Download - LINAC LOW ENERGY MODULATORS
LINAC LOW ENERGY MODULATORS
3-CELL TEST RESULTS & 9-CELL PLANHowie Pfeffer
11/13/13
3-CELL TEST SET-UP
2-STAGE DIAGRAM
3-CELL TEST 900 V/cell, 8.3 ohm load – no problem?!
3-CELL TEST Turn-on Ringing, cells switching within
600ns
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.
2-CELL TESTING Turn-on Ringing, cells within 600ns
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.
DIODE REVERSE CURRENT
MODIFICATIONS TO ALLOW DIFFERENT SWITCHING CURRENTS
2-CELL Switching at 300 amps
2-CELL Switching at
low currents
2-CELL “ON” time is important
3-CELL 3-cell
switching at 100 amps
3-CELL 3-cell
simultaneous turn- on at 300 amps.
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
9-CELL UNDER CONSTRUCTION
MODULATOR REQUIREMENTS
Beam Top Tilt (Adj.): +/- 5 kVBeam Voltage Step : 15 kV/usecFlattop/Beam Reg.: +/- 25 VRepeatability: +/- 10 VMax. spark Energy: < 5 Joules
Pulse rep rate: 15 HzMaximum Output: 35 kVMaximum step size: 1.5 kVLoad Resistance: ~100 ohmBeam 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.
9-CELL TEST WAVEFORMSProgram Voltage
Load Voltage
Triangle Square Wave
MATT’S VHDL BLOCK DIAGRAM
PULSE-WIDTH MODULATION
PWM FILTER
Unfiltered Filtered
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
IMPORTANCE OF CHARGE INHIBITMain Switch
ChargingSwitch
CapacitorCharged to950 volts
Current
CONCLUSIONS, 9-CELL TESTING 9-Cell construction on-going
All 9 cells individually tested Controls for ramping complete Regulation mode controls being designed
Goal: start system testing in late January