MODEL FC-66 FARADAY CUPSPECIFICATIONS
Faraday cup
inside groundshieldFC-66

Output
BNC
Aperture

Aperture size

5.1 mm dia.

Length70 mm, including BNCDiameter

13.2 mm dia. shield

Operating temperature

350ºC max.

Input power continuous

4 watts max. recommended

Figure 1 FC-66 Faraday Cup
INSTALLATION AND OPERATING PROCEDURE
INTRODUCTION
The Kimball Physics model FC-66 Faraday cup,connected to an ammeter, is used to collect and measurecharged particle current, such as the beam emitted from anelectron or ion gun. The FC-66 is UHV compatible and fullybakeable
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. An outer,grounded cylinder provides shielding.
An electricalconnection is made to the base of the Faraday cup,
terminating in a BNC. The current is then conducted to avacuum feedthrough and so to an ammeter.
A feedthrough on a 1a CF or larger flange, or a customdesigned feedthrough, is available from Kimball Physics as aseparate option.

High beam power can melt Faraday cup
components. Maximum recommended
beam power into the Faraday cup is
4 W continuous.

1. Position the Faraday cup in the target area inside thevacuum chamber.

2. Connect the output BNC on the back of the Faraday cup
to an electrical feedthrough on the vacuum system (anin-vacuum coaxial cable/ CF flange/ BNC assembly isavailable from Kimball Physics).

3. Connect the feedthrough to a suitable user-supplied
POWER INPUT CAUTIONS
For continuous measurement, the maximum beam
power recommended into the standard FC-66 Faradaycup is 4 watts. The Faraday cup temperature should not
be raised above 350ºC due to outgassing
The power input can be calculated by multiplying thebeam current times the electron acceleration voltage; forexample, 1 mA at 20 keV gives 20 W, which is much too highfor continuous measurement.
The temperature of the
Faraday cup increases approximately linearly with the power
input. A 2 W input results in approximately 150ºC, and a 4 W
input results in 300ºC.
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 20 W input for 10 sec into the FC-66 will raise the cup
temperature by approximately 150ºC. 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 in the impact area of the beam.
For example, an electron gun with an output of 1 mA at
10 keV focused to a 1 mm spot size has a power density of2
approximately 13 kW/cm . Assuming no heat flow, this would
bring the Faraday cup stainless steel at the spot to its melting
point in only 11 sec.

ammeter. NEVER collect beam current in the Faraday
cup unless the cup output is connected to anammeter or grounded. Without proper termination, theFaraday cup terminal will charge up to full beam energy
and electrical discharging will result.

4. Calculate the expected power input into the Faraday cup
and check that it is within the acceptable range. See thediscussion of power input cautions.

5. Measure electron or ion current with the ammeter.
BIASING THE FARADAY CUPThe Faraday cup can be electrically biased to reduce
scattering of electrons or ions collected in the Faraday cup
and to reduce secondary electron emission. For electrons ornegative ions, +50 V is typically adequate, and for positiveions, -50 V. This can be accomplished by placing a battery
between the vacuum feedthrough and the ammeter.
FC-66 info.01.doc
FC-66 drawing #: 292 FC-71 partial

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