Helicons - Our Last Year Christian M. Franck and Thomas Klinger Max-Planck Institut für Plasmaphysik Teilinstitut Greifswald Euratom Association Outline Introduction The VINETA experiment Distinguishing the discharge modes Strategy / Concept Measurements and Results October 29, 2 DPP 2, Mini-Conference on Applications of Helicon Sources
Christian M. Franck DPP 2 2 large volume widely configurable magnetic field (e.g. homogeneous, gradients, mirror, magnetic null points, etc.) high density, steady state (Helicon), high β (-5...-2) Versatile Experiment to investigate basic questions on plasma instabilities and plasma waves. October 29, 2 experimental features: conceptual idea: The VINETA experiment
Amplifier 2.5kW cw 6kW pulse RF source 2-3 MHz matching unit or matching unit pump & gas control DC Power Supply DC Power Supply DC Power Supply DC Power Supply remote control Technical Data / Performance: Diagnostic Tools on VINETA length: 4.5m (4 9 ) diameter: 4cm (5.7 ) magn. field: <mt rf input power 2.5kW cw 6kW pulse afterglow decay 2.5ms in use: Langmuir Probes Ḃ -Probes just finished/trial runs: Laser Induced Fluorescence under construction Microwave Interferometer October 29, 2 Christian M. Franck DPP 2 3
RF power October 29, 2 I wish I d know. Christian M. Franck Electron heating within skin depth near the plasma surface. Large antenna currents generate inductive electric field. Ionisation where electric fields are largest (antenna edge). RF voltage drops across the sheath between plasma and antenna. 3 4 5 6 5 x x B =8mT 8 x 2 i capacitive inductive helicon wave sustained DPP 2 5 radial position [mm] The three discharge modes n [m 3] 4 P=2W P=75W P=.9kW
B stainless vacuum vessel diameter 4mm removable glas shields Pyrex extension diameter mm matched HF matched HF Using an external helicon antenna always works Use as Reference Using an internal helicon antenna doesn t work so far. But why? ) dielectric / metal boundary condition 2) direct electric contact to plasma? 3) secondary electrons? 4) RF discharge in whole volume? Place glass cylinder inside antenna. Shield antenna completely with insulator. Use a balun transformer. Need for understanding how the above factors influence the discharge mechanism. October 29, 2 Christian M. Franck DPP 2 5
External antenna as reference concentrate on low powers first (capacitive mode; 4W, 45mT,.Pa) 5 5 2 3 Density (norm.).75.5.25.8.6.4.2 October 29, 2 Christian M. Franck DPP 2 6
Does insulation help? 5 5 2 3 Density (norm.).75.5.25.8.6.4.2 5 5 2 4 6 Density (norm.).75.5.25.8.6.4.2 external 8 4 4 8 5 5 2 3 4 5 Density (norm.).8.6.4.2.5 Phase B z (π).8.6.4 October 29, 2 Christian M. Franck DPP 2 7 uncovered internal internal 5 5 covered
Magnetic fluctuation ( Ḃ ) probe Measuring Magnetic Fluctuations.5.4.3.2. rel. sensitivity [arb.units] Hall sensor typ. <khz, special <8kHz induction loop (using Faraday s law) B U ind ----- ωb t when B = B exp( iωt ) 2 2 Kalibration der 3 Achs B Punkt Sonde B x (ch) B y (ch3) B (ch2) z Phase /Deg. shielding and compensation for electric field pickup with larger diameter probes down to 5kHz B (ch) x B y (ch3) B z (ch2) 5 5 2 25 3 35 4 45 5 frequency [MHz] semi-rigid Koax mini-transformer 7mm x 8mm.4-8MHz B probe 6 wdgs. 6mm diam. 4 2 2 4 2 2 Voltage /mv 4 6 windings, diameter 4mm, centre tapped to earth 7..2 φ = 2 Antenna /arb. units B z Voltage /mv φ = 95 Antenna /arb. units B (8 rotated) z 5 5 2 time /ns October 29, 2 Christian M. Franck DPP 2 Backup
next step: higher powers (inductive mode; 75W, 75mT,.2Pa) 5 5 5 5 5 5.8.6.4.2 I i,sat (ma) 5 5.75.5.25 October 29, 2 Christian M. Franck DPP 2 Backup 2