Patent Number: 047675918
Section: summary

BACKGROUND OF THE INVENTION It is often desirable to determine the energies and doses of particles incident on materials exposed to a plasma. For example, the successful design of a fusion reactor based on magnetic confinement requires an understanding of conditions at the boundary region between the magnetically confined plasma and the reactor wall. The particles of interest are usually hydrogen or deuterium with energies and fluxes typically in the range of 10 to 1000 eV and 10.sup.12 to 10.sup.18 particles per square centimeter per second, respectively. In the past, such studies have generally relied on ion beam analysis or sputter profiling measurements to determine the dose and energy of the particles retained in probe samples exposed to the plasma edge. However, such techniques require specialized facilities and considerable effort to perform the analysis. Additionally, ion beam analysis or sputter profiling cannot provide the desired information during exposure of the probe. A technique for measuring neutron doses is disclosed in U.S. Pat. No. 2,564,626 to MacMahon et al. This device includes a boron, lithium, aluminum or lead element which is exposed to a neutron beam. Changes in the resistance of the element resulting from nuclear transmutation indicate the neutron dose. However, this device cannot be used to detect energetic ions. Accordingly, it is an object of the present invention to overcome the disadvantages associated with devices and methods known in the prior art. An object of the invention is to provide a simplified means and method for determining the energy and integrated dose of energetic ions and neutral particles in a plasma. It is an object of the invention to provide means for obtaining desired information regarding a plasma continuously during exposure of a probe to the plasma. SUMMARY OF THE INVENTION The invention comprises improved means and method for determining the energy and flux of particles in a plasma which do not suffer from the drawbacks associated with the prior art devices and techniques. The inventive means and method are capable of determining energy and flux of both ions and neutral particles, and can provide desired information continuously during exposure of a probe to the plasma. Additionally, a probe in accordance with the invention is capable of detecting low energy particles at low doses, and represents a substantial improvement over the prior art. The invention is based upon an observed change in the electrical resistance of a thin carbon film upon bombardment of the film by particles. A probe in accordance with the invention comprises a carbon film which may be evaporated onto a substrate. Particles bombarding the film become implanted therein, increasing the resistivity of the film. Means are provided for passing an electrical current through the film, and for measuring the resulting voltage across the film. From the measured voltage, the resistance, and thus the number of particles implanted in the film, can be determined. A probe in accordance with the invention may be shielded from low energy particles by applying an insulating layer thereto or by imposing a positive bias voltage on the carbon film. Imposing the positive bias voltage may also serve to separate charged particles from neutral particles. In use, a probe in accordance with the invention is exposed to a plasma edge, whereby the carbon film is exposed to the particles of the plasma. The desired information, from which particle energy and flux may be determined, may be continuously obtained during exposure of the probe to the plasma by monitoring the electrical resistance of the probe.