Date post: | 01-Jan-2016 |
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INVESTIGATING
Run just 2-cells - simplify Bottom (ground-level) cell switching in
2nd place – measurements easier. Isolated charging circuits with high-
value resistors – eliminate other possibilities.
Still had oscillations when spaced less than 600 ns.
POSSIBLE EXPLANATION?
After various failures, started to suspect problem due to diode turn-off before diode is fully on.
Seen similar problems with FETs – turning them off shortly after turning them on.
If so, then problem should go away if diode is at higher current during turn-off.
3-CELL TESTING CONCLUSIONS
No problems with 3-cell operation as long as initial turn on timing is done properly.
Simultaneous turn-on yields 2700 volts with 250 ns rise time (10% - 90%).
9-cell construction under-way
MODULATOR REQUIREMENTS
Beam Top Tilt (Adj.): +/- 5 kVBeam Voltage Step : 15 kV/usec
Flattop/Beam Reg.: +/- 25 VRepeatability: +/- 10 V
Max. spark Energy: < 5 Joules
Pulse rep rate: 15 Hz
Maximum Output: 35 kVMaximum step size: 1.5 kV
Load Resistance: ~100 ohm
Beam Length/Time: 110 usec
Load (Tube) Voltage
40-Cell Marx Output
12-Cell Reg. Output
9 – CELL TESTING
Nine steps up to 8 kV Load = 100 ohms. First step on for 2 us before subsequent steps. One step followed by 8 cells simultaneously. Uses existing driver electronics. Delays between control edges and outputs.
Short circuit protection Delay turn-off by 8 us after receiving trip signal.
This allows overcurrent circuits on gating boards to turn off slowly.
9 – CELL TESTING (CONT.)
Interleaved Regulator operation Uses Matt’s circuit. Doesn’t require ultimate resolution. Pulse one cell ON, then make 4 kV 8-cell
interleaved pulse. 1 us spacing, 2 us minimum time, 8 us cycle,
96 us pulse length. Filter circuit?
Short circuit response Freeze signals for 8 us then turn OFF.
SPECIAL CONCERNS
Short circuit testing Critical function – there will be arcs
Charging diode transients during pulse Diode Qrr results in reverse current during
turn-off
Charge inhibit function Ensure charge switch and main switch are
never on at the same time - disaster
SHORT CIRCUIT CONCERNSSPARK CURRENT AND ENERGY WITH NO CURRENT LIMITTING
37,000 amps 55 Joules (50 volt arc)
18 usec