DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
Login | Review | Submit | Logout | Help
Questions about this website? Contact Andrew LeBlanc Questions about ROF? Contact Chuck Greenfield
| Title | 446: Hydrogen Sensor Diagnostic for DIII-D | ||
| Name: | Dean Buchenauer ( |
Affiliation: | Sandia National Laboratories |
| Research Area: | Hydrogenic Retention | Presentation time: | Not requested | Co-Author(s): | D. Buchenauer, R. Bastasz, D. Rudakov, P. West, C. Wong |
| Description: | Understanding plasma-surface interactions in DIII-D requires knowledge of the flux and energy spectrum of plasma particles striking the walls and divertor. A diagnostic comprised of a number of solid-state hydrogen sensors, placed at various
locations in DIII-D, could provide much of this information. It is proposed that an initial component for this diagnostic be built and tested on the DiMES probe. It will be used to characterize the hydrogenic (H and D) particle flux striking the divertor floor in DIII-D at the DiMES location. Successful operation of the DiMES hydrogen sensor would lead to incorporation of hydrogen sensors in so-called smart tiles installed in the walls of DIII-D. | ||
| Experimental Approach/Plan: | |||
| Background: | The sensors are metal-insulator-semiconductor (MIS) devices that can be configured as diodes, capacitors, or transistors. The detection mechanism is similar in each case. Hydrogen striking the sensor surface is accommodated into a metal overlayer, which is typically made from a Pd alloy that promotes rapid transport of the hydrogen to the metalinsulator interface. Hydrogen trapped at this interface induces a dipole and changes the barrier height in the device. The change can readily be sensed by measuring the leakage current, flatband, or gate voltage. Fluences as low as 1012 H/cm2 have been measured in vacuum. For energetic particle detection, the metal layer thickness should be greater than the range of the most energetic particle impacting the device. This is typically on the order of 100 nm. An additional layer of a hydrogen impermeable material, such as Au, can be added on top of the
active metal layer to reduce the energy of particles entering the sensor, filter out low-energy particles, and to provide a means for energy discrimination. Each configuration has particular characteristics that must be considered when implementing the diagnostic. The diode is the most basic structure and is the simplest to operate. However, its temperature sensitivity makes it suitable for use only in isothermal locations. The capacitor is a more robust device whose response is relatively temperature insensitive. It requires a measurement circuit capable of generating capacitance-voltage (CV) curves and determining the appropriate flatband voltage. The transistor is the most complicated structure, but can be integrated into complex circuitry. All of these MIS sensor configurations are small in size and operate at low power. Depending upon the operating temperature and exposure conditions, these sensors function either as fluence or flux monitors. Diagnostic Description: The diagnostic is small enough to mount inside a regular sized DiMES sample. Initially, an individual sensor chip containing four redundant devices, configured as capacitors, will be embedded in a graphite-faced DiMES sample with a small viewing hole. A prototype has been designed and built. Nine connections to the sensor diagnostic are needed, four sensor contacts, a common, and two leads each for a small heater and thermocouple. The connections will be made using the existing DiMES electrical cable. External instrumentation will consist of a low-voltage supply, a capacitance meter, and interface electronics, which will be connected to a computer in communication with the DIII-D data system. There are three possible modes of operation: dosimetric, real-time, and chopped. The dosimetric mode, in which the sensor integrates the incoming flux and reports a single H dose value based on measurements made before and after the exposure period, is the most straightforward mode to implement. | ||
| Resource Requirements: | Piggyback initially, possibly 1/2 day experiment if tests are successful | ||
| Diagnostic Requirements: | Edge and divertor diagnostics and magnetics | ||
| Analysis Requirements: | Magnetic analysis for stored energy and strike point location, divertor spectroscopy and IR analysis, and divertor probes and Tompson data | ||
| Other Requirements: | -- | ||
Print this page