May 20th Research Update

The ion gauge failed on the MOT chamber today. For some reason the grid was contacting the collector. We cooled down the ion pump in preparation for venting the chamber. Is there anything that won’t go wrong with this chamber? Why has it worked all this time? How did something inside the vacuum change? Perhaps we started something by banging on the window to get it off.

In other efforts we successfully melted the lead in the vapor cell on both windows. However, we learned something about temperature gradients in the cell. At first we were afraid that a rush of suddenly molten lead would rush to the center of the cell, up around the lip, and down into the Sr if we had the cell at too steep an angle to prevent the lead from falling against the window. That was never a valid fear apparently.

I initially brought just the window up to 340 C. This was enough to soften the lead enough to fall through against the inner diameter of the cell which is a little narrower than the inner diameter of  the gasket. So we brought the cell up 380 C. This was enough to get the gasket to fall a little farther in and produce a small molten bubble at the bottom. Bringing up the temperature on the valve to 380 and the cartridge heater to 350 C only improved things slightly.

Not until we covered the windows with aluminum foil caps did the lead suddenly melt. The shallowness of the angle left most of the lead at each end in a puddle up against the windows. We could have easily used a much steeper angle and gotten most of the lead away from the windows. Also I would have made the foil just big enough to cover the inner diameter of the cell as opposed to the gaskets.

The noticeable difference in temperature between the heater and the lead made me wonder about the window. Not only were there significant temperature gradients from the center to the edge, there were significant gradients from one side of the glass to the other. At the glass to metal interface the temperature was less than 300 C when the heater was at 380, and the center of the glass was at ~170 C. This means that running at temperatures to get a Sr vapor puts the center of the window as the coldest part of the cell even when the windows are the heat source. We’re going to machine covers for the windows to prevent them from air currents and to reduce the gradients across them.