Transcript "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
γCavity
experiments are sensitive to axions in the range
1 μev – 100 μeV
a
B
G. Rybka Vistas in Axion Physics 2012
2
Axion Search Big Picture
Source: T. Dafni, PATRAS 2010 (modified)
G. Rybka Vistas in Axion Physics 2012
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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)
Power from Axion-Photon Conversion
2
BV
P
QC mn
TS
•
•
•
•
B = Magnetic Field Strength
V = Volume of cavity(ies)
Q = min{QL,Qa}
Cmn = Form factor
Stored Energy (B2V)
2
BV
P
QC mn
TS
• 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
Cavity Q
2
BV
P
QC mn
TS
• 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
System noise temperature
2
BV
P
QC mn
TS
• 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)
Cavity form factor
2
BV
P
QC mn
TS
• 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
Push to higher frequency
Or:
For ADMX,
r = 21 cm
f = 550 MHz
L = 100 cm
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.)
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)
ADMX operated a 4 cavity array
Did not fill the cavity volume well
Cavities must operate at the same
frequency
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
Need up to 32 cavities
Covers about 1 decade in axion mass
Detecting higher axion masses
Higher frequency resonant structures
fres ~ 10 x f0 ~ 3 GHz
Yale experiment- single small cavity
• Cu resonant cavity at 34 GHz, cooled to T=4 K, tunable,
TE011 mode.
Vistas in Axion Physics 2012
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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