Development Status of The ILC Marx Modulator Page: 3 of 4
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each of the channels at 30 MS/s with an 8-bit resolution into a
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Individual modules are extensively tested for quality control
purposes. Over-voltage testing of switch modules, shown in
Fig. 4, exemplifies this testing regiment. Switch modules are
tested with a high inductance load, which at turn-off causes
the switch voltage to increase until clamped by the over-
voltage protection circuit. In the Fig. 4 data, the switch was
closing against 2 kV and 140 A. Without the over-voltage
protection circuit limiting the IGBT voltage to 3.7 kV, the
inductive voltage would have greatly exceeded the 4.5 kV
breakdown voltage of the switch.
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Fig. 4. Switch module over-voltage test results, Chl is switch voltage, Ch2 is
Qualified modules are assembled into Marx cells, which are
individually tested to 12 kV and 150 A. Cells are tested under
normal operating conditions and short-circuit conditions to
evaluate the effectiveness of the over-current protection
circuitry. An example of the latter is shown in Fig. 5. The
initial load conditions are 12 kV and 140 A. After the load is
shorted, the voltage collapses and the current rapidly rises.
Once the current exceeds the threshold of 180 A, the fire
switches are gated off, and after a turn-off delay of -1.4 ps the
current decreases from a peak of 600 A. The maximum dI/dt,
300 A/ps, corresponds to 11 kV across the series inductor.
Once cells have been qualified, they are assembled into the
Marx. The vernier regulator is still under development, so the
modulator is run with only coarse pulse flattening at present.
Infrastructure issues currently prevent operation at full power,
so the modulator is run at either the full 1.6 ms pulse width
and reduced PRF or at 5 Hz and reduced pulse width. The
former is illustrated in Fig. 6. The turn on, and off, of the
cells is staggered to reduce the peak current into the long
cable that connects the modulator to the load in the test
configuration. In the ILC implementation the cable would be
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Fig 5. Over-current testing of an individual Marx cell, Chl is load current,
Ch2 is load voltage, Ch4 is dl/dt.
eliminated, as could be the triggering delays, shortening the
rise and fall times.
The next phase of modulator testing will focus on fault
tolerance, primarily arc-down testing of the output to simulate
klystron breakdown. In addition to the single test results
shown above, testing has been successfully completed with a
2-cell configuration and preliminary results have been
obtained with a 4-cell configuration, as shown in Fig. 7. The
voltage is monitored at the output and mid-point of the Marx
along with the modulator current. With initial load conditions
of 32 kV and 40 A, the load was shorted and the peak current
rose to 450 A. At present, the cells individually detect and
react to the over-current condition. System over-current
detection will be added to the controls to provide a back-up.
Detailed examination of the voltage waveforms indicates
good voltage sharing between cells under this transient
condition. These measurements will be extended to full
voltage in the 4-cell configuration and then to 8, 12, and 16-
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rig. 6. Marx output wit coarse natth
voltage, and Math3 is load power.
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Nguyen, M; Beukers, T.; Burkhart, C.; Larsen, R.; Olsen, J.; Tang, T. et al. Development Status of The ILC Marx Modulator, article, June 7, 2010; [California]. (digital.library.unt.edu/ark:/67531/metadc1014483/m1/3/: accessed May 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.