MODEL FC-71A FARADAY CUPSPECIFICATIONS

Output BNC

Ground, Retarding andSuppression Grids

Mountingbracket
Grounded shield with
Faraday cup insideFC-71AFour0-80
screwsFig. 1 Faraday Cup FC-71A with optional phosphor
screen and grids attached
INTRODUCTION
The Kimball Physics model FC-71A 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 71A is UHV compatible and fully bakeable.
The Faraday cup consists of a hollow stainless steel cylinderclosed at the base, with an appropriately-sized aperture forcollecting the electrons or ions. An outer, grounded cylinder
provides shielding. An electrical connection is made to the baseof the Faraday cup, terminating in a BNC. The current is thenconducted to a vacuum feedthrough and so to an ammeter.
The FC-71A usually has a 5.0 mm diameter aperture in a 1
inch square plate. Custom aperture sizes are available, up to9.5 mm dia.
A 12 inch in-vacuum cable and BNC feedthrough on a 1?
CF flange, or a custom-designed feedthrough, is available fromKimball Physics as a separate option.
For continuous measurement, the power entering thestandard FC 71A Faraday cup must not exceed 4 watts. TheFaraday 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 plate of the FC
71A 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 500 eV for electrons and 1000 eV 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 orexposure may cause phosphor screen damage.
OPTIONAL GRIDS
The FC 71A Faraday cup can have a set of three grids sothat it can be used as an energy analyzer. These grids are, in311 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

Aperture size

5.0 mm dia. standard
custom available, 9.5 mm max.

Plate size

2.54 cm squareLength

6.95 cmOperating temperature

3500 max

Input power continuous

4 watts max. recomended

Grid voltage

1000 V max. on retarding grid(custom available)

the order that the electrons or ions go through them: ground,retarding, and suppression, labeled G, R and S in the diagramwith 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 the technique
is used to analyze the energy spread of ions from a typicalion gun. The Faraday cup current at any given retarding grid
potential represents the total current due to ions with energy
greater than that potential. The data in Fig. 2 can bedifferentiated to yield an energy profile (Fig. 3) that indicatesthe ion energies present in the beam. The standard Faraday
cup can have up to 1000 V applied to the retarding grid; forhigher voltages, optional larger insulators are available.
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 or
negative 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.

MODEL FC-71A FARADAY CUPPOWER INPUT CAUTIONS
For continuous measurement, the maximum beam
power recommended into the standard FC-71A Faraday cup
is4 watts. The Faraday cup temperature should not be raised0
above 350 C due to outgassing.
The power input can be calculated by multiplying the beam
current by the electron acceleration voltage; for example, 1mA
at 20 keV gives 20 W, which is much too high for continuousmeasurement. The temperature of the Faraday cup increases
approximately linearly with the power input. A 2 W input results
in approximately 1500C, and a 4 W input results in 3000C.
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-71A will raise the cup temperature byapproximately 1500C. The cup will cool from 2000C 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 spot size of 1 mm 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 sec.

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

INSTALLATION AND OPERATING PROCEDURE
1. Position the Faraday cup in the target area inside thevacuum chamber. A mounting bracket is provided.
2. Connect the output BNC on the back of the Faraday cup
to an electrical feedthrough on the vacuum system (an invacuum 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 discharging will result.
4. Optional Faraday cup grids:a.
For the retarding grid (R in Fig. 4): Connect a
0 to 1000 V power supply (negative for electrons or
negative ions, and positive for positive ions) througha vacuum electrical feedthrough to the pins on thesecond grid plate using Teflon-coated stainless steel
wire with a spring connector end.
b.
For the suppression grid (S in Fig. 4): Similarly
connect a 0 to 50 V power supply to the pin on thethird grid plate.
5. Calculate the expected power input into the Faraday cup
and check that it is within the acceptable range. See thediscussion of power input cautions.
6. Measure electron or ion current with the ammeter.
REPLACEMENT OPTIONAL APERTURE PLATES, GRIDS
OR PHOSPHOR PLATES

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
and insulators (ceramic spacers) in order on the screws.
3. Replace the desired aperture plate, grid or phosphor
screen, then reassemble.

Phosphor

BNCGFig. 3 Calculated energy profile of ions in beam
(Graph derived from data in Fig. 2)

RS

Shield w/ Faraday
Cup insideGRIDS

Fig. 4 Diagram of Faraday cup with optional grids andphospher screen plate
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

2