Imaging Bolometer Research Articles

Research and Development of Imaging Bolometers

An overview of the research and development of imaging bolometers giving a perspective on the applicability of this diagnostic to a fusion reactor is presented. Traditionally the total power lost from a high temperature, magnetically confined plasma through radiation and neutral particles has been measured using one dimensional arrays of resistive bolometers. The large number of signal wires associated with these resistive bolometers poses hazards not only at the vacuum interface, but also in the loss of electrical contacts that has been observed in the presence of fusion reactor levels of neutron flux. Imaging bolometers, on the other hand, use the infrared radiation from the absorbing metal foil to transfer the signal through the vacuum interface and out from behind a neutron shield. Recently a prototype imaging bolometer known as the InfraRed imaging Video Bolometer has been deployed on the JT-60U tokamak which demonstrates the ability of this diagnostic to operate in a reactor environment. The application of computed tomography demonstrates the ability of one imaging bolometer with a semi-tangential view to produce images of the plasma emissivity. In addition, new detector foil development promises to strengthen the foil and increase the sensitivity by an order of magnitude.

Spatial Variation of the Foil Parameters from in Situ Calibration of the JT-60U Imaging Bolometer Foil

We obtained the local foil properties of the JT-60U imaging bolometer foil (a single graphite-coated gold foil with an effective area of 9 × 7c m 2 and a nominal thickness of 2.5 µm) such as the thermal diffusivity, κ ,a nd the product of the thermal conductivity, k ,a nd the thickness, t f ,b y calibrating some parts of the foil. Calibration of the foil was made in situ using a He-Ne laser (∼27 mW) as a known radiation source to heat the foil. The thermal images of the foil are provided by an infrared (IR) camera (microbolometer type). The parameters are determined by finite element modeling (FEM) of the foil temperature and comparing the solution to the experimental results. In this work we apply this calibration technique to investigate the spatial variation of the foil parameters. Significant variation in the local temperature rise of the foil due to local heating by the laser beam indicates a spatial variation of the foil parameters κ, k and t f .T his variation is possibly due to nonuniformity in carbon coating and/or the thickness of the foil.

Millimetre continuum observations of southern massive star formation regions – II. SCUBA observations of cold cores and the dust grain emissivity index (β)

We report the results of a submillimetre continuum emission survey targeted towards 78 star formation regions, 72 of which are devoid of methanol maser and UC HII regions, identified in the Swedish ESO Submillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA) millimetre continuum survey of Hill et al. At least 45 per cent of the latter sources, dubbed ‘mmonly’, detected in this survey are also devoid of the mid-infrared MSX emission. The 450- and 850-μm continuum emission was mapped using the Submillimetre Common User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emission is detected towards 97 per cent of the 78 sources targeted as well as towards 28 other SIMBA sources lying in the SCUBA fields. In total, we have identified 212 cores in this submillimetre survey, including 106 previously known from the SIMBA survey. Of the remaining 106 sources, 53 result from resolving a SIMBA source into multiple submillimetre components, whilst the other 53 sources are submillimetre cores, not seen in the SIMBA. Additionally, we have identified two further mmonly sources in the SIMBA images. Of the total 405 sources identified in the SIMBA survey, 255 are only seen at millimetre wavelengths. We concatenate the results from four (sub)millimetre continuum surveys of massive star formation, together with the Galactic plane map of Pierce-Price et al. in order to determine the dust grain emissivity index β for each of the sources in the SIMBA source list. We examine the value of β with respect to temperature, as well as for the source classes identified in the SIMBA survey, for variation of this index. Our results indicate that β is typically 2, which is consistent with previous determinations in the literature, but for a considerably larger sample than previous work.

