MODEL FC-72AFC-72A FARADAY
FARADAY CUP
CUP
MODEL

Phosphor screenwith standardaperture‘Grid platewith pin forvoltageconnection

Grounded shield
Faraday cup insideOutput
BNC
Four 0-80
Ground platescrews
with custom
size aperture
Fig. 1 Faraday Cup FC-72A with optional phosphor
screen and grid shown separately
INTRODUCTION
The Kimball Physics model FC-72A 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-72A 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.
Anelectricalconnection 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.
The FC-72A usually has a 11.28 mm diameter aperture2(1 cm area) in a 2 inch square plate. Custom smaller
aperture sizes are available.
A 12 inch in-vacuum cable and BNC feedthrough on a1⅓ CF flange, or a custom-designed feedthrough, is available
from Kimball Physics as a separate option.
For continuous measurement, the power entering thestandard FC-72A Faraday cup must not exceed 4 watts. The
Faraday cup temperature should not be raised above 350ºC
due to outgassing.
OPTIONAL PHOSOPOR SCREEN
A phosphor screen made with high luminosity P22
phosphor (ZnS:Ag) is available as an option on the front plateof the FC-72A Faraday cup. The use of a grounded phosphor
target screen is helpful as it allows visual, real-timeobservation of the spot. The phosphor screen emits a pale
blue light (photons) when bombarded by high energy particles(threshold value approximately 500 eV for electrons and1000 eV for ions). The screen can be used for gun alignment
and to set the proper operating parameters necessary toobtain maximum beam uniformity or minimum spot size. Notethat excessive current or exposure may cause phosphor
screen damage.

SPECIFICATIONS
FC-72A
2Aperture size

11.28 mm dia.=1 cm area
Custom smaller dia. available

Plate size

5.08 cm square

Length

7.16 cmOperating temperature

350ºC max.

Input power continuous

4 watts max. recommended

Grid voltage

1000 V max on retarding grid(custom available)

OPTIONAL GRIDS
The FC-72A Faraday cup can have a set of three grids
so that it can be used as an energy analyzer. These grids
are, in the order that the electrons or ions go through them:ground, retarding, and suppression, labeled G, R and S in thediagram with Fig. 4 on page 2.
The ground grid is attached to the phosphor screen /initial aperture plate, and both are grounded by being
connected to the shield. There is also a final, non-grid plate
as part of the grounded shielding cylinder.
A variable potential is applied to the retarding grid toanalyze the energies of electrons or ions. The grid voltage isnegative for electrons/negative ions, or positive for positiveions. The current into the Faraday cup is measured while theretarding grid voltage is varied from zero to a voltage whichcuts off the current completely.
This is illustrated in Fig. 2 on page 2, where thetechnique is used to analyze the energy spread of ions from atypical ion gun. The Faraday cup current at any given
retarding grid potential represents the total current due to ionswith energy greater than that potential. The data in Fig. 2 canbe differentiated to yield an energy profile (Fig. 3) thatindicates the ion energies present in the beam. The standard
Faraday cup can have up to 1000 V applied to the retarding
grid; for higher voltages, optional larger insulators areavailable.
When a low voltage is applied to the suppression grid, it
can be used either to suppress secondary and scatteredelectrons or to suppress scattered ions, so that current is notlost. The suppression grid also reduces the capacitive
coupling between the retarding grid and the Faraday cup.
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 positive
ions, -50 V. This can be accomplished by placing a battery
between the vacuum feedthrough and the ammeter.

FC-72A info.04.doc
FC-72 drawing #200MK010A

311 KIMBALL HILL ROAD, WILTON, NH 03086-9742 USA
Tel: (603) 878-1616 1-888-546-7497 Fax: (603) 878-3700
e-mail: info@kimphys.com Web: www.kimballphysics.com

MODEL FC-72A FARADAY CUP cont.
POWER INPUT CAUTIONSINSTALLATION AND OPERATING PROCEDURE

For continuous measurement, the maximum beam
power recommended into the standard FC-72A Faraday
cup 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 theFaraday 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 40 sec into the FC-72A 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.

FARADAY CUP CURRENT (nA)

150

FARADAY CUP CURRENT vsRETARDING GRID POTENTIALHigh 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-suppliedammeter. NEVER collect beam current in the Faradaycup 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. Optional Faraday cup grids:
a. For the retarding grid (R in Fig. 4): Connect a090n0gra phs
Energy Profile500

ILG-2C ION GUN

b.

100

0 to 1000 V power supply (negative for electrons or
negative ions, and positive for positive ions) througha vacuum electrical feedthrough to the pin on thesecond grid plate using Teflon-coated stainless steel
wire with a spring connector end.
For the suppression grid (S in Fig. 4): Similarly
connect a 0 to 50 V power supply to the pin on thethird gird plate

5. Calculate the expected power input into the Faraday cup

50and check that it is within the acceptable range. See thediscussion of power input cautions.

6. Measure electron or ion current with the ammeter.
00100

200

300

400500

REPLACING OPTIONAL APERTURE PLATES,GRIDS OR PHOSPHOR SCREENS600

RETARDING POTENTIAL (V)Fig. 2 The use of the Faraday cup as an energy analyzer
measuring ion beam current

1. Loosen the four 0-80 gold-plated, socket head screws on
the front plate of the Faraday cup, and remove the nuts.

2. Lift off the Faraday cup from the back, keeping the platesI / V (nA/V)

090n0gra phs

6

ENERGY PROFILE-Energy Profile500
FARADAY CUP CURRENT vsRETARDING GRID POTENTIAL DIFFERENTIATED

5ILG-2C ION GUN

and insulators (ceramic spacers) in order on the screws.

3. Replace the desired aperture plate, grid, or phosphor
screen, and then reassemble.

432
100100200300400500600Fig. 4 Diagram of Faraday cup with optional grids andphosphor screen plate.

RETARDING POTENTIAL (V)Fig. 3 Calculated energy profile of ions in beam
(Graph derived from data in Fig. 2)-2-