ASTROD Variations in Capacitor Number and Placement
The first ASTROD I built used one capacitor. This limits the amount of gap that can be used since there does not seem to be enough voltage buildup, but mostly decreases the intensity of the beam. It still produced effects, though, and I never seriously compared the operations of using one versus multiple capacitors (I only experimented a day or two with a single cap setup before I was building and incorporating more.)
The circuit for a single capacitor is different than that for multiple capacitors. In it, the spark gap is put where C1 is and the single capacitor is put where the spark gap/2-cap assembly is.
In the current design, Alan Trent recommended putting all the capacitors in parallel with the spark gap and not having the "up-and-down" one parallel to the tube. But, using the one in parallel with the tube made the circuit work better for me. Using more than three capacitors, ones made differently from mine, or if other factors in the circuit are affecting it, it may work better to connect them all in parallel.