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Abstract:
The CLOUD (Cosmics Leaving OUtdoor
Droplets) experiment at CERN (European Council for
Nuclear Research) investigates the nucleation and growth
of aerosol particles under atmospheric conditions and
their activation into cloud droplets. A key feature of the
CLOUD experiment is precise control of the experimental
parameters. Temperature uniformity and stability in the
chamber are important since many of the processes under
study are sensitive to temperature and also to contaminants
that can be released from the stainless steel walls by upward
temperature fluctuations. The air enclosed within the 26 m
3
CLOUD chamber is equipped with several arrays (“strings”)
of high precision, fast-response thermometers to measure its
temperature. Here we present a study of the air temperature
uniformity inside the CLOUD chamber under various
experimental conditions. Measurements were performed
under calibration conditions and run conditions, which
are distinguished by the flow rate of fresh air and trace
gases entering the chamber at 20 and up to 210 L min
−
1
,
respectively. During steady-state calibration runs between
−
70 and
+
20
◦
C, the air temperature uniformity is better
than
±
0.06
◦
C in the radial direction and
±
0.1
◦
C in the
vertical direction. Larger non-uniformities are present during
experimental runs, depending on the temperature control
of the make-up air and trace gases (since some trace gases
require elevated temperatures until injection into the cham-
ber). The temperature stability is
±
0.04
◦
C over periods of
several hours during either calibration or steady-state run
conditions. During rapid adiabatic expansions to activate
cloud droplets and ice particles, the chamber walls are
up to 10
◦
C warmer than the enclosed air. This results in
temperature differences of
±
1.5
◦
C in the vertical direction
and
±
1
◦
C in the horizontal direction, while the air returns
to its equilibrium temperature with a time constant of about
200 s.
