PENN ARRAY FRIDGE CYCLING PROCEDURE
===================================

08may03 bsm
 -distilled from mark supanich emails
 -this is their current lab procedure
	precise numbers may be different
	in the production dewar.

========================================

degrees of freedom you manipulate
----------------------------------
*4He heat switch (turning on a 50 KOhm resistor closes the switch;
turning it off opens it; a few minutes delay is required for the state
to be acquired.  the heat switch itself is a pot with liquid helium
and some charcoal absorber... when you turn the resistor on the helium
evaporates and the pot as a whole becomes a good thermal link--
confusingly similar to the fridge itself but different in function)
*3He heat switch (ditto)
*4He heater (420 Ohm resistor-- mounted on 4He charcoal) 
*3He heater (420 Ohm resistor-- mounted on 3He charcoal)

monitor points
--------------
*4He charcoal temperature
*3He charcoal temperature
*4He evaporator
*3He evaporator (aka "fridge"-- this is the temperature of the focal
plane)
*Base plate temperature (this is the terminus of the 1st stage
refrigerator [pulse tube cooler], and is what the 4He fridge operates
off of)

initial state
-------------

*both heat switches "open".  during normal operation and cooldown,
both heat switches will be closed, and they are typically only opened
to cycle the sorption fridge.

========================================

-wait for Base Plate T<3.5 K. this will usually be the case in normal
operation, but not necessarily during cooldown of the entire dewar
from room temperature.

-turn off power to both heat switches and wait 5 minutes for them to
"open"

-Apply 10 V to 420 ohm Heater on 3He Charcoal and 30 V to 420 ohm
heater on 4He Charcoal

-Turn 3He heater off when charcaol reaches 35 K

-Turn 4He Heater off when charcoal reaches 50 K

-Wait until 4He evaporator reaches 3.5 K then apply 10 V to 50 KOhm
heater on 4He Hs to turn it on

-Wait until 4He evaporator reaches 0.75 K then apply 10 V to 50 KOhm
heater on 3He Hs to turn it on

-Cycle complete when 3He Fridge reaches 0.25 K

-Total cycle takes approxiametly 6.5 Hrs (more like 5.5 if we only
wait for the 3He evap to get to 0.300 K)