Accelerator High-Frequency High-Voltage Resonant Power Supply Design

The drive towards more compact, efficient, and high-performance particle accelerators for industrial, medical, and research applications has spurred significant innovation in the supporting power systems. Among these, the development of high-frequency, high-voltage resonant power supplies for radiofrequency (RF) cavities and other accelerator structures represents a critical area of engineering focus. These supplies are tasked with providing the precise, high-power RF fields necessary for particle acceleration or beam manipulation, moving beyond traditional low-frequency, line-transformer-based designs. The core challenges lie in achieving high efficiency at multi-kilovolt and kilowatt levels, maintaining exceptional amplitude and phase stability, and ensuring reliable operation in demanding experimental or production environments.

The fundamental shift to high-frequency operation, typically in the range of tens to hundreds of kilohertz (kHz) and into the low megahertz (MHz) range, offers several key advantages over 50/60 Hz systems. The primary benefit is a drastic reduction in the size and weight of magnetic components, specifically the step-up transformer and any filtering inductors. This allows for more compact RF cavity designs and easier integration into space-constrained facilities. Furthermore, higher switching frequencies facilitate faster control loop response, enabling tighter regulation of the cavity field against beam loading effects and external disturbances. The design, however, pivots on the effective implementation of resonant topologies to manage the inherent challenges of switching losses and electromagnetic interference (EMI) at these elevated frequencies.

The selection of the resonant converter topology is paramount. Common configurations include the Series Resonant Converter (SRC), Parallel Resonant Converter (PRC), and the more advanced LLC resonant converter. The LLC topology, with its two inductors and a capacitor, is often favored for accelerator applications due to its ability to maintain zero-voltage switching (ZVS) over a wide load range. This characteristic is essential for high efficiency at high frequencies. ZVS ensures that the primary switching elements, typically high-power MOSFETs or IGBTs, turn on when the voltage across them is nearly zero. This eliminates the majority of the switching losses that would otherwise be prohibitive at frequencies above 100 kHz, especially at power levels exceeding several kilowatts. Achieving and maintaining ZVS requires meticulous design of the resonant tank components (Lr, Lm, Cr) and precise control of the switching frequency relative to the resonant frequency of the tank.

The high-voltage output stage presents another layer of complexity. The resonant transformer must not only provide the necessary voltage gain but also withstand the high dielectric stresses and manage the significant inter-winding and intra-winding capacitances that become pronounced at high frequencies. The parasitic capacitance of the transformer's secondary winding, combined with the capacitance of the connected RF cavity or load, becomes part of the resonant circuit itself. This load-dependent capacitance can shift the effective resonant frequency, challenging the control system's ability to maintain optimal ZVS conditions. Therefore, the transformer design employs careful winding techniques, such as sectionalized or layered windings with electrostatic shielding, to minimize parasitic capacitance and control its distribution. The core material is selected for low loss at the operational frequency, with ferrite being a common choice for kHz to MHz operation.

Voltage regulation and stability are non-negotiable requirements. The amplitude of the RF field in an accelerator cavity directly influences the energy gain of the particle beam. Any ripple or drift translates directly into beam energy spread or instability. A high-frequency resonant supply employs a closed-loop control system that typically regulates the output voltage by modulating the switching frequency. Since the gain of an LLC converter is a function of the switching frequency to resonant frequency ratio (fs/fr), the controller adjusts fs to correct for deviations in the output. This frequency modulation must be extremely precise and jitter-free. The feedback path often uses a high-voltage probe and a phase-locked loop (PLL) system to measure the actual cavity field, comparing it to a highly stable reference. The controller's bandwidth must be sufficient to reject disturbances caused by the pulsed nature of the beam loading, which can occur at repetition rates from Hz to kHz.

The integration of the power supply with the accelerator control system is a critical consideration. Modern accelerators require precise synchronization of the RF field with other subsystems like beam injection and diagnostics. The high-frequency power supply must accept external timing and phase reference signals, often at the accelerating frequency or a sub-harmonic. Its phase-locking capability must be fast and accurate to ensure the RF field is at the correct phase when the particle bunches arrive. Any phase error results in inefficient acceleration or even beam deflection. This necessitates a control architecture where the resonant converter's driver and frequency modulation circuitry are slaved to a master low-noise oscillator.

Protection and fault management are designed with the understanding that the load is both expensive and sensitive. An RF cavity presents a dynamic load that can exhibit voltage standing wave ratio (VSWR) changes or even arcs under fault conditions. The power supply must incorporate comprehensive protection features: over-current detection that responds to abnormal beam loading or arcs, over-voltage protection for the cavity, and temperature monitoring for the switching devices and transformer. In the event of an arc in the cavity, the system must be capable of removing the high-voltage drive within microseconds to prevent damage. This is often achieved by a fast interlock that disables the gate drivers, coupled with crowbar circuits or fast switches to dissipate stored energy safely.

Finally, the management of electromagnetic compatibility (EMC) is a major design driver. A high-frequency, high-power switching converter is a potent source of conducted and radiated emissions. To prevent interference with sensitive beam diagnostics and other laboratory instrumentation, the supply requires extensive filtering on all input and output lines. The entire assembly is typically housed in a meticulously designed shielded enclosure, with careful attention to grounding schemes and the filtering of all penetrations. The resonant topology itself, by smoothing current and voltage waveforms, aids in reducing harmonic content compared to hard-switched converters, but supplementary filtering is always necessary. In essence, the design of a high-frequency high-voltage resonant power supply for accelerators is a multidisciplinary exercise in precision power electronics, high-voltage engineering, and control theory. It balances the demands of efficiency, stability, speed, and reliability to produce the finely controlled electromagnetic fields that are the heartbeat of modern particle acceleration.