Star formation in Sandqvist 187 and 188

We present 1.2 mm continuum data of the filamentary dark cloud Sandqvist 187/188 - also known as the Norma cloud - taken with SIMBA (SEST Imaging Bolometer Array). The data are complemented by measurements from the 2MASS, MSX and IRAS surveys. The extended 1.2 mm emission traces the optical dark cloud down to a visual extinction of AV = 3.7 mag. We derive a mean column density of N(H2) = 1.4 × 10 22 cm −2 , equivalent to a visual extinction of AV = 7.6 mag and a total mass of the cloud of 340 M� . We also find six compact millimetre sources, labelled MMS 1 to 6, five of which coincide with known stellar objects. MMS 6, however, only has a very weak MSX counterpart at 14.56 µm and an absorption feature at 8.28 µm consistent with a visual extinction of AV > 145 mag. Dust temperatures, luminosities and gas masses for all compact millimetre sources are given. In an effort to further constrain the uncertain distance to Sandqvist 187/188, we analyse its stellar content based on 2MASS colours and find an improved lower limit of 440 ± 50 pc.

Calibration and sensitivity of the infrared imaging video bolometer

The infrared (IR) imaging video bolometer (IRVB) is an imaging bolometer which uses a large (9 cm×9 cm) thin (1 μm) gold foil and an IR camera to provide images of radiation from the plasma. Calibration of the IRVB using a lamp has been performed to compensate for any nonuniformities in the foil’s thickness and its thermal properties due to blackening of the foil with graphite to improve the IR emissivity. This calibration revealed close to expected values for the calibration coefficient proportional to the product of the thermal conductivity and the foil thickness in the central region of the foil, while these values were anomalously high near the foil edge. The calibration coefficient proportional to the thermal diffusivity is a factor of 2 smaller than the expected value at the center and drops further at the edge of the foil. Using a derived expression for the IRVB noise equivalent power, a sensitivity comparison shows the IRVB using current IR technologies to be ∼200 times less sensitive than an equivalent conventional resistive bolometer operating under ideal conditions.

Bolometric images of the three-dimensional structure of asymmetric radiative collapse in LHD

The infrared (IR) imaging bolometer has the merit of being able to measure wide two-dimensional images of the plasma radiation. Two IR imaging video bolometers (IRVB, one type of IR imaging bolometer system) were installed on the large helical device (LHD). Using these IRVBs, the asymmetric radiation collapse that is similar to the multifaceted asymmetric radiation from the edge was measured in LHD. From the results of the asymmetric collapse study in LHD, the high radiation region is localized on the inboard side and under the midplane on the vacuum vessel, but this region is not the nearest point from the plasma to the first wall. From other reports on LHD, it is written that the wall recycling is not a trigger for the asymmetric radiation onset, which is confirmed by this new result.

Infrared imaging video bolometer for the large helical device

A new type of infrared imaging bolometer, known as the infrared imaging video bolometer (IRVB) has been under development at the National Institute of Fusion Science (NIFS) over the last year. A prototype of this diagnostic has been designed and constructed at NIFS and installed on the large helical device (LHD) and successfully operated. This diagnostic utilizes a 66×90×0.001 mm gold foil mounted in a frame to detect the radiation and neutrals from the plasma which are incident on the foil through a 1 cm diameter pinhole. The resulting temperature distribution on the foil is measured using an AGEMA THV 900 LW infrared camera having 136×272 pixels with a frame rate of 15 Hz and a nominal sensitivity of 80 mK. An image of the foil and surrounding frame consisting of 120×160 pixels is resampled down to 12×16 pixels using a linear interpolation scheme. Using a numerical technique the spatial and temporal derivatives of the temperature distribution on the foil are calculated and the incident power density on the foil is determined using a calibration of the foil obtained by means of a HeNe laser. The resulting 10×14 pixel view of the plasma radiation at a 15 Hz frame rate has a noise equivalent power density of 0.5 mW/cm2. The IRVB was mounted on a LHD tangential port for the 1999 campaign. An image of the plasma radiation during a discharge using the inboard vacuum vessel wall as a limiter shows radiation localized near the limiting surface.

