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Plasma ion sources for radioactive molecular ion beams

Plasma ion sources for radioactive molecular ion beams. Pekka Suominen 2010 CERN. Contents. Two new plasma ion sources tested at offline separator COMIC (Compact Microwave and Coaxial) 2.45 GHz coaxial ¼-wave resonator antenna HELICON (miniature RF-source with B-field)

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Plasma ion sources for radioactive molecular ion beams

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  1. Plasma ion sources for radioactive molecular ion beams Pekka Suominen 2010 CERN

  2. Contents • Two new plasma ion sources tested at offline separator • COMIC (Compact Microwave and Coaxial) • 2.45 GHz coaxial ¼-wave resonator antenna • HELICON (miniature RF-source with B-field) • 100 MHz helical antenna • Technical developments • Double stage extraction (accel-accel) • Radiation hard coaxial RF power line for frontend 7 (new FE for GPS)

  3. Why? • Interest for CO2 and other molecular beams • Ideal extraction for Carbon and Oxygen isotopes • High purity (separated as CO2 and broken in EBIS) • Main things we aim for • High gas efficiency • Stable and easy operation • Reasonable emittance • Compact size & robust construction (high radiation environment)

  4. COMIC”Compact Microwave and Coaxial”Q-COMIC”Quartz-COMIC”

  5. Q-COMIC • Developed by P.Sortais / LPSC, Grenoble • Plasma ignition with 30 W of 2.45 GHz Microwaves • Quartz plasma chamber developed for ISOLDE D = 50 mm

  6. Q-COMIC in ISOLDE target unit Pictures: (c) P. Sortais 17.8 KV LPSC testbench 0.20 mA

  7. Experimental setup at Offline

  8. Main results for Q-COMIC

  9. HELICON”A helicon is a low frequency plasma wave 10 kHz(Ignited with RF power of 1 … 100 MHz)”

  10. Helicon vs. RF plasma source • RF source normally has no axial B-field • Helicon requires B-field • Introduces low frequency travelling waves in the plasma  electrons can ”surf” with the wave and gain energy  ionization by electron bombardment (c) Nasa / VASIMR

  11. ISOLDE Helicon source

  12. RF impedance matching • Matching is difficult due to stray capacitances • Normally coaxial line is around 100 pF/m and 200 nH/m Coaxial line affects strongly to the resonant frequency For long coaxial line one cannot get resonance at the antenna Match-box 50 cmcoax line Antenna

  13. Maxwell-Boltzmann energy distribution Relative e-density CO2 breaks CO2 1st ionization potential Argon Helium Xenon Electron energy [eV]

  14. Double stage extraction (accel-accel) Puller @ 0V-potential Base @ HV (for example 30 kV) Intermediate electrode @ HV - 3kV (27 kV to ground)

  15. Rad hard RF Coaxial line for FE7 • We need a solution, which can survive 5 years at online

  16. Outlook • More understanding of the plasma is necessary • Langmuir probing, Beam energy distribution measurements, plasma simulations • New gases to study molecule breaking • for example N2O (6.3 eV to break / 12.9 eV to ionize) • COMIC • More power • New Solid-state MW-generator 180 W, 2450 MHz +/- 25 MHz • Effect of magnetic field at the extraction • HELICON • Tests of different antennas • Improved RF impedance matching • plate capacitors in vacuum • Variable magnetic field (coils) • Effect of magnetic field at the extraction • Tests by varying the plasma electrode position

  17. Special thanks to: P. Sortais, LPSC-Grenoble, France O. Tarvainen, JYFL, Finland Thank you for your attention ! Do not hesitate to contact me for additional information pekka.suominen@cern.ch

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