Closed-Loop High-Voltage Control for Neutron Generator Beam Current Intensity

 

The core challenge lies in the complex physics of the ion source. The beam current is a function of several interdependent variables: the gas pressure within the source, the filament current or RF power for plasma generation, the extraction voltage applied to draw ions from the plasma, and the final acceleration voltage. Minor fluctuations in any of these parameters can cause significant drift in beam current. For instance, changes in filament emission over time, variations in gas feed rate, or localized heating of electrodes can alter the plasma density and the source's impedance. A fixed acceleration voltage cannot compensate for these changes, leading to an unstable or drifting neutron output.

 

A closed-loop system addresses this by continuously measuring the actual beam current. This measurement is typically achieved using a Faraday cup or a toroidal current transformer (beam current transformer) integrated into the beam path. The measured current signal is fed back to a high-speed digital controller. This controller compares the measured value against a user-defined setpoint. The error signal, the difference between setpoint and measurement, is processed by a control algorithm—commonly a Proportional-Integral-Derivative (PID) controller with additional feed-forward or adaptive elements. The output of this algorithm dynamically adjusts the command signal to the high-voltage power supply's regulation circuitry.

 

The high-voltage supply itself must be designed for this demanding role. It must have a wide and linear control bandwidth, allowing its output voltage to be modulated quickly and precisely in response to the controller's commands. The supply's output ripple and noise must be exceptionally low, as high-frequency noise can be coupled into the beam, causing micro-variations in energy that broaden the neutron energy spectrum or cause instability in the ion source plasma. Furthermore, the supply must be capable of handling the unique load characteristics presented by the accelerator structure, which can behave as a complex, sometimes non-linear, capacitive and resistive network. Arc-down events within the tube, which are inevitable at high voltages, must be managed by the supply's protection circuits without causing a prolonged interruption that would force the control loop to reset.

 

The implementation of the control loop requires careful consideration of time constants. The ion source plasma has its own response time to changes in power. The beam transport and measurement introduce a delay. The high-voltage power supply has a finite slew rate. An overly aggressive control loop can become unstable, hunting around the setpoint and causing oscillations in the beam current. Therefore, the control parameters must be tuned to the specific dynamics of the neutron generator, often requiring empirical testing and modeling. Advanced systems may employ state-space control or model predictive control to better handle the system's non-linearities and delays.

 

The benefits of such precise closed-loop control are substantial. It enables reproducible neutron yields from one experiment to the next, which is vital for quantitative analysis in applications like neutron activation analysis or porosity logging in oil wells. It allows for the implementation of complex beam time structures, such as pulses or ramps, by defining a corresponding current setpoint profile. Most importantly, it protects the neutron generator hardware. By preventing excessive beam current, which can overheat the target or induce excessive X-ray emission, the system enhances operational safety and extends the lifetime of critical components like the tritium target. In essence, the closed-loop high-voltage control system transforms the neutron generator from a static device into a stable, programmable source of neutrons, unlocking its full potential for scientific and industrial applications.