-bombarded mode. The entire active area of the imager was covered with 130 nm of thermally grown gate oxide while only the clocked half of each pixel was additionally covered with a 500 nm polysilicon electrode. No protective overcoat was grown over the imager. A 20 kV electron beam was focused onto the imager to a total dose in excess of 120,000 primary electrons per pixel. Both the parallel and serial clocks were operated between -15 V and +2 V throughout, and no adjustment in any of the operating parameters was required. However, flat band shifts on the order of 2 V were detected. Single primary electron events were clearly detected with a signal-to-noise ratio exceeding 10. In excess of 90% of the secondary charge generated by a primary event was collected in a single pixel. The standard virtual phase imager with only the protective overcoat deleted can be used with a photocathode in the electron-bombarded mode for observing low to moderate light levels and can act as a true photon counter.

Evaluation of RCA thinned buried channel charge-coupled devices /CCDs/ for scientific applications

The RCA 512 x 320 pixel buried channel CCD has been employed in a number of astronomical observations ii the past year and the astronomical results are presented in several companion papers. This paper reports on recent laboratory evaluation of an experimental version of this CCD which has a lower input capacitance on-chip amplifier. The data presented includes readout noise measured in several ways, charge transfer efficiency as a function of signal level, and dark current as a function of temperature. The response of the RCA buried channel CCD to single energetic electrons and soft X-rays is also presented.