Beam-Diagnostics Summary

Beam Diagnostics

Diagnostic instruments are essential for monitoring and assessing any beam experiment. These diagnostics provide information on the state of the beam and on the progress and results of experiments performed on the beam, monitoring critical beam parameters such as current, size, energy, and emittance. A number of beam diagnostics are currently in use in the HIF program:

Rogowski coils to measure time-dependent net beam current
Faraday cups to measure intercepted beam current
Profile monitors, such as Kapton film and optical imaging on glass screens, harps, and grids, to measure beam profiles and spatial distributions
Capacitive probes to infer time-dependent moments of the transverse beam distribution
Gated beam imager to measure time-dependent beam distribution
Emittance scanner to measure the transverse beam emittance

A 32-channel Faraday cup array, shown here, is used to map the radial beam profile of the 2-MV Injector. This and other devices will be used to diagnose the High-Current Experiment (HCX), which will have a beam current similar to that expected in the front end of a driver.

Recently, scientists have begun evaluating whether free electrons in a fusion driver would be trapped by the beam space charge and degrade the performance of the accelerator. Free electrons can be produced in induction accelerators by two mechanisms: collisions of halo ions or free electrons with the beam-pipe wall, and collisional ionization by beam ions of background gas in the accelerator. Due to the high current possible in induction accelerators, the beam potential can be thousands of volts and is expected to trap most free electrons created within the beam. Several new diagnostics have been proposed for HCX to measure the generation and trapping of electrons, and the production of secondary particles at the wall:

Instrumented target to measure gas evolution and secondary electron production
Gridded ion-energy analyzer to measure energy of background ions expelled from the beam
Low-frequency interferometer to measure trapped electron density

In new and planned experiments, such as the Neutralized-Transport Experiment (NTX) and the Integrated Beam Experiment (IBX), further diagnostic development will be required to monitor additional beam characteristics:.

Beam-energy spectrometer to measure the time evolution of the beam energy and beam-energy distribution. This device may be a time-of-flight spectrometer or a magnetic spectrometer.
Electron-beam probe to measure space-charge potential at final focus of NTX. The potential is measured by monitoring the electric-field-induced deflection of an electron beam injected across the path of the heavy-ion beam.

Beams in HIF experiments to date have had sufficiently low current and energy that diagnostic devices could be inserted into the beam without damaging the apparatus or producing unwanted x-rays. In the future, as beam energies increase, a growing interest in non-intercepting diagnostics is expected. These diagnostics might be optically based and depend on remote sensing of interaction between beam ions and residual gas or an injected gas jet. An active diagnostic device of this sort might also be based on laser-induced fluorescence. Such a device would use an intense short-pulse laser to excite beam ions. The light emitted by these ions as they return to their ground state could be used to determine the local beam density and emittance. The main drawbacks of this device are its projected cost and the problem of finding a good match between available laser frequencies and excited states in a beam ion.