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

2. G. Rybka Vistas in Axion Physics 20122 Cavity Axion Searches Asztalos et al. PRL 104, 041301 (2009) Cavity experiments are sensitive to axions in the range 1 μev – 100 μeV ADMX experiment a B γ

3. G. Rybka Vistas in Axion Physics 20123 Axion Search Big Picture Source: T. Dafni, PATRAS 2010 (modified)

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{Q L,Q a } C mn = Form factor

6. Stored Energy (B 2 V) 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 TM 010 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 ~ 150 MHz 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 Higher frequency resonant structures f res ~ 10 x f 0 ~ 3 GHz

18. Yale experiment- single small cavity Cu resonant cavity at 34 GHz, cooled to T=4 K, tunable, TE 011 mode. 18Vistas 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