Is the tip of the anode blocked as well? There are plenty of images of brems emission from the tip of the anode as well as the base. My working hypothesis is electrons that were once confined during the pinch escape the pinch region when the current falls below the confinement threshold. That explains why they are late in time and why they are hard. Another option is current restriking to the anode after the pinch but I’ve never observed any electrical evidence to support a current restrike near the tip of the anode. An imaging diagnostic would help verify the shield placement. I never trust detectors without images to verify what they are seeing.
I attached an image of an SS304 anode with a hydrogen pinch. The image is integrated over 100 shots at 250 kA. The image is of the >10 keV spectrum. The anode wall is too thick to see the anode base but the tip of the anode is clearly visible. The really striking feature is the lack of a pinch. Too little mass density in the pinch to produce significant brems while Fe does a fine job of converting electrons to x-rays. Put argon in instead of hydrogen and you get a nice visible pinch.
I suggest switching to a gas that does not produce neutrons to verify that your x-rays are not really gamma rays. SS304 vacuum chamber can produce (n,gamma) reactions that lead to 800 keV photons. The reaction relies on fast neutrons so it could explain the time lag in the second pulse. Neutrons are slow compared to photons. Have you considered using a stepwedge spectrometer to measure your photon spectrum? They are cheap and easy to setup. The data analysis requires a little work, but flat response films like GAF film reduce some of the problems. It is perfect for machines that can replace the films between shots. Film to digital reduction does not require developing with GAF as it visibly darkens from white to gray. A modest quality scanner is all that is required to convert from film to digital data.
FYI, I sent Steve on the Sandia team our PoP paper to get his thoughts.
Assymetric, You raise a lot of good points. First, we do include the tip of the anode in the field of view—about half the circumference is visible to our detectors. Your images show that something is lighting it up in x-ray emission. For our first x-ray pulse, it is hard to rule out some contribution, but I am not sure it is large. You can’t see the pinch, but we have four times as much current as you and the x-ray output will scale as I^5, while the beam energy will be about I^3, so relatively our pinch will be brighter. It is hard to see how the edge of the beam can be very bright, since from the damage done to the anode it looks like the beam is a Gaussian that is at most 20 degrees in half width, so at 90 degrees it should be pretty weak.
For our x-ray pinhole imaging we did not try integrating many shots—we should. So far we did not get an image with single shots.
We are sure our x-ray pulses are not gammas from neutrons hitting the vacuum chamber wall. The neutrons clearly come later than the first x-ray pulse.
The step wedge idea seems a good one. Do you have any references on that?
I will check on a step wedge reference and report back. I am familiar with the step wedge concept from my grad school days as it was included in course work.
Don’t rule out neutrons just yet. I thought the same thing initially and the little buggers ruined a week. If you consider that the neutrons are traveling at ~2E7 m/s and your chamber is something like 10 cm from the source, you have a delay of 6 ns before the neutrons hit the first wall. In your Physics of Plasma paper, you show two x-ray pulses that could be 6ns apart at the peaks. The neutrons produce an ~850 keV photon in some fraction of the reactions with the wall. That photon would give the appearance of a hard x-ray spectrum when it was really nuclear in origin. Another thought is reactions with your copper anode. We have done some calculations that suggest our hardest x-rays might be from both the anode and the chamber. More to do to confirm. A test with hydrogen would lend some insight into the problem. Just double the operating pressure to keep the mass the same as D2.
I was surprised to see the x-ray emission from the anode rim but I’ve recorded shots in most of the common PF gases and the feature is universal. I can’t speak to its absolute strength but some images I’ve collected show the rim is as bright as the base of the anode when looking down/up at the anode. I didn’t use a step wedge to look at the x-ray energy in each region.