So I did a simple comsol model of laser heating of a silicon disk, with only radiation, to see the temperature variation at steady state, which could be the limiting factor for high Q at 123 K, due to the thermalelastic effect.
The model just uses a simple 2 inch disc, at 0.028 cm thick, with the flats not incorporated in yet.
I had to search for silicon thermal conductivity and heat capacity at low temperatures, settling with k= 800 W/(m K) and C_p= 300 j/(kg K) from refering to papers. Will check LIGO documents for more accurate versions.
I put an arbitary boundary condition of constant temperature of 123 K on a spot .2 mm in radius, to simulate a beam.
Other arbitary values include 77 K for ambient and a surface emissivity of 0.5.
The laser is off center, because that it where the laser will enter the current setup.
We can see that the power required is .02 W, which seems reasonable.
The model is consistent with the analytic model I made with the laser beam at the centre of the disc. See last two figures.
I'm still trying to get the time dependence to work, as it is just giving me nonsense right now.
Some thoughts: beam radius affects the temperature variation quite significantly, with a fat beam (1 mm radius) having half the temperature variation as a beam of .2 mm radius
I think the halo is just a trick of the eye, but I could be wrong.
Things to do:
Find the time scale of the system, as we want to modulate the laser to adjust the temperature, which will then be run though the mode ringer to measure Q to find the zero crossing
Change the heat source to be an actual laser
Add in the solid mechanics part
Add in the sapphire lens underneath