Infrared imaging video bolometer

A new concept for an infrared imaging bolometer is proposed which provides full video (two-dimensional) imaging of the radiated power from the plasma. This concept preserves all the advantages (compared to conventional metal foil resistive bolometers) of the previously proposed and tested segmented mask infrared imaging bolometer (SIB). It avoids the problems associated with the copper mask of the SIB, while giving a full frame video image of the plasma radiation with improved experimental flexibility regarding the pixel size. Analysis of the noise equivalent power shows that compared to a SIB with the same pixel area, the infrared imaging video bolometer is 2–5 times more sensitive with improved spatial resolution. These benefits are gained at the expense of the mechanical support, which the mask provides for the foil in the SIB. A numerical algorithm is used to solve the two-dimensional heat diffusion equation for the foil and determine the time-dependent spatial distribution of incident power on the foil from the infrared (IR) video camera measurements of the foil temperature. Testing the algorithm using a Gaussian model of the incident power shows that it can accurately reproduce the Gaussian model to within 6%. A simple scheme to absolutely calibrate the entire foil is described and several design points are detailed pointing out the range and experimental flexibility of the diagnostic using currently available IR camera technology.

Development of imaging bolometers for long-pulse MFE experiments (invited)

We have developed the concept of an imaging bolometer, capable of operation with 100’s of individual channels, while relying on optical (infrared) readout of the temperature rise in a thin foil. A thin gold foil (0.5–5 μm thick) is sandwiched between pieces of copper. The copper mask (a large thermal mass) has a hole pattern drilled into it to form many “individual pixels,” effectively forming many separate sensors. This segmented foil/mask combination is exposed on its front side to plasma radiation through a cooled pinhole camera geometry. Simultaneously, a high-resolution infrared camera monitors any temperature change on the backside of the thin foil. A sensitive infrared (IR) camera views the foil through an IR telescope/periscope system, and is shielded from the magnetic and nuclear radiation fields, either by distance and/or material shielding. A simple time-dependent design algorithm, using 1D heat transport to a cold boundary, has been written in MathCad, which allows us to select optimal material and geometries to match the expected plasma conditions. We have built a compact prototype with 149 channels, and tested it successfully both in a vacuum test stand in the laboratory, and on a plasma in the CHS at the National Institute for Fusion Science, subjecting it to electron cyclotron heated and neutral beam injection heated conditions. A water-cooled version has been built for the new LHD. Since the IR imaging bolometer uses only metal parts near the plasma, and has no need for wiring or wiring feedthrus, it is intrinsically radiation hard, and has direct application to ignition device to test engineering concepts (ITER), or ITER-class experiments.

A radiation-hard, steady state, digital imaging bolometer system

The concept and design of a new type of bolometer system which can function with excellent spatial resolution and good time resolution in the next generation of long-pulse (or steady state), harsh-neutron environment, fusion plasmas, is outlined. It uses a cooled pinhole-camera design, employing a robust, passive, segmented radiation absorber. In one scenario, IR emission from the absorber's front surface is relayed by metal mirror optics to a shielded, high resolution IR video camera with ±0.01°C temperature resolution. In a second scenario, the matrix is viewed from its back side, in order to eliminate any extraneous IR radiation from the vessel or plasma. The system can make thousands of simultaneous `pixel' measurements at up to 50–60 Hz, with no wire leads through the vacuum interface.

Design of an imaging bolometer system for the large helical device

We describe a radical design for a bolometer system employing infrared (IR) imaging of a segmented-matrix absorber in a cooled-pinhole camera geometry, which we will prototype and demonstrate on the large helical device (LHD).1 LHD will be operational in early 1998, with an l=2 superconducting winding, a major radius of 3.9 m, a minor radius of 0.5–0.65 m, and input powers ranging from 3 MW (steady state) to 30 MW (pulsed). The bolometer design parameters are determined by modeling the temperature of the foils making up the detection matrix using a two-dimensional time-dependent solution of the heat conduction equation. This design will give a steady-state bolometry capability, with modest (60 Hz) time resolution, while simultaneously providing hundreds of channels of spatial information. No wiring harnesses will be required, as the temperature-rise data is measured via a 12-bit, ±0.025 °C resolution, 3–5 μm band, 256×256 pixel IR camera. The spatial data will be used to tomographically invert the profile of the highly shaped stellarator main plasma and divertor radiation, in conjunction with more conventional fanned arrays of traditional bolometers.