MODEL FC-73A FARADAY CUPINTRODUCTION
The Kimball Physics model FC-73A 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-73A is mounted on a 2¾ CF flange for convenientattachment to the vacuum system. The Faraday cup is UHV
compatible and fully bakeable.
The Faraday cup consists of a hollow stainless steel cylinderclosed at the base, with an appropriately sized aperture for collecting the electrons or ions. The FC-73A has a standard aperture diameter of 5.0mm; however, custom aperture sizes areavailable. A grounded aperture holder provides shielding. The
Faraday Cup is electrically connected directly to a BNC whichthen can be connected to an ammeter.
For continuous measurement, the power entering thestandard FC-73A Faraday cup should not exceed 4 watts. The
Faraday cup temperature should not be raised above 350°C
due to outgassing.
GRIDS
The FC-73A Faraday cup has a set of three grids so that it
can be used as an energy analyzer. These grids are, in theorder that the electrons or ions go through them: ground, retarding, and suppression, labeled G, R and S in the diagram withFig. 1.
The ground grid is attached to the aperture plate, and isgrounded by connection to the shield.
A variable potential is applied to the retarding grid to analyze
the energies of electrons or ions. The grid voltage is negative forelectrons/negative ions, or positive for positive ions. The current
into the Faraday cup is measured, while the retarding grid voltage is varied, typically in a range from the accelerating voltage10% to the accelerating voltage+10% or any voltage which cuts
off the current completely.
The is illustrated in Fig. 2, on the reverse page, where thetechnique is used to analyze the energy spread of electron froman electron gun. The Faraday cup current at any given retarding

Specifications
FC-73A
Aperture size
(Custom avail.)
Shield size

5.0 mm dia.
(9.5 mm max.)
3.18 cm dia.

Insertion length

2.6 cm with shield

Operating temp.

350°C max.

Input power continuous

4 watts max. recommended

Grid voltage

5000 V max. retarding grid

grid potential represents the total current due to particles withenergy greater than that potential. The data in Fig. 2 can bedifferentiated to yield an energy profile (Fig. 3) that indicates theenergies present in the beam. The standard FC-73A Faraday
cup can have up to 5000V applied to the retarding grid.
When a low voltage is applied to the suppression grid, it canbe used either to suppress secondary and scattered electrons orto suppress scattered ions from the Faraday cup so that current
is not lost. The suppression grid also reduces the capacitive
coupling between the retarding grid and the Faraday cup. At low
beam energies, the suppression grid voltage must not be toohigh or it can interfere with the beam.
OPTIONAL PHOSPHOR SCREEN
A phosphor screen made with high luminosity P22 phosphor(ZnS:Ag) is available as an option on the front plate of theFC-73A Faraday cup. The use of a grounded phosphor target
screen is helpful as it allows visual, real-time observation of thespot. The phosphor screen emits a pale blue light (photons)
when bombarded by high energy particles (threshold value
approximately 500eV for electrons and 1000eV for ions). The
screen can be used for gun alignment and to set the proper
operating parameters necessary to obtain maximum beam
uniformity or minimum spot size. Note that excessive current or
exposure may cause phosphor screen damage.
BIASING THE FARADAY CUP
If there is no suppression grid, the Faraday cup can be electrically biased to reduce scattering of electrons or ions collectedin the Faraday cup and to reduce secondary electron emission.
For electrons or negative ions, +50V is typically adequate, andfor positive ions, -50V. This can be accomplished by placing abattery between the vacuum feedthrough and the ammeter.

Figure 1: Faraday cup FC-73A with outer shield removedPhosphor

BNCGRGRIDSS

Shield w/ FaradayCup inside

MODEL FC-73A FARADAY CUPPOWER INPUT CAUTIONS
For continuous measurement, the maximum recommended beam power into the standard FC-73A Faraday cup is 4watts. The Faraday cup temperature should not be raised
above 350°C due to outgassing.
The power input can be calculated by multiplying the beam
current times the electron acceleration voltage; for example, 1mA
at 20 keV gives 20 W, which is much too high for continuous
measurement. The temperature of the Faraday cup increases
approximately linearly with the power input. A 2 W continuous
input results in approximately 150°C, and a 4 W input results in300°C.
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 the heat capacity of the cup, a 20 W
continuous input for 40 sec will raise the cup temperature byapproximately 150°C. The cup will cool from 200°C to room temperature in about 15 min.
Care must always be exercised when using 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 of approximately
13 kW/cm2. Assuming no heat flow, this would bring the Faraday
cup stainless steel at the spot to its melting point in only 11 μsec.

INSTALLATION and OPERATING PROCEDURE

CAUTIONHigh beam power can melt Faradaycup components. Maximum recommended
Beam power into the Faraday cup
Is 4 W continuous

1. Bolt the 2¾ CF Faraday cup flange to a port on the vacuum
chamber, sealing with a new copper gasket.
2. Connect a suitable user-supplied pico-ammeter to the output
BNC on the flange labeled FC. (See Fig. 1).
NEVER collect beam current in the Faraday cup unlessthe output BNC is connected to an ammeter or grounded.
Without proper termination of the Faraday cup, the BNC
center terminal will charge up to full beam energy andelectrical discharging will result.
3. Optional Faraday cup grids:
a. For the retarding grid: Connect a user-supplied
power supply that provides voltages approximately
10% higher than beam energy (5000 V max.) to theflange MHV labeled RET. (See Fig. 1)
b. For the suppression grid: Connect a 0 to 50 V power
supply to the flange BNC labeled SUPP. (Connection
types and voltage limits are marked on theconnectors).
4. Calculate the expected power input into the Faraday cup andcheck that it is within the acceptable range. See thediscussion of power input cautions.
5. Measure electron or ion current with the user suppliedpico-ammeter.
REMOVING/ REINSTALLING THE OUTER SHIELD

Fig 2: The use of FC-73 Faraday cup as an energy analyzer measuring
electron beam current

1. Remove the outer shield by unscrewing the 3 2-56 x 1/8"
Philips screws.
2. Replace the shield by putting it over the Faraday cup andreplacing the 3 screws. (See Fig. 1).

REPLACING APERTURE PLATES, GRIDS, OR PHOSPHORSCREENS
1. Remove the outer shield as above.
2. Disconnect the two internal spring electrical connections fromthe plates. (See Fig. 1).
3. Remove the four 0-80 socket head screws on the front of the
Faraday cup, and lift off the entire aperture holder with theplates.
4. Carefully lift off the plates and insulators (ceramic spacers),
noting the original order of the plates.
5. Replace the desired aperture plate, grid, or phosphor screen,and then reassemble.
Fig 3: Calculated energy profile of electrons in beam (Graph derived
from data in Figure 2)