Multi Resonance Probe/Plasma Absorption Probe

The head of the Multipole Resonance Probe (MRP) con­sists of two metallic hemispheres working as electrodes of a small an­ten­na and separated by a dielectric layer. The head’s diameter is 6 mm and is en­clo­sed by a dielec­tric tube (thickness of 1 mm) which is im­mer­sed in the plas­ma. This makes the probe insensitive against ceramic coatings which is needed in many applications. A self-developed electronic or, as an alternative, a net­work ana­ly­zer feeds an rf si­gnal sweep to the an­ten­na and dis­plays the fre­quen­cy de­pen­dence of the power ab­sorp­ti­on. This method is known as active plasma resonance spectroscopy (APRS). From the ab­sorp­ti­on spec­trum the value of the elec­tron den­si­ty is cal­cu­la­ted. In comparison to other APRS probes, the MRP shows a geometrical and electrical symmetry which significantly decreases the complexity of the model. The measurement range of the electron density is between 10¹³ m^-3 and >10¹⁸ m^-3. Time resolution lies in the range of sub ms, depending on the used electronic.

The plas­ma ab­sorp­ti­on probe (PAP) was in­ven­ted as an eco­no­mi­cal and ro­bust dia­gnostic de­vice to de­ter­mi­ne the elec­tron den­si­ty di­stri­bu­ti­on in tech­ni­cal plas­mas. It con­sists of a small an­ten­na en­clo­sed by a dielec­tric tube which is im­mer­sed in the plas­ma. A net­work ana­ly­zer feeds a rf si­gnal to the an­ten­na and dis­plays the fre­quen­cy de­pen­dence of the power ab­sorp­ti­on. From the ab­sorp­ti­on spec­trum the value of the elec­tron den­si­ty is cal­cu­la­ted. The ori­gi­nal eva­lua­ti­on for­mu­la was based on the dis­per­si­on re­la­ti­on of plas­ma sur­face waves pro­pa­ga­ting along an in­fi­ni­te dielec­tric cy­lin­der. In this let­ter the aut­hors pre­sent the ana­ly­sis of a less idea­li­zed con­fi­gu­ra­ti­on. The cal­cu­la­ted spec­tra are in good qua­li­ta­ti­ve agree­ment with their ex­pe­ri­men­tal coun­ter­parts, but dif­fer con­s­i­der­a­b­ly from those pre­dic­ted by the sur­face wave an­satz. An eva­lua­ti­on sche­me which takes our fin­dings into ac­count will im­pro­ve the per­for­mance of the PAP tech­ni­que fur­ther.

The measurement range of the electron density is between 10¹⁴ m^-3 and 10¹⁸ m^-3. Time resolution lies in the range of sub ms.