USA As Represented by Secretary of the Navy

David D Blackwell

Richard F Fernsler

David N Walker

Scott G Walton

David R Boris

William E Amatucci

An apparatus and method for determining plasma parameters such as plasma electron density n. The probe apparatus includes an LC resonance probe comprising an inductive element and a capacitive element connected in series. The capacitive element of the probe can be in the form of a parallel plate capacitor, a cylindrical capacitor, a spherical capacitor, or any other suitable capacitor. The configuration of the probe apparatus gives it a characteristic resonance frequency &#x3c9;which can be determined by a circuit analysis device. When the capacitive element of the probe apparatus is placed in a plasma, the probe exhibits a new resonance frequency &#x3c9;, which is different from &#x3c9;because of the dielectric constant &#x2208; of the plasma. The difference in resonance frequencies can be used to determine plasma density n, where 

LC resonance probe for determining local plasma density

An rf probe is placed within a plasma and an rf signal from a network analyzer for a given dc bias voltage Vis applied The frequency applied by the network analyzer, &#x3c9;, is less than the plasma frequency, &#x3c9;, and therefore is not in the resonant absorption range (&#x3c9;=&#x3c9;) used to determine electron density in typical rf impedance probe operation. Bias voltages at the applied frequency are applied to the probe in a series of voltage steps in a range which includes the plasma potential. At each bias step, a value of Re(Z), the real part of the plasma's complex impedance, is returned by the analyzer. A local minimum in the real part of the impedance Re(Z) occurs where the applied bias voltage Vequals the plasma potential &#x3c6;. The plasma potential &#x3c6;can be found by taking the first derivative of Re(Z) with respect to Vp, and finding the value of Vp at which within error tolerances.

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Alexandria, VA, United States of America

David D Blackwell