Faraday cup

A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup is named after Michael Faraday who first theorized ions around 1830. A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup is named after Michael Faraday who first theorized ions around 1830. It is installed in Space probes (Voyager 1, & 2, Parker Solar Probe, etc), or used in Mass spectrometry, and others. When a beam or packet of ions hits the metal it gains a small net charge while the ions are neutralized. The metal can then be discharged to measure a small current proportional to the number of impinging ions. Essentially the Faraday cup is part of a circuit where ions are the charge carriers in vacuum and the Faraday cup is the interface to the solid metal where electrons act as the charge carriers (as in most circuits). By measuring the electric current (the number of electrons flowing through the circuit per second) in the metal part of the circuit the number of charges being carried by the ions in the vacuum part of the circuit can be determined. For a continuous beam of ions (each with a single charge) where N is the number of ions observed in a time t (in seconds), I is the measured current (in amperes) and e is the elementary charge (about 1.60 × 10−19 C). Thus, a measured current of one nanoamp (10−9 A) corresponds to about 6 billion ions striking the faraday cup each second. Similarly, a Faraday cup can act as a collector for electrons in a vacuum (for instance from an electron beam). In this case electrons simply hit the metal plate/cup and a current is produced. Faraday cups are not as sensitive as electron multiplier detectors, but are highly regarded for accuracy because of the direct relation between the measured current and number of ions.This device is considered a universal charge detector because of its independence from the energy, mass, chemistry, etc. of the analyte. The Faraday cup utilizes a physical principle according to which the electrical charges delivered to the inner surface of a hollow conductor are redistributed around its outer surface due to mutual self-repelling of charges of the same sign – a phenomenon discovered by Faraday. The conventional Faraday cup is applied for measurements of ion (or electron) flows from plasma boundaries and comprises a metallic cylindrical receiver-cup – 1 (Fig. 1) closed with, and insulated from, a washer-type metallic electron-suppressor lid – 2 provided with the round axial through enter-hollow of an aperture with a surface area S F = π D F 2 / 4 {displaystyle S_{F}=pi D_{F}^{2}/4} . Both the receiver cup and the electron-suppressor lid are enveloped in, and insulated from, a grounded cylindrical shield – 3 having an axial round hole coinciding with the hole in the electron-suppressor lid – 2. The electron-suppressor lid is connected by 50 Ω RF cable with the source B e s {displaystyle B_{es}} of variable DC voltage U e s {displaystyle U_{es}} . The receiver-cup is connected by 50 Ω RF cable through the load resistor R F {displaystyle R_{F}} with a sweep generator producing saw-type pulses U g ( t ) {displaystyle U_{g}(t)} . Electric capacity C F {displaystyle C_{F}} is formed of the capacity of the receiver-cup – 1 to the grounded shield – 3 and the capacity of the RF cable. The signal from R F {displaystyle R_{F}} enables an observer to acquire an I-V characteristic of the Faraday cup by oscilloscope. Proper operating conditions: h ≥ D F {displaystyle hgeq D_{F}} (due to possible potential sag) and h ≪ λ i {displaystyle hll lambda _{i}} , where λ i {displaystyle lambda _{i}} is the ion free path. Signal from R F {displaystyle R_{F}} is the Faraday cup I-V characteristic which can be observed and memorized by oscilloscope i Σ ( U g ) = i i ( U g ) − C F d U g d t {displaystyle i_{Sigma }(U_{g})=i_{i}(U_{g})-C_{F}{frac {dU_{g}}{dt}}} . (1) In Fig. 1: 1 – cup-receiver, metal (stainless steel). 2 – electron-suppressor lid, metal (stainless steel). 3 – grounded shield, metal (stainless steel). 4 – insulator (teflon, ceramic). C F {displaystyle C_{F}} – capacity of Faraday cup. R F {displaystyle R_{F}} – load resistor.