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Progress Report. Nick Shipman - Thursday, 01 March 2014. Some recent plots included in the Apparatus re-write.

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progress report

Progress Report

Nick Shipman - Thursday, 01 March 2014


Some recent plots included in the Apparatus re-write

The power in the HRR circuits resistors and the current drawn form the resistor depends on the voltage, capacitance and pulse width. There are also power limitations which means at certain voltages we have to reduce the rep rate.

These plots show the maximum save rep rate, as well as the power in the resistors and current drawn from the power supply at this rep rate.

The capacitance of the FGS is much higher than in system I which means we have to reduce the rep rate at a lower voltage to stay within the power limitations. Exceeding these limitations would risk damage to the circuit.


Some recent plots included in the Apparatus re-write

To study the effect of the pulse length on the BDR it was necessary to reduce the bleed resistance to make the voltage after the switch opens fall faster.

If the bleed resistance is lower it dissipates more power for a given voltage.

The choice was between 1 or 2 4k resistors and hence the power limit of the bleed resistance was half as big when using just one resistor.

The graphs on the left show how the repetition rate varies with the voltage and pulse length.

Although a 4k resistor would have given a faster fall time it was decided to use a 8k resistance so that we were not limited as much in repetition rate.


Pulse Width vs. BDR

If these results prove to be repeatable they are quite remarkably close to the behavior observed in RF accelerating cavities. Despite the much longer pulse lengths and very different geometry.

Before these experiments I was of the opinion that the BDR~t^6 scaling law observed in RF was somehow related to the number of RF cycles and we would therefore not see an effect in a DC experiment.

This was a reasonable guess especially considering the vast majority of DC breakdowns happen right at the beginning of the pulse as opposed to in accelerating structures where they are more evenly spread.

Pulse Length us

Defining Pulse Length at the 90% level

BDR [#BDs/pulse]

Pulse Length us

Defining Pulse Length at the 95% level

BDR [#BDs/pulse]


Effect of Magnetic Field Perpendicular

To Electrodes – 15um

Blue is without field red is with 0.5T parallel to electrodes. Is the BDR consistently higher with field?


Effect of Magnetic Field Parallel

To Electrodes – 15um


Run number

Blue is without field red is with 0.5T parallel to electrodes.


Magnet Test I – 60um – 5th

Whilst attempting to condition the new electrodes overnight we accidentally had a very large number of BDs. This seems to have caused the same problem we had with the previous electrodes in that the BDR was very unstable and it was difficult to find the ‘right’ voltage.

Run Number

BDR #BDs/pulse



Magnet Test II – 60um – 7th

Run Number

BDR #BDs/pulse

5555 V

5500 V


Zero Field – 60um – 8th

Run Number

BDR #BDs/pulse

5250 V



Magnet Test III– 60um – 8th

Run Number

BDR #BDs/pulse


parallel field i 60um 9th
Parallel Field I – 60um – 9th

Run Number

BDR #BDs/pulse



average bdr problem example
Average BDR Problem - example

My gut instinct tells me the magnet has no effect, however due to clustering the simple 1/root(n) error analysis tells us we have a statistically significant effect.

Clustering can give very high BDRs and in addition these data points often consist of a large number of BDs (due to the short time necessary to take the data so they therefore have a very high weighting.

error analysis
Error Analysis
  • The problem is how to calculate the errors
  • Due to clustering 1/root(n) is not sufficient
  • We can assume a Poisson distribution but we know this is incorrect.
  • We can use a measured distribution but we only have 2 at different BDRs and they are both different (due to difference in amount of clustering)