High-Voltage Exchange System for Accelerator Beam Stripping Foils
Within the injector chains of medium to high-energy particle accelerators, stripping foils serve a critical function. These thin membranes, often composed of carbon or similar low-Z materials, are used to change the charge state of an ion beam. For example, a negatively charged hydrogen ion beam can be passed through a foil, stripping away its two electrons to produce a positively charged proton beam suitable for further acceleration. These foils are subjected to intense radiative and thermal load from the beam, leading to gradual degradation, thinning, and eventual failure. Manual replacement necessitates a full accelerator shutdown, which can last for days. Therefore, a High-Voltage Exchange System is engineered to allow for the remote, rapid, and precise replacement of these foils without breaking the accelerator's vacuum, a key enabler for high facility availability.
The system is fundamentally a specialized robotic manipulator operating within a high-voltage terminal, which can be at potentials ranging from hundreds of kilovolts to several megavolts relative to ground. The stripping foil is mounted on a precisely positioned frame or cartridge. Multiple such cartridges are stored on a carousel or linear magazine inside the high-voltage vessel. When the beam diagnostics indicate excessive foil scattering, energy loss, or when a pre-determined operational time expires, the control system initiates an exchange sequence.
The first step is the safe handling of the high voltage. The beam is typically diverted or shut off. The high-voltage power supply for the terminal must be ramped down in a controlled manner, but critically, the terminal itself may remain electrically floating or be held at a reduced but still significant potential to maintain vacuum integrity and avoid discharge events. The exchange mechanism itself is electrically part of the high-voltage terminal. Its motors, sensors, and control electronics must be powered from batteries or via isolation transformers and optical links that withstand the full terminal potential. Every actuator, from the carousel stepper motor to the gripper solenoid, is designed for operation in high vacuum and in the presence of strong electric fields.
The exchange sequence is automated. A robotic arm or linear actuator engages with the used foil cartridge. It must unlatch it from its operational position with micron-level precision to avoid damaging the delicate foil or its mounting. The cartridge is then retracted and placed into a used storage slot. The carousel indexes, presenting a fresh foil cartridge. The arm then inserts this new cartridge into the precise beamline axis. The alignment tolerance is extraordinarily tight; a lateral error of even a few hundred microns can cause the beam to clip the edge of the foil frame, generating excessive scattering, x-rays, and rapid localized heating. The system often incorporates in-situ alignment verification, such as using a low-current pilot beam or capacitive sensing, to confirm correct positioning before releasing the cartridge.
Once the new foil is locked in place, the system performs a self-check. The high-voltage supply is then commanded to ramp back to its operational level. The entire process, from initiation to being ready for beam, is designed to complete within minutes, contrasting sharply with the day-long shutdown required for manual intervention. The system's design must also account for worst-case scenarios, such as a foil breaking during handling. Contingency procedures and mechanisms, like a separate debris containment cassette, are integrated to prevent fragments from contaminating the beam tube.
This high-voltage exchange system exemplifies the marriage of precision mechanics, ultra-high vacuum engineering, and high-voltage engineering. It directly contributes to the uptime and operational flexibility of major accelerator facilities, enabling more productive research schedules and ensuring stable beam delivery for applications ranging from nuclear physics to medical isotope production. The ability to perform this maintenance remotely also provides a significant radiological safety benefit, as activated components are handled without direct human exposure.
