Glow discharge
Glow discharge is started manually
when the tank pressure has reached 8x10-6 mbar. The diffusion pump plate valve
is closed, a
bleed valve causes
the pressure to rise to 10-3 mbar and the voltage and the current will rise
gradually to a peak of 2.8 kV and 1.3 A. This will last for a period of 7
minutes.
Meissner cooling
After glow discharge the tank pressure drops back quickly as the bleed valve
is closed and the diffusion pump plate valve opened. Liquid Nitrogen will
now start flowing through the Meissner
coils just below of the filaments. It takes about 20 minutes to cool the
coils down.
Tank pressure before evaporation
Before performing the evaporation the tank pressure is 6 - 8 x10-6 mbar.
Aluminum and filaments
There are 120 filaments in the tank,
located in a ring of 4.2 m in diameter, 2.0 m above the target.
Each coils (10 turns) is 75 mm long and 7.5 mm in diameter and is loaded with
two 55 mm long and 1.5 mm thick aluminum wires.
They are clamped to the filament on one side to make
sure they don't fall out just before evaporation. It happens sometimes that
drops of aluminum form and fall down during evaporation.
Evaporation
For evaporation all coils are heated up together. The ramp up of the filament
current is programmed and the maximum value of 7500 A (total current) is
reached after 25-30 seconds and will settle to 6000 A afterwards. After about
60 sec. all aluminum has deposited on the target and after 90 sec. the
filament current is switched off.
A total of 63 g of aluminum is evaporated, 3.5 g being deposited on the
13m² surface of the WHT primary mirror to create a reflective surface
100 nm thick.
Venting Procedure
Pumping continues while warming up the Meissner coils (20 minutes). Then the
tank pressure in increased to 10-1 mbar. After two hours we vent to 100 mbar
and leave it there for another 12 hours before venting to atmospheric pressure
and opening.
Aluminizing small mirrors
Uniform coating thickness has been optimized for the 4.2m primary mirror.
Smaller mirrors should be
supported on stands, such their
surface is sitting approximately at the same level in the aluminizing tank as
the surface of the 4.2m mirror.
I appreciate your comments, please contact Juerg Rey, juerg@ing.iac.es