[Kevin Sevcik]

1. The cap goes between C1 and C2. When you're firing, you want high current provided by a high voltage through a low resistance. Your HV power supply couldn't supply the current on its own, so you charge up a cap and use it as a source of high current (for a very short time). Ideally, you disconnect the HV power supply from the cap just before you fire so the power supply doesn't melt trying to supply that kind of current.

2. You can trigger the SCR directly from the anode. That is, if you had a switch (with a resistor in series) connecting the anode to the gate, and you closed the switch, your coil would fire. Of course if you're serious about a multi-stage coil gun, you'd be triggering the SCR with yet other solid state devices like some transistors, etc.