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Big Bang Blackbody Simulator

The Cosmic Microwave Background (CMB) is radiation left over from the Big Bang, and as such it provides us with a direct view into the early universe. By mapping out the CMB we are able to view the universe as it appeared when it was only 400,000 years old.

The CMB radiates as an almost perfect blackbody at a temperature of 2.7 Kelvin. It is extremely important to be able to simulate the CMB as a blackbody source to allow us to test instrumentation intended for CMB observation.

The purpose of the Big Bang Blackbody Simulator (BBS) is to construct a blackbody 'cold load' to measure the microwave response of superconducting Transition-Edge Hot-Electron Microbolometers. These detectors will allow us to measure the faint polarization signals in the CMB that are expected to be the result of gravitational waves generated in very first moments of the universe.

Finline Structure
The finline structure of the Big Bang Blackbody Simulator

The BBS consists primarily of a 100 Ohm chip resistor mounted on a copper finline structure. The resistor absorbs incident radiation and radiates as a blackbody when heated. The entire BBS will be mounted in a copper waveguide. The copper finlines reduce impedance in the transition from the resistor to waveguide. Once mounted in the copper waveguide, the BBS will be placed near the detectors in a cryostat and cooled to below 1 Kelvin. The BBS can then be heated to generate a well calibrated source of microwave radiation.

If these polarization signals are detected, they will allow us to study physics at energy scales far beyond the reach of particle accelerators. The Transition-Edge Hot-Electron Microbolomoters are also a candidate technology for the CMBPol satellite, which NASA may launch at the end of this decade.
Finline Picture
Actual size of the BBS

Radiation Coupling: Gaussian Optics

A major challenge we face with the BBS is coupling its radiation to THM detector. Radiation from the BBS must pass through waveguides, free space, and antennas, encountering different impedance levels and electromagnetic symmetries. Our beam coupling patterns are based on a special type of optics called Gaussian optics. We use Gaussian optics because geometrical optics assumes that the wavelength is small compared to the size of the structures in the optical design, which is not the case with our design. Gaussian optics allows us to model the long wavelengths of the CMB.

Once the microwave radiation leaves the BBS, it travels through a waveguide into a horn antenna, where it converges to what is called a Gaussian beam waist. In geometrical optics, we make the approximation that beams converge to a single point, in Gaussian optics, however, the beam reaches a minimum size of finite width at its beam waist.

After the microwave radiation leaves the horn, it briefly spreads out through free space. It then enters a focusing lens made of a plastic called Rexolite, which focuses the beam to a new beam waist of about 120% larger than the original waist in the horn. The new beam waist is located right at the front of the silicon hyper-hemispherical lens. This final lens then focuses the beam into the double slot antenna, the first component of the THM.

Last updated: 7/27/10

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