Resonant Cavity

The resonant cavity is a circular cylinder, 1 meter long and 0.5 meter diameter, made from copper-plated stainless steel and containing up to 2 moveable tuning rods. The electric field within the cavity is sampled by an electric field probe connected to the ultra-low-noise receiver electronics. We search for axions by slowly scanning the cavity resonant frequency across a frequency range by adjusting tuning rods. Metal tuning rods increase the cavity resonant frequency as they are translated nearer to the cavity center.

Here is a graphical representation of the electric field as the rods move towards the center. Note that the magnitude of the electric field is not represented accurately, meaning the yellow portions of the plot at the start of the animation are not the same magnitude as yellow portions in the middle of the animation.

This is a graph showing the resonant frequency of the cavity vs. the angle of the rods, where the angle is 0 for a rod when it is closest to a wall and 180 when it is closest to the center. 360 degrees is when both rods are closest to the center.

Magnet

The magnet is a superconducting solenoid. It consists of 37700 niobium titanium windings (bore 60 cm, stroke 1 m), and has a self-inductance of 534 Henrys. At full-field strength, the central field is 7.92 Tesla, and the energy stored in the magnetic field is about 15 MJoules. By comparison, the world's largest stored-energy magnet as of 5 years ago was made by Oxford Instruments, with stored-energy of 27 MJoules.

Receiver

The ultra-low noise microwave receiver is the centerpiece of this experiment. Using the world's quietest amplifiers in this frequency region (made by Richard at NRAO), the receiver was built (by Leslie) in the mid-90's. Briefly, the receiver downconverts microwave power from within a 50 kHz bandwidth about the cavity resonant frequency to the same bandwidth centered in the audio at 35 kHz. At each tuning rod setting, the cavity resonant frequency is precisely determined by a swept transmission measurement. The receiver electronics then acquires a power spectrum representing the electromagnetic field in the cavity about the resonant frequency f0. Each single-sided power spectrum consists of four hundred 125 Hz frequency bins covering the bandwidth of 50 kHz about the cavity resonant frequency. Ten thousand such sequential spectra at the same tuning setting are averaged together in the hardware. The set of averaged power spectra for each of the cavity resonant frequencies, together with other experimental parameters constitutes the raw data and is saved to disk. As discussed earlier, the experimental signature of axions is excess power in a bandwidth of about 10-6 f0, corresponding to a peak of about 6 bins wide in a power spectrum. The dominant background is broadband Johnson (thermal) noise from the cavity and the receiver electronics.