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…