Capacitive bridge for Debye length measurements

Very sensitive capacitive bridge, measures the delta between C3 and C4, the key is the floating input voltage from the 4 transformers.
R1 is the input impedance of the AD8310 detector.
L9 is there to reduce the drive current.

The 4 transformers make the 10MHz input signal floating (and multiply the input by 4), since C1 and C2 are equal (use NP0 and a trimmer) the voltages over C4 and C3 are equal if C4=C3, any current going through R1 must be the difference in current between C3 and C4.
When C4 = (1+d)*C3, where d<<1 the Thevenin voltage is V1 * d, and the Thevenin impedance (without L9, and without R1) is a reactance of  0.5 * C3
The attainable resolution with such a circuit should be around 10 aF.
A higher resolution is possible, but that would require a higher frequency and noise sensitive air-core transformers.
In this example it is a variation is capacitance which gives an output signal, when C4 is made of isolated electrodes in the plasma, the (very small) dielectric losses and variation in reactance could be used to measure the Debye length.
The capacitor in the plasma (C4) consists of two isolated electrodes, C3 = C4 at vacuum/air, when the space between the electrodes is a plasma with a low Debye length (conducting) C4 will have a higher capacity but low dielectric losses. (output signal high and in phase with input signal), when the Debye length becomes comparable to the distance between the plates, the dielectric losses will be higher and C4 will be close to C3, so that will give a low output signal and a higher phase-shift.
De plasma can be a simple DC discharge.
Since it uses only one stable frequency (10MHz in the example) noise filtering should be simple.

Looking for a place to build a lab

Running out of space for my equipment and tools.

Looking for a lab place in Leiden-Wassenaar.

Need about 80 m^2, please mail me ( linuxificator at gmail com) if you know anything.

TPU240 TCP600 and maxigauge

A few words... the "octavio" is not going to work, sorry.
Got a new plan, have a new setup, all CF rings, I'm using my old setup for plasma cleaning.

Way too busy to spend time on this blog, this weekend second try in plasma cleaning (first try too many things broke down)

Here is TCP600, TPU240, DUO008B, maxigauge, mechanical feedthrough stuff etc, gets into the E-7 range in about 30 minutes without bake-out.

lamps and controllers just arrived for bake-out inside chamber, also got my CoSm magnets for the ion guns.

Pressure control

I tried to control the pressure by placing an electric valve between the DUO008b and the turbo pump, and switching the valve on and off with the TPG251 single gauge controller.

It worked, but it wasn't really stable, for example; setting HI en LO on the TPG251 to 6E-5 and 5E-5 resulted in a situation where the pressure dropped from 6E-5 to 1E-5 when the valve opened.

So then I connected the relais of the TPG251 to the external on/off function of the TCP121 turbo pump controller.

And that worked perfect!

Just switching the turbo pump on and off was enough to control the turbo speed, pressure was controlled within 10% for all desired values in the 1E-3 to 1E-6 mbar range.

The TPG251 has an external input for setting HI and LOW, should be easy to control it from the computer.

Xray transformer under oil

Have been busy with the Xray transformer.
It's under oil now, the outgassing with vacuum took some time, lot's of bubbles :)

I tested the transoformer with 10V input, output was about 700V, I expected more, but maybe it's not that linear at low input voltages.

Negative HV supply for PMT's

The idea of making a high voltage with a DC-DC converter and a voltage multiplier can also be used to generate a negative supply for PMT's:

There is no need to use a P-channel mosfet, just reversing the diodes in the voltage multiplier reverses the polarity of the output.
The feedback circuit needs to be adapted to a negative output, when the PCB's for the positive supply are in, I will extend the design and make one ciruit board which can generate -3kV...+3kV with jumpers to switch polarity (changing in and out of the voltage multiplier and changing the feedback circuit).

10mm polycarbonate for the 85000V supply

Have been working on the (very small..) saw table today, sawing some polycarbonate sheet for the high voltage supply:

It's big enough for the Xray transformer which is going to be immersed in transformer oil (the real thing, 60kV/mm), to get the air out I will use vacuum, which will create a force of about 2900N on the biggest sides. The sides will be glued together with epoxy, the edges are 45 degrees.

I also got a preview of the circuit board for the neutron counter power supply (eurocircuits.nl):

Circuit board voor HV supply

The SNM-10 and another detector tube need about 1600V.
This is the circuit board:

Designed with Eagle.
The circuit is VERY DANGEROUS, it can deliver a few milliamps at 3kV, the capacitors are charged at 3kV. If you build this thing, make sure that you know what you are doing!
Eagle BRD file
Eagle SCH file

40nF 100kV

For the 85000V supply, a capacitor bank:

The capacitors are 10nF, 20kV, so the 110 capacitors should be 44nF (5x series of 22x parallel).
The cap meter shows 40nF total, only 10% off. The bank will be in oil, and I still need to find the right resistors (5x 200Mohm, 2W, or enough other values make 200Mohm)

85000 volts

Did some measurements on the new XRT transformer.
It's supposed to be 1200W, 85000V @14mA.

