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Cavities and Magnets Working Group

Cavities and Magnets Working Group. Darin Kinion (LLNL) 4/26/2012. Cavity Axion Searches. Asztalos et al. PRL 104, 041301 (2009). ADMX experiment. Cavity experiments are sensitive to axions in the range 1 μ ev – 100 μ eV. a. γ. B. Axion Search Big Picture.

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Cavities and Magnets Working Group

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  1. Cavities and Magnets Working Group Darin Kinion (LLNL) 4/26/2012

  2. Cavity Axion Searches Asztalos et al. PRL 104, 041301 (2009) ADMX experiment Cavity experiments are sensitive to axions in the range 1 μev – 100 μeV a γ B G. Rybka Vistas in Axion Physics 2012

  3. Axion Search Big Picture Source: T. Dafni, PATRAS 2010 (modified) G. Rybka Vistas in Axion Physics 2012

  4. Big Questions: • Should we still be looking for axions? • Yes! • Should we be using microwave cavities to search for axions? • Yes! • What mass (frequency) range should be to goal? • No overwhelming theoretical consensus • Stick to “natural” range for existing cavity amplifier technology (100 MHz – 40 GHz)

  5. Power from Axion-Photon Conversion • B = Magnetic Field Strength • V = Volume of cavity(ies) • Q = min{QL,Qa} • Cmn = Form factor

  6. Stored Energy (B2V) • NHMFL – very interesting array of large volume, high-field magnets • Built magnets could possibly be utilized, but no natural fits for ADMX • New magnets – very expensive ($7M-$50M) • Retrofit, renovation of existing magnet could be problematic

  7. Cavity Q • ADMX-HF exploring the idea of using superconducting thin films to reduce losses in cavity walls and tuning rods • Requires homogenous B field to reduce radial component • Factor of 6 improvement possible

  8. System noise temperature • Combination of physical temperature and first amplifier (mostly) noise temperature • Superconducting amplifiers provide near quantum limited performance up to ~ 8-10 GHz • HFETs above 8-10 GHz, pending future amplifier developments • Quantum Noise ~ hf/kb – above 6 GHz reduce need for dilution refrigerators (He3 systems)

  9. Cavity form factor • Drives choice of mode typically TM010 for the right-circular cavity • Higher modes provide path to higher frequencies, as well as in situ testbed for new amplifiers • Extensive use of Finite Element software

  10. Push to higher frequency For ADMX, r = 21 cm • f = 550 MHz L = 100 cm Or:

  11. Length cannot get too long • The longer the cavity, the more TE modes there are in the tuning range. • With metal tuning rod, there are also TEM modes at ~ integer*c/2L ~ 150MHz for 1 m L • Typical values L ~ 5r = 2.5*diameter Modes for r = 3.6 cm, L = 15.2 cm cavity. d is the distance the metal rod is from the center. (Divide frequencies by 6 for ADMX.)

  12. Push to higher frequencies • As the cavity(ies) get smaller the question becomes what to fill the remaining magnet volume with • Multiple cavities • Small cavities with TM010-like modes • More magnet wire (increase B0 as volume shrinks)

  13. ADMX operated a 4 cavity array Did not fill the cavity volume well

  14. Cavities must operate at the same frequency

  15. Segmented Resonator • ~¼ scale prototype • TM010 frequency = 2.7 GHz • Q  25,000 (300K) • V  5 liters • Scaled to ADMX, would have f = 850 MHz • 4 segment resonator would have f = 1.1 GHz

  16. Need up to 32 cavities Covers about 1 decade in axion mass

  17. Detecting higher axion masses fres ~ 10 x f0 ~ 3 GHz Higher frequency resonant structures

  18. Yale experiment- single small cavity • Cu resonant cavity at 34 GHz, cooled to T=4 K, tunable, TE011 mode. Vistas in Axion Physics 2012

  19. Summary • Current searches are underway • ADMX & ADMX-HF • Yale search at 30+GHz • Incremental improvement possible in B,V but very expensive • Factor of 6 possible in Q • Strategy for covering higher frequencies is a real issue

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