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Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
Hi All,
Last November we invited a group of HV/Tesla Coiling enthusiasts and Theo Gray of Popular Science Magazine to help us make more Lichtenberg Figures. Various specimens of 1/2" - 4" thick acrylic (PMMA) specimens were internally charged by directly injecting electrons via a 5 MeV electron beam accelerator. The charged specimens were then manually discharged by poking them with a sharp conductive point. During the photo shoot we determined that we could (VERY carefully) handle fully charged specimens up to 12" x 16" x 1". Specimens of this size can store as much as 1 -1.3 kJ of electrostatic energy between the positively charged outer surfaces and the negative space charge region within. The on-line article, and a link to a short video clip that shows some large specimens being discharged, can be found here:
We also measured the discharge current from a number of 4" x 4" x 3/4" specimens using a special holding fixture that connected a HV wire (through a Pearson wideband CT) between a sharp discharging point to the fixture which held the specimen between a pair of metal plates. A Tektronix TDS 3014B 100 MHz DSO was then used to capture the waveforms. During a series of measurements we found that the discharge current was not a single pulse, but typically consisted of 2 - 5 distinct, overlapping current peaks as the discharge apparently propagated in a discontinuous manner, sequentially discharging untapped regions of excess charge. One of these is shown below. In the following waveform, the overall discharge transient lasts for about 120 ns, consists of four overlapping peaks, with the highest peak at about 580 amperes. The theoretical charge plane voltage (in the center of the specimen prior to discharging) was estimated to be about 1.5 MV versus the specimen's exterior surfaces. Captured positive ions form a large surface charge on the exterior surfaces of the specimen which screens much of the sE-field from the stranded negative charge within. As a result, the specimen becomes a "plateless", high energy capacitor. Even a smaller 4" x 4" x 3/4" specimen can theoretically hold ~45-50 joules. The excess charge does bleed off via the PMMA's bulk leakage resistance. However, by using high quality material (and other tricks) we found that we were able to successfully discharge a number of specimens 15-20 minutes after initial irradiation with good results.
We are planning to do another production run next month. One experiment will be to discharge 4" x 4" x 3/4" specimens within a strong magnetic field supplied by a 400 pound laboratory electromagnet. We hope to induce a spiral pattern in the resulting discharge, hopefully similar to the electrodeposition effects shown below:
During an earlier experiment, we used a pair of large ferrite magnets but we were unsuccessful.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
Bert wrote ...
One experiment will be to discharge 4" x 4" x 3/4" specimens within a strong magnetic field supplied by a 400 pound laboratory electromagnet. We hope to induce a spiral pattern in the resulting discharge, hopefully similar to the electrodeposition effects shown below:
Some random thoughts.... Wouldn't you need to use the magnet during the exposure process? I thought the pattern was essentially created as a pattern of electron paths within the block, which was then 'developed' by the discharge causing heat/shock fractures along the previously created paths. Or is it the case that the charge field in the block is very uniform and the patterns created as the electrons try to find their way out? Can you 'tap' a charged block anywhere regardless of orientation when irradiated and have the 'tree root' be at that point?
I wonder how much effect a magnetic field could have on electrons which are effectively trapped in a solid - do they have any degree of freedom to move under the influence of the magnetic field? By the time any significant current starts flowing, perhaps the electrons have already found their pathway through the plastic, and so not be very vulnerable to magnetic fields. Perhaps an external electrostatic field might have more of an effect? What if you put two irradiated blocks next to each other and discharged one of them?
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
Some random thoughts.... Wouldn't you need to use the magnet during the exposure process? I thought the pattern was essentially created as a pattern of electron paths within the block, which was then 'developed' by the discharge causing heat/shock fractures along the previously created paths. Or is it the case that the charge field in the block is very uniform and the patterns created as the electrons try to find their way out? Can you 'tap' a charged block anywhere regardless of orientation when irradiated and have the 'tree root' be at that point?
Irradiating the specimen simply charges it up. There's no internal breakdown occurring at this stage unless the specimen prematurely discharges as it's being irradiated. A charged-up specimen looks identical to an uncharged specimen except for a bit of X-Ray darkening (solarization). Prior to discharging, the excess charge is fairly evenly distributed within a thin (~1/16 - 3/32" thick) space charge layer that's ideally located about half way through the acrylic. Once breakdown is manually initiated, the discharge begins propagating through the charged acrylic. Since the leading edges of the propagating discharge are positive relative to the internal acrylic, the discharge pattern that forms is actually a positive Lichtenberg Figure. Whether the applied magnetic field will be sufficient to cause a change in the trajectory of the propagating fronts of the discharge remains to be seen. Because we can control where the root of the discharge appears, we have some control over the eventual shape of the discharge(s).
I wonder how much effect a magnetic field could have on electrons which are effectively trapped in a solid - do they have any degree of freedom to move under the influence of the magnetic field? By the time any significant current starts flowing, perhaps the electrons have already found their pathway through the plastic, and so not be very vulnerable to magnetic fields. Perhaps an external electrostatic field might have more of an effect? What if you put two irradiated blocks next to each other and discharged one of them?
