I received the SCR and relay this morning, I installed them and test the system again. However, the high voltage still could not be controlled. Mike helped to check the circuit board again.
Craig loaned us the used computer (400 MHz, Pentium II, better than our old one), which can be used to grab the wave froms from the oscilloscopes. I installed Tektronix WaveStar and successflully obtained the wave forms, much faster than the old machine.