The smallest and simplest practical particle accelerator are pyroelectric crystal linear accelerators. They are typically cylindrical rods of lithium tantalate

The smallest and simplest practical particle accelerator are pyroelectric crystal linear accelerators. They are typically cylindrical rods of lithium tantalate (or niobate) cut so their pyroelectric face is exposed on the ends (~1 to 2 cm long). Put one of these in mild vacuum (~10^-4 torr) and use a resistor to periodically heat it up and let it cool down. During the temperature changes moving charge will be exposed on the crystal's pyroelectric faces. This exposed charge creates voltage gradients which can be used to accelerate ions or electrons (alternately) up to 200-300 keV, about 10x the peak speeds of the linked laser accelerator (over ~5x longer distance). People have used them as linear accelerators with deuterated plastic targets to accelerate deuterium ion gas residuals and achieve toy-scale desktop fusion. https://en.wikipedia.org/wiki/Pyroelectric_fusion https://en.wikipedia.org/wiki/Pyroelectric_fusion They were also used as a portable x-ray source in a commercial x-ray fluorescence spectroscopy kit called "Cool-X", https://www.amptek.com/internal-products/obsolete-products/cool-x-pyroelectric-x-ray-generator https://www.amptek.com/internal-products/obsolete-products/c... If you powered the resistive heater with a battery the device could literally be only a few cm long and maybe a cm radius. Very easily held in the palm of the hand. It's not a penny, but it's the full device and it's energy storage.