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2) flow system
The mechanical laser mount forms a sealed block around the laser and the inlet/exhaust
jets. A pump draws on an exhaust jet pulling flow through the inlet jet and across the
laser mode. The inlet jet is an aerodynamically focused assembly with a sample nozzle of
500
µ
m diameter and a sheath nozzle of 760
µ
m diameter. The tip of the sheath jet sits
within 1 mm of the edge of the laser mode. Sample flows are between 1 and 100 sccm
and the sheath flow is typically 700 sccm. Particle velocity depends on sheath flow rate,
but is on the order of 50 to 100 m/s. The particles are confined to a region of space whose
extent is approximately 10 % to 20 % of the laser beam diameter which is 0.5 mm (
e
-2
intensity diameter). This yields a sizing resolution of approximately 2% to 5% of the
particle size. (See Figure)
PUMP
Mass flow controller for
sample flow
control/measure
electronic
flowmeter
for sheath
flow
measure
mechanical
needle valve
for sheath
flow control
filter
sample
inlet
sheath flow
confined sample stream
laser mode
optical
block
Figure: Schematic diagram of flow system
