Cavities at higher and lower frequencies
C. Hagmann, J. Hoskins, I. Stern, A.A.
Chisholm, P. Sikivie, N.S. Sullivan, and D.B.
Tanner
University of Florida
Vistas in Axion Physics
April 2012
ADMX
Basic cavity is a right circular cylinder
Or:
For ADMX,
r = 21 cm
 f = 550 MHz
L = 100 cm
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
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)
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
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.)
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
Cavities must operate at the same frequency
Vistas in Axion Physics
April 2012
ADMX
ADMX operated a 4 cavity array
Did not fill the cavity volume well
Vistas in Axion Physics
April 2012
ADMX
Segmented resonator
•
•
•
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
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
Need up to 32 cavities
Covers about 1 decade in axion mass
Vistas in Axion Physics
April 2012
ADMX
Cavities must be added in phase
Vistas in Axion Physics
April 2012
ADMX
Pound Drever-Hall (pdh) reflection locking
•  Cavity reflects (promptly) waves that are not on
resonance
•  Reflection dip at resonance, along with phase change
Vistas in Axion Physics
April 2012
ADMX
Phase-modulated light
Vistas in Axion Physics
April 2012
ADMX
As cavity tunes, phase of reflected carrier shifts
Vistas in Axion Physics
April 2012
ADMX
Each cavity can be driven to resonance at the carrier
5 GHz oscillator
N-way splitter
φ
200 kHz sine
Directional couplers
Amplifier
Mixer
Cavities
Tuning rod actuator
Vistas in Axion Physics
April 2012
ADMX
Above a few GHz
Vistas in Axion Physics
April 2012
ADMX
Detecting higher axion masses
Higher frequency resonant structures
Vistas in Axion Physics
April 2012
fres ~ 10 x f0 ~ 3 GHz
ADMX
Can be tuned
•  For ADMX-HF; frequencies in ADMX
would be 1.5 to 1.66 GHz
Vistas in Axion Physics
April 2012
ADMX
C2Q varies with tune
Vistas in Axion Physics
April 2012
ADMX
Approching 1 meV
•  Synthesize static
magnetic field with q = qγ
•  Current varies along z
Vistas in Axion Physics
April 2012
ADMX
Conceptual design
•  Dimensions
~3mx3mx6m
•  Wire spacing
~ 2 mm
•  Number of wires
~ 4 x 106
(but only ~3000 planes)
•  Currents
~ 200 A
Vistas in Axion Physics
April 2012
ADMX
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
Vistas in Axion Physics
April 2012
ADMX
THE END
Vistas in Axion Physics
April 2012
ADMX