Enhancing Cavity Performance at Varying Frequencies

1. Cavities at higher and lower frequenciesC. Hagmann, J. Hoskins, I. Stern, A.A. Chisholm, P. Sikivie, N.S. Sullivan, and D.B. TannerUniversity of Florida

2. Basic cavity is a right circular cylinder For ADMX, r = 21 cm • f = 550 MHz L = 100 cm Or:

3. Lower frequency • 30 m diameter, 3 T magnets have been built for energy storage • base frequency = 8 MHz • Capacitor for LC resonator • Hz to MHz • Each pair of plates must be in a grounded cage to avoid | -> | <- | -> | <- | 0 form factor

4. Signal strength • Power from the cavity is • QL ~ 70000(GHz/f)2/3 (ASE) and Qa ~106 • gγ ~ 0.97 (KSVZ); gγ ~ 0.36 (DFSZ)

5. 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.)

6. Power will decrease with frequency • Single cylinder: • Power decreases because the volume decreases as f -3, the Q decreases as f -2/3 while the mass increases as f. • Can use multiple cavities, tuned together and added in phase

7. Cavities must operate at the same frequency

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

9. Partitions reduce scale, increase frequency Efficient use of magnetic volume compared to, e.g., 4 parallel cylinders. Tune by moving rods from corner to center in each partition Segmented resonator

10. 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

11. Segmented Resonator • ~¼ scale prototype • TM010 frequency = 2.7 GHz • Q  25,000 (300K) • V  0.005 m3 • 3D model • Comsol • Single rod gives TM010 frequency range = 2.7 – 3.4 GHz

12. Segmented Resonator • ~¼ scale prototype • TM010 frequency = 2.7 GHz • Q  25,000 (300K) • V  0.005 m3 • 3D model • Comsol • Single rod gives TM010 frequency range = 2.7 – 3.4 GHz

13. Segmented Resonator • ~¼ scale prototype • TM010 frequency = 2.7 GHz • Q  25,000 (300K) • V  0.005 m3 • 3D model • Comsol • Single rod gives TM010 frequency range = 2.7 – 3.4 GHz

14. Segmented Resonator • ~¼ scale prototype • TM010 frequency = 2.7 GHz • Q  25,000 (300K) • V  0.005 m3 • 3D model • Comsol • Single rod gives TM010 frequency range = 2.7– 3.4 GHz • Corresponds to 870 – 1100 MHz for ADMX

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

16. Cavities must be added in phase

17. Cavity reflects (promptly) waves that are not on resonance Reflection dip at resonance, along with phase change Pound Drever-Hall (pdh) reflection locking

18. Phase-modulated light

19. As cavity tunes, phase of reflected carrier shifts

20. Each cavity can be driven to resonance at the carrier N-way splitter 5 GHz oscillator f 200 kHz sine Directional couplers Amplifier Mixer Cavities Tuning rod actuator

21. Above a few GHz

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

23. Can be tuned • For ADMX-HF; frequencies in ADMX would be 1.5 to 1.66 GHz

24. C2Q varies with tune

25. Approching 1 meV • Synthesize static magnetic field with q = qg • Current varies along z

26. Conceptual design • Dimensions ~ 3 m x 3 m x 6 m • Wire spacing ~ 2 mm • Number of wires ~ 4 x 106 (but only ~3000 planes) • Currents ~ 200 A

27. Summary • To go beyond the basic right circular cylinder tuning range and to retain the basic sensitivity at higher frequencies is not easy • Mode crossings must be accounted for (Ed Daw has 3 slides) • Good ideas are needed

28. THE END