Microdischarges in solid dielectrics

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Discussion Overview

The discussion revolves around the phenomenon of microdischarges in solid dielectrics, particularly in the context of conductors under high voltage (approximately 2500V) interacting with various dielectric materials such as plastics and ceramics. Participants explore the characteristics of these microdischarges, their potential causes, and the implications for measurement setups.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes observing low-frequency microdischarges (0.01 Hz) with charge levels between 0.05 pC and 0.2 pC, which disrupt their measurement setup.
  • Another participant mentions experiencing breakdowns at joints between dielectrics and copper waveguides under high RF power, attributing light emission to secondary electron emission and gas ionization.
  • A different participant argues that their observations occur in a static environment with fields below ionization thresholds, yet they still detect small discharge pulses every few minutes.
  • Concerns are raised about the possibility of measurement setup issues, with questions about the measurement methods and potential external triggers for the discharge events, such as ESD or AC mains noise.
  • One participant introduces the concept of secondary emission and multipactor effects, suggesting that even below breakdown thresholds, these phenomena could lead to ionization or luminescence, potentially explaining the observed discharges.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between their observations and the conditions under which they occur. Some believe the phenomena are unrelated, while others suggest potential connections through secondary emission effects. The discussion remains unresolved with multiple competing perspectives on the causes and implications of the microdischarges.

Contextual Notes

Participants note the importance of material purity and the presence of defects in dielectrics, which could influence the occurrence of microdischarges. There is also mention of the need to monitor field conditions during discharge events to better understand the underlying mechanisms.

vanesch
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Hi all,

I'm looking for information/theories/data/phenomenology about micro discharges in dielectrics (and surfaces).

I'll explain the problem: we have some conductors under a potential of ~2500V which are (of course) in contact with solid dielectrics (types of plastic, ceramics...), and we observe low-frequency (~0.01 Hz) microdischarges on them of the order of ~0.05 pC - 0.2 pC which, well, screw our measurement setup.
Now, I know some basic techniques to diminish it, like using guard rings and stuff like that, but I'd like to know more about the phenomena themselves, and I can't find any material about it.

So any pointers are welcome.
 
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vanesch said:
Hi all,

I'm looking for information/theories/data/phenomenology about micro discharges in dielectrics (and surfaces).

I'll explain the problem: we have some conductors under a potential of ~2500V which are (of course) in contact with solid dielectrics (types of plastic, ceramics...), and we observe low-frequency (~0.01 Hz) microdischarges on them of the order of ~0.05 pC - 0.2 pC which, well, screw our measurement setup.
Now, I know some basic techniques to diminish it, like using guard rings and stuff like that, but I'd like to know more about the phenomena themselves, and I can't find any material about it.

So any pointers are welcome.

Not sure about this, but you might have the same problem that we have.

We have a traveling wave dielectric tube with coupling to copper waveguides. We detect breakdowns at the joints where the dielectric meets the copper when we have roughly 3 MW of RF power going through. We see light glow when this happens and we attribute this to secondary electron emission causing some form of gas ionization (the whole system is under 10^-7 Torr). So this is in addition to the breakdown at the joints. In high gradient studies, the "triple point" region is usually (depending on geometry), the region most susceptible to problems. So in this case, one has the dielectric-metal-vacuum point. In our simulation using Microwave Studio, this part has one of the highest field and thus susceptible to such breakdown/sparks.

Zz.
 
I don't think both phenomena are related: I'm in a quiet, all-static environment, and normally, the fields are way below ionizing any gas, and the dielectrics are stressed way below their breakdown fields. In other words, I shouldn't see anything, but I do: I see small discharge pulses of a few tens of femtocoulombs, one every few minutes.
 
Interesting problem. 0.01Hz is pretty suspicious, though. Any chance it's a measurement setup issue? How are you measuring these discharges? Are there any other paths that the discharges might be taking? Are there any other things that seem to be possible triggers, or at least are correlated with the discharge events? Like ESD events nearby, or HVAC thermostat trips, or AC mains noise?

The only semi-related issue that I can think of for the dielectric breakdown would be some disclinations or other breaks/pinhole paths through the dielectric. How pure is the material? What is it?
 
vanesch said:
I don't think both phenomena are related: I'm in a quiet, all-static environment, and normally, the fields are way below ionizing any gas, and the dielectrics are stressed way below their breakdown fields. In other words, I shouldn't see anything, but I do: I see small discharge pulses of a few tens of femtocoulombs, one every few minutes.

But you don't need to get to the breakdown threshold to get secondary emission. All you need is a few dark current electrons that are above the first secondary electron energy crossover that can induce multipactor. We have observed such phenomenon without causing any permanent damage to our dielectric, and dielectric are notorious for this because their secondary emission coefficient are generally way higher than metals.

The onset of multipactoring is enough cause either ionization or some form of luminescence for light emission, even for a short burst. This could be what you are detecting. It would help if you can monitor the field in the tube and see if there's a sudden drop in such a field when this occurs.

Zz.
 

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