Faraday Cups

Accurately measure ion beam currents with NEC Faraday cups.

NEC manufactures various types of Faraday cups that are ideal for applications requiring accurate measurement of ion beam intensity in cases where the ion beam diameter is one inch (2.54 cm) or less.

Applications

To monitor ion beam currents, NEC offers 3 standard models of Faraday cups: FC18, FC50 and FC28.

FC18: For applications involving high energy or high current beams. Designed to handle 1 kW of continuous beam power.
FC50: For applications requiring maximum beam clearance when the cup is retracted. Capable of handling 50 kW of beam power.
FC28: For applications involving nanosecond pulsed beams. Designed to measure the coincidence and/or duration of pulsed beams. (This model is not retractable.)

Faraday Cup Design

The configuration of NEC Faraday cups varies from model to model dependent upon power rating and application requirements. However, all NEC Faraday cups are equipped with the following features:

Electron suppression assemblies that can be biased for accurate beam current readings. The suppressor is necessary to return secondary electrons, which affect the beam current readings.
Tantalum collector to limit neutron production.
Molybdenum beam aperture to protect the insertion mechanism and assures proper operation of the suppression assembly.
Pneumatic cylinder to control the position of the Faraday cup, which is actuated by air pressure. The motion is transmitted through a bellows seal to move the Faraday cup to one side of the beam path.
Ultra-high vacuum compatibility.

NEC provides three standard models of Faraday cups, with custom versions based on any of these designs available upon request. Below are brief descriptions of each model.

Model	Beam Power Rating	Aperture dia.	Features
FC18	1000 W	1.0″ (2.54cm)	Highest Power Faraday Cup For applications requiring high energy or high beam currents. Unique tantalum cone design that decreases the effective power density on the tantalum surface to eliminate local hotspots. Indirectly water-cooled cone that allows the use of ordinary tap water.
FC50	50 W	1.0″ (2.54cm)	Faraday Cup for Existing Housings & Maximum Beam Clearance For applications requiring maximum beam clearance when the cup is retracted. Mounted on a single flange with feedthroughs to allow it to be mounted in an existing vacuum chamber. Cooling by radiation only. No water cooling needed. 2″ (50 mm) stroke. Custom insertion lengths and custom housings available.
FC28	100 W	0.375″ (0.95cm)	Fixed Faraday Cup for Pulsed Ion Beams Designed as a timing device by measuring the coincidence and/or duration of pulsed ion beams. Includes a tungsten grid at the exit of the suppressor assembly to limit the spread of the delivered pulse, which is affected by the transit time of the beam across the gap between the grid and the target. Fixed Faraday cup that cannot be removed from the beam path and therefore is limited to usage at the end of the beamline.

The above beam power ratings are for a continuous beam with a diameter of 3mm or greater and with appropriate cooling.

Required Faraday Cup Accessories

If connecting the Faraday cup to an existing NEC control system with ACT/CAMAC, a complete system includes a Faraday cup for actuation and log amplifiers for current measurement. When not connecting the Faraday cup to an NEC control system with ACT/CAMAC, a complete Faraday cup system includes a Faraday cup, a controller for actuation, and a picoammeter for current reading.

Faraday Cup Local Controller:

Provides control for up to four Faraday cups
Allows the user to control the actuation of the Faraday cup, determine which cup(s) are in/out, and view the current readout from the selected cup (with the use of a picoammeter)
Provides electron suppression to the cup in use

Current Readers:

Picoammeter: Provides an 0-2 V signal to the local controller to display current on the analog meter on the front of the controller. NEC can provide or suggest a suitable picoammeter model for local controllers upon request.
Logarithmic Amplifier: Converts current readings from the Faraday cup through the controller to the control computer. Various logarithmic amplifiers for remote controllers are available from NEC, which are based on the following parameters:

Beam Polarity (Positive or Negative)
Input A.C. Voltage (115 V or 230 V)
Internal on/off relay (Yes or No)
Output Voltage (typically 0-6 V)