Beam Profile Control via High-Voltage Shaping in Compact Neutron Sources
In the realm of compact neutron source development, the generation of a stable and well-defined proton or deuteron beam is paramount for neutron production via nuclear reactions. My five decades in high-voltage engineering have consistently shown that the beam profile at the target is not merely a function of ion source extraction optics, but is profoundly influenced by the temporal and spatial stability of the accelerating potential. For compact sources, where real estate is limited and beam lines are short, the ability to shape the beam through dynamic high-voltage manipulation becomes a critical tool.
The fundamental challenge lies in the beam's phase space dynamics. As a pulsed ion beam is accelerated through a column, its transverse emittance is susceptible to degradation from lens aberrations and space charge forces. These effects are exacerbated if the accelerating voltage exhibits ripple or droop during the pulse. A common misconception is that only the final beam energy matters. In reality, the instantaneous voltage profile during the beam's transit defines the energy spread and the convergence or divergence of the beam. For instance, a voltage that sags even by a fraction of a percent over the pulse duration will impart a time-dependent energy variation, causing different longitudinal slices of the beam to focus at different points. This leads to a larger effective spot size on the target, reducing neutron flux density.
High-voltage pulse shaping, therefore, moves beyond simple voltage stabilisation to active profile engineering. By deliberately manipulating the waveform of the accelerating potential, we can introduce a controlled energy-time correlation. This technique, often referred to as 'voltage ramping' or 'debunching', can counteract the natural energy spread introduced in the ion source. If the beam from the source has a positive energy-time correlation (head of the pulse has lower energy than the tail), applying a decelerating ramp to the accelerating field can align the energies of all particles upon arrival at the target. This requires a high-voltage pulser capable of generating precise, arbitrary waveforms with nanosecond-scale rise times and minimal overshoot. The power supply here ceases to be a simple DC source and becomes an active beam optics element.
Furthermore, the beam profile is acutely sensitive to the voltage stability of intermediate electrodes in multi-stage acceleration columns. In a typical Cockcroft-Walton or tandem accelerator configuration, intermediate electrodes are biased to create an equipotential gradient. Any instability or ripple on these bias supplies creates time-varying electrostatic lenses. These dynamic lenses can induce periodic focusing and defocusing of the beam, effectively 'pumping' the beam envelope and leading to halo formation. Halo particles represent not only a loss in beam current but also a source of activation and background noise in the neutron experiments. Designing the resistive divider networks and the associated filtering for these intermediate stages requires a deep understanding of the interplay between RC time constants and the beam pulse repetition frequency. A poorly damped divider can ring at the pulse frequency, creating a standing wave of voltage along the column that severely distorts the beam profile.
The practical realisation of such beam profile control demands a holistic design approach. The high-voltage deck must be conceived as a system where the power converter, the pulse-forming network, and the accelerating structure are modelled together. Stray capacitances to ground, which are often neglected in circuit simulations, become dominant paths for transient currents that can distort the voltage waveform. I have seen many projects fail because a beautifully designed pulser was connected to an accelerator column without considering the column's own impedance and its effect on the pulse shape. The use of fast, capacitive voltage dividers integrated into the column base is essential for monitoring the true voltage applied to the beam, allowing for closed-loop corrections on a pulse-to-pulse basis. This level of precision, moving from millisecond to microsecond and nanosecond stabilisation, is the new frontier in maximising the brightness and utility of compact neutron sources for materials science and imaging applications.
