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A Faraday cup collects beam electric charge on a conducting electrode and measures the resulting electric current or accumulated electric charge. For complete collection of particles carrying known electric charge $q$, $Q=Nq$ and $I=qdN/dt$, providing a direct relation between the electrical signal and particle delivery.

The collector intercepts the beam. Its geometry and applied fields must account for escaping secondary electrons, which alter the net electric charge left on the cup. Background electric currents and incomplete collection also affect the inferred particle number. ^[ref]

The heliosheath, usually meaning the inner heliosheath, is the region of shocked solar wind between the termination shock and the heliopause. Solar wind plasma entering it has been slowed and compressed, and its flow turns as it approaches the boundary with the interstellar medium. The termination shock and heliopause are different boundaries: crossing the shock does not yet mean leaving solar-wind material.

The Voyager 2 plasma experiment provided direct measurements around several shock crossings between 30 August and 1 September 2007. The observed thermal plasma was cooler than a simple picture of complete conversion of the incoming flow energy into thermal-ion heat would suggest. The researchers inferred that much of the energy instead went into pickup ions or other energetic particles.^[ref]

The instrument used four Faraday cups, three facing approximately into the flow. Currents recorded at different particle energies and viewing directions were fitted with moving Maxwell-Boltzmann velocity distributions to estimate proton temperatures. This measures a selected part of the particle population; it does not directly measure all the energetic ions that may carry the remaining energy.^[ref]

John D. Richardson studies space plasmas, including the solar wind and its interaction with the interstellar medium. MIT identifies him as a principal investigator of the Voyager plasma experiment and describes earlier work on plasma transport in planetary magnetospheres.^[ref]

His 2008 Voyager 2 study analyzed plasma measurements across the termination shock. The thermal ions were cooler than a simple picture of bulk-flow energy becoming thermal-ion heat would suggest. The result made the partition of energy among particle populations central to understanding the shock.^[ref]

The experiment illustrates how a remote boundary becomes a local measurement. Four Faraday cups sampled particle currents over energy-per-charge ranges; combining differently oriented detectors allowed the team to infer plasma density, flow and temperature. These inferred properties depend on the measured spectra and distribution model, rather than being direct readings of a thermometer.^[ref]

Ernest Orlando Lawrence and Milton Stanley Livingston reported repeated ion acceleration in 1932. Their evacuated brass chamber used one driven hollow cyclotron dee; the grounded box and slotted divider supplied the other electrode. Filament electrons ionized hydrogen near the gap. A perpendicular magnetic field bent the ions; a high-frequency oscillator drove the gap. Deflecting plates and slits selected particles for a Faraday cup, read by an electrometer.^[ref]

Scanning the magnetic field produced narrow resonance peaks. The optimum deflecting voltage tracked the kinetic energy predicted from field and orbit radius, supporting the reported $1.22\mathrm{MeV}$ protons. Source width and slit geometry limited this check; it was not a direct energy measurement of every particle.^[ref]

This combination distinguishes producing a circulating electric current from establishing its kinetic energy. The electrometer measures collected electric charge per unit time; it cannot by itself tell whether the collected particles are fast. The separate deflection selection supplies a speed-dependent test. Agreement of two consequences of the model, the cyclotron resonance condition and the cyclotron energy and orbit radius relation, makes the inference stronger than either a current signal or a predicted orbit alone.

Milton Stanley Livingston built the early cyclotrons with Ernest Orlando Lawrence; the 1939 Nobel Prize in Physics went to Ernest Orlando Lawrence. His lecture, The evolution of the cyclotron, was delivered on 11 December 1951.^[ref][ref]

A mass spectrometer detector turns arriving ions into a recorded electrical signal. It is distinct from the mass analyzer that makes mass-to-charge ratio information distinguishable. The measured response can depend on collection, conversion and amplification as well as on how many ions arrived.

A Faraday cup collects electric charge directly. Ideally, a flux $\Phi$ of ions with known charge $q$ gives $I=q\Phi$. For $10^6$ singly charged positive ions per second, the current is $1.602176634\times 10^{-13}\mathrm{A}$, about $0.1602\mathrm{pA}$. Escaping secondary electrons and background currents must be accounted for when inferring particle delivery.^[ref]

An electron multiplier instead uses secondary emission to amplify the initial conversion signal. The manufacturer's detector description shows discrete dynodes, a continuous-dynode alternative, and a conversion dynode before the multiplier.^[ref] In an illustrative equal-stage model, ten stages with mean yield $\delta =4$ give mean gain $G=4^{10}=1048576$. This is a model calculation, not a measured gain for a named instrument.

Detector gain and detection efficiency answer different questions: the size of an output event and the probability of registering an arriving ion. Increasing gain cannot recover missed ions. At high signals, mass spectrometer detector saturation can also invalidate proportionality between ion delivery and recorded response. Quantitative peak comparisons therefore need a tested response range and appropriate instrument calibration, not merely a large signal.