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Leicester lab study
CAVIAR spectroscopy meeting at UCL 30th April 2010
Stephen Ball & Mark Watkins
Heated source
Water reservoir heater
Hot = 50C
Psat (H2O) at 50 C = 0.2 Atm
Psat(H2O) at 20 C = 0.02 Atm
Valve heater
Hotter ~ 80C
No more condensation in valve
Slit design
Previously:
rubber seal against slit
Leaks! Sticky opening!
armature
standard valve
“poppet”
pin hole orifice
1 1 30mm volume
exchangeable slit(100m 30mm)
face plate
New design
Standard valve with slit attached to face plate: reproducible operation
Slit valve performance
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
0.00002
0.00004
0.00006
0.00008
0.00010
0.00012
0.00014
0.00016 Likely pumpsaturation region
Optimisedopening time
Nominalopening
Valve opening optimised at: - 327 s - 2.5 bar backing pressure in Ar
Io
n G
aug
e P
ress
ure
/ T
orr
Opening Time / s
2.5 bar 5.0 bar 8.5 bar
Ringdown time = 40 s
Clean pumping system
Mist filter
Foreline trap
Eliminates oil contamination on optics – cavity now maintains ringdown time for days
Typical cavity parameters
Mirror reflectivity Absorption path length # of passes thru cavity(740-770nm)
10km 15000
10km
66cm66cm
3cm
i.e ~ 25 fewer molecules intercepted by light beam
H2O spectra at reduced pressure
H2O (~1.5%) in Ar at room temp; PTOTAL = 26 Torr
[H2O] = 1.3 1016 molecules cm3
Stdev(resid) = 2.7109 cm1
H2O (~1.5%) in Ar at room temp; PTOTAL = 106 Torr
[H2O] = 5.2 1016 molecules cm3
Stdev(resid) = 6.2109 cm1
Comparison with Shillings et at BBCRDS paper
Water dimer signal?Heated water reservoir heater:
Psat (H2O) ~ 0.2 Atm cf 0.02 Atm at 20 C
10
Lines around 740nm are weaker than band centre, 10
yet oscillator strength (H2O)2 H2O(…depends if rotational cooling narrows dimer bands)
Mole fraction of dimer in source?
0.01-0.05
Things we’ve not thought of
0.1
NET 0.1-0.5
O2 spectra at reduced pressure
Another (stronger) absorber to put through valve to aid alignment
Peak differential cross sections:
O2 = 51024 cm2 molec1 at 760nm band head (at our resolution)
NO2 = 31021 cm2 molec1 around 745nm
O2 (100%) PTOTAL = 10.2 Torr
[O2] = 3.3 1017 molecules cm3
Stdev(resid) = 7.2109 cm1
Where now?
• Mark’s contract finishes today
• Engineering, technical side & spectroscopic method => as good as it can be for this experiment
• Leicester CAVIAR end date = 30/10/2010(No cost extension to synchronise with other groups?)
• BBCRDS OK to say in Leicester over the summer
• Vacuum chamber available until at least Oct
• Steve wants to do experiments…
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