HV pulse generator

This pulse generator was designed to drive my N2 TEA Laser with a variable pulse rate of about 1-50Hz without the need for a HV rectifier. It produces HV pulses which can charge the laser's capactitance of 3.4nF to 18kV. Without load it produces 3.5cm long sparks.

The pulse generator works by charging the MP capacitor C5 with mains voltage (<=325V) and discharging it through the car ignition coil L3 via the SCR U2.
The charging resistor R4 consists of ten 270Ohms/9W resistors in series adding to 108Ohms/90W. The mains voltage is rectified by D1-D4. I salvaged the noise supressors C1/C2, C6/C7 and L1/L2 from an old switchmode power supply, so I don't have have any information on L1 and L2. The car ignition coil has three connectors: +, gnd and HV. I'm using it in reverse direction so it produces negative pulses which are more suitable for the N2 laser.
The trigger pulses for the SCR are produced by R1, R2, C3 and U1. As soon as C3 is charged to 33V, the trigger diode's breakdown voltage, the SCR is triggered. D5 (actually consisting of a 160V and a 51V Zener diode in series) decreases the charging voltage and thus the required power rating of R1 and capacity of C3. C4 provides a constant charging voltage and R3 discharges it after S1 is turned off. D6 prevents C4 from being discharged into C5.