That's really a good question - one that currently does not appear to have an answer. Assuming that the avalanche process at the tips of the discharge involve liberated higher energy electrons, we suspect that there should be an effect, but it may require a magnetic field considerably stronger than we can create using an iron-core lab electromagnet (~18 kG). Hopefuly, we'll find out in March.
jpsmith123 wrote ...
Hi Bert,
I'm curious, what kind of accelerator are you using?
Regards, jpsmith123
For several years we have been renting beam time on a commercial 5 million electron volt 150 kW materials processing DC accelerator. The specific machine we currently use is called a Dynamitron, made by Radiation Dynamics, Inc. Details about the Lichtenberg creation process can be seen here:
The theory of operation of the Dynamitron (compared to other types of accelerators) can be found here:
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Awesome Bert! I'll have to check out that issue!!!!
I just got around to machining a nice LED illuminator with my new milling machine for my Lichtenburg figure. Using a 5W luxeon LED it looks spectacular, even under full daylight!
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
[quote]
I wonder how much effect a magnetic field could have on electrons which are effectively trapped in a solid - do they have any degree of freedom to move under the influence of the magnetic field? By the time any significant current starts flowing, perhaps the electrons have already found their pathway through the plastic, and so not be very vulnerable to magnetic fields. Perhaps an external electrostatic field might have more of an effect? What if you put two irradiated blocks next to each other and discharged one of them?
That's really a good question - one that currently does not appear to have an answer. Assuming that the avalanche process at the tips of the discharge involve liberated higher energy electrons, we suspect that there should be an effect, but it may require a magnetic field considerably stronger than we can create using an iron-core lab electromagnet (~18 kG). Hopefuly, we'll find out in March. [/quote] Might you get a stronger field by placing a couple of big neodymium magnets either side of the block? Obviously some care would be needed to avoid them banging into the block and initiating the discharge...
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
Tesladownunder wrote ...
Hey, where did this one come from? As Bert would recognise, it's not one of his. I took this pic a few weeks ago but didn't get around to posting it.
BTW, good media coverage and good science as usual.
Hi Peter - thanks for the kind words.
OK... where DID it come from??
The specimen has been manually discharged along at least three sides, but none of the resulting discharges was able to span the entire area. I've seen this occur in specimens that low levels of stored charge, due either to too low a beam dosage or excessive charge leakage (poorer material, high ambient higher temperature). We normally irradiate the entire specimen at once using a scanning e-beam from a DC accelerator. However, if the specimen was made using a smaller research of medical pulsed accelerator, it's also may be that (because of the smaller effective beam diameter) smaller portions of the specimen were sequentially irradiated and discharged. It's also interesting that the two larger discharge regions overlap, and that both are dendritic discharges. This strongly suggests that whoever made the specimen may have irradiated (and discharged) only a portion of the specimen at a time, discharged it, then repeated this process for a different region. Multiple charge depths may have also been employed, particularly if beam penetration depth was not about halfway through the specimen.
Peter, if you look at the specimen edge-on, are all of the discharges are in the same plane? If not, it's likely that the person who made the specimen may have used the following sequence: irradiate left portion of specimen, discharge from left edge, flip 180 degrees (to expose the opposite surface to the beam), irradiate bottom portion, discharge from bottom edge, and then finally discharge from right edge (only small residual charge left over).
If a specimen breaks down as it's being being irradiated, OR if you re-irradiate a previously discharged region, the resulting patterns range from being a chaotic tangle, very densely dendritic, "moss agate" ultra-fine dendritic, to (very rarely) multiple "snowflake-like" patterns instead of the long branching lightning-like patterns seen in your specimen. This strongly suggests that a given region of your specimen was only irradiated once OR that it was flipped 180 degrees to form at least two independent charge layers. For comparison, some examples of chaotic, dense dendritic, moss agate, and snowflake patterns can be seen below:
Do you know anything about the history/creation of your specimen?
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I thought that might get your interest. It is from Doug Baker who ran a company in Western Australia, doing vacuum and medical technology work. The specimen was 30 years old and originated from a medical Linac which was 5Mev as he recalls. They did lots of them in an unofficial capacity. I asked him about the output window. I can't remember the details (? mica) but it was tiny compared to your huge titanium foil aperture.
I visited Doug recently when he expressed interest in making a TC to drag the kids or grandkids away from computer games. (I told him that doesn't work in my experience of N=2).
I knew that you would see the multiple discharges and recognise a less powerful setup.
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
mikeselectricstuff wrote ...
Might you get a stronger field by placing a couple of big neodymium magnets either side of the block? Obviously some care would be needed to avoid them banging into the block and initiating the discharge...
Could we? Perhaps if we used soft steel flux concentrators along with a pair of large NIB magnets. NIB magnets alone would have a max Br of about 14 kG, while the lab electromagnet we planned to use can hopefully go up to ~18 kG. Rather than drag the heavy electromagnet and DC power supply to the accelerator facility (~500 miles one way), we're also looking at charging up specimens and then keeping the cold using dry ice. in a cooler. When kept at lower temperatures, charged specimens made from high quality cast PMMA can retain sufficient charge so that they can be discharged days, weeks, perhaps even months, later.
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Lichtenberg Figures are in March issue of Popular Science Magazine
