I am trying to charge a transmission line (Z0=4.4 ohms, propagation delay=1.4 ns) by a HV (1-1000 V, 600 mA) power supply through a 1700 V rated IGBT (S1). The charged transmission line gets discharged through a photo conductive switch (S2) to a 4.4 ohm resistive load. There is another identical transmission line between the load and S2. The IGBT is triggered by ISO5852s based gate driver evaluation board. The timing signal for S1 and S2 is provided by an external pulse delay generator. The simplified circuit diagram and the timing diagram for S1 and S2 are shown in the figure below.
When I probe the transmission line placed before S2 after S2 is triggered off (after 150.5 us), I am getting a huge oscillation which is bigger than the HV charging voltage (50 V) although S1 (IGBT) and S2 is off at that time. As a result of that, the transmission line is being charged again although it should be isolated from the power supply at that time as S1 and S2 are off at that time. I am also getting oscillation across the transmission line after S2 (ideally I should get voltage output only for the period S2 is on and a little bit time after that). I am wondering what is the source of these oscillations. The wire that is connecting the collector terminal to the HV supply is about 100 cm long connecting wire. Is it a reason of this oscillation? However, if I remove the IGBT (S1) and charge the transmission line directly, I do not see such high ringing after S2 is triggered off.