Compact High Voltage Multiplier Circuit Design for Portable Neutron Generator High Voltage Power Supply
Portable neutron generators produce neutrons through nuclear reactions initiated by accelerating deuterium or tritium ions into a metal hydride target. The acceleration requires high voltage potentials of tens to hundreds of kilovolts, traditionally achieved with large, stationary high voltage systems. Portable applications including field analysis, well logging, and security scanning demand compact, lightweight high voltage power supplies that can operate from battery power. The compact high voltage multiplier circuit design enables this portability while maintaining the voltage and current requirements for neutron production.
The neutron generation reaction most commonly used in portable generators is the deuterium tritium fusion, which produces 14.1 MeV neutrons. Deuterium ions are accelerated into a tritium containing target, where the fusion reaction occurs with a probability that depends on the ion energy. Higher acceleration voltages increase the reaction cross section and the neutron yield, motivating the highest practical voltage within size and power constraints. Typical portable generators operate at 80 to 200 kilovolts.
The Cockcroft Walton voltage multiplier provides a well established approach for generating high voltage DC from a lower voltage AC input. The circuit consists of multiple stages of capacitors and rectifiers arranged in a ladder structure. Each stage adds the peak input voltage to the output, with the total output voltage approximately equal to the number of stages times the peak input voltage. The circuit requires no magnetic transformer for voltage step up, enabling compact implementation.
Size reduction in Cockcroft Walton multipliers comes from several design optimizations. Operating at higher frequency reduces the capacitor values needed for a given output current and ripple, as the charge transferred per cycle is smaller. Modern designs use frequencies from tens to hundreds of kilohertz, enabling capacitor values in the nanofarad range rather than the microfarad range required at line frequency. High frequency operation also reduces the transformer size if an input transformer is used for isolation or impedance matching.
Capacitor technology selection affects both the size and the reliability of the multiplier. Ceramic capacitors offer high energy density and low equivalent series resistance but may have voltage coefficients that reduce effective capacitance at high voltage. Film capacitors have stable capacitance but larger volume for the same rating. Specialized high voltage capacitors with optimized dielectric thickness and geometry provide the best combination of size and performance. The capacitor voltage rating must exceed the stage voltage with adequate margin for transients.
Rectifier selection for high frequency multiplier operation requires fast switching devices. Silicon diodes have reverse recovery times that can cause losses at high frequency. Silicon carbide diodes offer faster switching and lower reverse recovery losses, improving efficiency at high frequency. The diode voltage rating must exceed twice the peak input voltage for the multiplier topology. Series connection of diodes can achieve higher voltage ratings, but voltage balancing networks are needed to ensure equal sharing.
Stage number optimization balances output voltage against voltage drop under load. The theoretical output voltage is the product of stages and input voltage, but the actual output drops under load due to the source impedance of the multiplier. The source impedance increases with the cube of the number of stages, making multipliers with many stages impractical for high current applications. The optimal stage number provides the required voltage at the operating current with minimum circuit complexity.
Ripple reduction techniques enable cleaner output voltage for applications sensitive to voltage variation. The simple half wave Cockcroft Walton has significant ripple that increases with load current. Symmetrical multiplier topologies with positive and negative sections reduce ripple through cancellation. Filtering stages at the output further reduce ripple at the cost of additional components. The ripple specification depends on the neutron generator sensitivity to voltage variation.
Encapsulation and insulation design are critical for reliable operation at high voltage in a compact package. The multiplier components operate at potentials ranging from ground to the full output voltage, requiring insulation between stages and from stages to the enclosure. Potting with high dielectric strength materials provides insulation and mechanical support. The potting material must have low outgassing, good thermal conductivity, and compatibility with the operating temperature range. Void free potting is essential to prevent partial discharge that can degrade insulation.
Thermal management in the compact package removes heat from the rectifiers and other lossy components. The power dissipation scales with the output current and the operating frequency. Passive cooling through conduction to the enclosure and radiation to the environment may suffice for low power designs. Higher power designs may require forced air cooling or liquid cooling. The thermal design must maintain component temperatures within ratings over the specified operating conditions.
