| |
Faraday cup system
for measuring lowest ion beam current (for positive charged particles
only)
comprising of the Faraday Cup Amplifier, Faraday Cup and vacuum
feedthrough.
Amplifier
with direct display in µA/cm² (analog), measurement
range: 3,
10, 30, 100, 300 and 1000 µA/cm², Bias voltage -10V,
analog
output 0-5V (0-100% FS).
Faraday
Cup FC-70A of Kimball Physics, Input max 2 Watts, Aperture
Ø1,596mm
(area 2mm²), max operation temperature 150°C
The
Kimball Physics model FC-70C Faraday cup, connected to an ammeter, is
used to collect and measure charged particle current, such as the beam
emitted from an electron or ion gun. The FC-70C is UHV compatible and
bakeable to 250ºC.
The
Faraday cup consists of a hollow stainless steel cylinder closed at the
base, with an appropriately-sized aperture for collecting the electrons
or ions. The standard aperture diameter is 1.596 mm which yields a 2.0
mm2 area; however, custom aperture sizes are available. An outer,
grounded box completely encloses the Faraday cup to provide shielding.
An electrical connection is made from the side of the Faraday cup to a
shielded, Kapton-insulated coaxial cable with an unterminated end. The
current is then conducted through a user-supplied vacuum electrical
feedthrough to an ammeter.
A
feedthrough on a CF-16 (1⅓“ OD) or larger flange, or a custom
designed feedthrough, is available from Kimball Physics as a separate
option.
To reduce
scattering of electrons or ions collected in the Faraday cup and to
reduce secondary electron emission, the Faraday cup can be electrically
biased. For electrons or negative ions, +50 V is typically adequate,
and for positive ions, -50 V. This can be accomplished by placing a
battery between the vacuum feedthrough and the ammeter.
Power input cautions:
For
continuous measurement, the maximum beam power recommended into the
standard FC-70C Faraday cup is 100 milliwatts. The Faraday cup
temperature should not be raised above 150ºC due to outgassing.
The
wattage can be calculated by multiplying the beam current times the
electron acceleration voltage; for example, 1 mA at 1000 eV gives 1 W,
which is much too high for continuous measurement. The temperature of
the Faraday cup increases approximately linearly with the power input.
A 50 mW input results in approximately 150ºC.
To use the
Faraday cup at high power, measure currents briefly and then let the
Faraday cup cool down before repeating the measurement. Due to heat
capacity of the cup, a 500 mW input into the FC-70C will raise the cup
temperature from room temperature to approximately 150ºC in 80
sec. The cup will cool from 200ºC to room temperature in about 15
min.
Care must
always be exercised with a highly focused beam, as a high power density
can bring the Faraday cup to melting temperature. Spot size should be
greater than 1 µm.
Features /
Options of the Faraday Cup:
- input:
100 mW max continuous
- aperture: 1,596mm
diameter (2mm2 area)
- shield size: 53 mm x 22 mm x 10 mm (2.1” x 0.9” x 0.4”)
- Cable length 30 cm, Optional: 1 m or 3 m
- other
aperture sizes available on request
- max.
operating temperature: 150°C
- cup
assembly completely enclosed in grounded shield
- UHV
Kapton-insulated cable
- vacuum to 10-10 Torr
- Feedthrough
CF16 (1.33"OD) with BNC on the air side and Microdot on the vacuum side
of the flange.
|
