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/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.

9-CELL TEST WAVEFORMS

Program 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 INHIBIT

Main 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