LINAC LOW ENERGY MODULATORS
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Transcript of LINAC LOW ENERGY MODULATORS
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LINAC LOW ENERGY MODULATORS
3-CELL TEST RESULTS & 9-CELL PLANHowie Pfeffer
11/13/13
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3-CELL TEST SET-UP
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2-STAGE DIAGRAM
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3-CELL TEST 900 V/cell, 8.3 ohm load – no problem?!
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3-CELL TEST Turn-on Ringing, cells switching within
600ns
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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.
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2-CELL TESTING Turn-on Ringing, cells within 600ns
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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.
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DIODE REVERSE CURRENT
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MODIFICATIONS TO ALLOW DIFFERENT SWITCHING CURRENTS
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2-CELL Switching at 300 amps
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2-CELL Switching at
low currents
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2-CELL “ON” time is important
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3-CELL 3-cell
switching at 100 amps
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3-CELL 3-cell
simultaneous turn- on at 300 amps.
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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
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9-CELL UNDER CONSTRUCTION
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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
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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.
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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.
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9-CELL TEST WAVEFORMSProgram Voltage
Load Voltage
Triangle Square Wave
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MATT’S VHDL BLOCK DIAGRAM
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PULSE-WIDTH MODULATION
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PWM FILTER
Unfiltered Filtered
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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
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SHORT CIRCUIT CONCERNSSPARK CURRENT AND ENERGY WITH NO CURRENT LIMITTING
37,000 amps 55 Joules (50 volt arc)
18 usec
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IMPORTANCE OF CHARGE INHIBITMain Switch
ChargingSwitch
CapacitorCharged to950 volts
Current
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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