And that's not 2x42500V, no, it's really one secondary winding, so with the 4x 80kV diodes I have, I can even make a symmetrical -80kV, 0, +80kV supply!

The DC resistance of the secondary winding is only 78k, so that's only 15W at 14mA, primary resistance is very low, seems like 1200W continious power capability to me :)

more HV supply stuff

Still working on the HV supply, did some test measurements, and designing a pcb in eagle (cadsoft).
The circuit is really working well!
This is the eagle schematic, pcb will follow soon when I have al the parts.

It is working much better than a normal voltage multiplier, and I was wondering how much stages could be added, I tried 11 stages, the circuit still works fine in the simulation, with about 3.6W of output:

The next image shows the "why", the driving voltage peaks are very short spikes, the lower line of capacitors have a very low impedance for these spikes (or in other words, the capacitors form a high-pass filter, but only a very small part of the energy of a delta function is in the lower frequencies)

It should be possible to add a lot more stages without loosing much energy.

HV supply for neutron counter or geiger müller tubes

I tried to make an HV supply for counter tubes before, but making the transformer took some time, and it didn't work well.
Based on the ideas in my previous post I tried to combine a switched DC-DC converter with a voltage multiplier, this is the good-old-pen-and-paper result:

You can click on the image to see it enlarged.
I'm using the MAX1771E, and the circuit is actually quite simple.
The only non-trivial component is the 1M resistor between the 1.5V REF en the FB input, it is needed to lift the FB input above 50mV on startup, so it forces the MAX1771E into non-bootstrapped mode, even if the output voltage is 0.
Without it the MAX1771E won't start unless the supply voltage is below 12V.
When the output voltage is stable, the voltage on the REF and FB are both 1.5V, so the 1M resistor does not introduce any error.
The off-switching of the MOSFET creates a high dU/dt peak at the drain, the lower line of capacitors have a very low impedance for this, because of the high dU/dt.
The upper line of capacitors are charged by the peaks, but they don't have any AC feed, so the output voltage at the end of the upper line is stable.
The peak drain voltage is limited by the energy in L and the combined Cdg and Cds of the MOSFET (about 1n for the NDF10N60ZG, dI/dt is not the limiting factor), peak can be calculated with LI^2=Cmosfet * U^2, where I is the max current set by the current sense resistor.
I started with 30mOhm current sense, giving an Imax of 3.33A, and a peak of about 500V (calculated) or 670V (measured).
Output current was 0.5mA at 1600V.
It worked great, but the MOSFET was getting a bit hot, maybe be because of breakdown, but probably because the on resistance is about 0.7 ohm.
Also, the circuit keeps pumping in power (about 1.25A at 12V), even without load.
With a sense resistor of 55mOhm everything worked fine, and the input current without load dropped to 250mA.
So the next step is using a MOSFET with a lower on resistance, bigger HV capacitors, and better diodes.
I've ordered the SiHG47N60E MOSFET's (64 mOhm), AU2PK diodes and got some 100nF/1kV caps from ebay.
This is a simulation of the maximum output power at >1600V (1637V) into 200K.
Seems to be ok, 13.4W out at 12W in, a little bit impossible but at least the simulation doesn't show any big losses.
I used 7us on, 2.3us off and 100ns rise and fall time to simulate the max power output signal of the MAX1771, this gives a peak current of 4A in the inductor.

Adding a stage and simulate with a 2M load gives an output voltage of 3800V at 1.9mA, but the really great thing is that the voltage step per stage doesn't get less at higher voltages, this is because of the high dU/dt of the spike at the drain of the mosfet, the impedance of the capacitors of very low for this spike. I wonder how many stages can be added before each stage starts to step up less than the previous one.

SNM-10 Neutron detector

Got a B10-lined neutron detector "SNM-10" or "CNM-10".
It works in a "corona mode", which means noisy current (about 2... 2.5 uA) at 1650V in series with 50 MOhm.
The pulses will be about 20V or so, but I'm planning to measure the current with a common-base circuit, to avoid the capacitive load over the high series resistor.
This is what it looks like:

For the high voltage I will combine a switched DC-DC step up converter with a voltage multiplier, the schematic only shows the idea, I will use the MAX1771 not the LTC:

We can do the same thick without a transformer:

Better helium plasma

The previous movie only showed the plasma at low voltages before the feedthough broke.
We made a new feedthrough, fused quartz, should be able to handle 50kV.
This time we go beyond sparks, a stable plasma can be seen, it's clearly red (helium), and near the end (around 7.5 kV, about 3.5 mA) some Xray spots can be seen.

Balloon feed

Sounds great "balloon feed", we connected the 18 bar helium to a hose, big bang.. So we switched to a complete new concept the "Balloon feed" (tm) :)
It's a pink helium balloon connected to the needle valve which is connected to the venting input of the turbo pump.
When running the turbo in standby (66%) it will pump heavy gasses much better than lighter ones, so we feed it He (4) for now, because it's the same mass as D2, so pressure regulation with the needle valve should be the same for He en D2, when we start with deuterium electrolysis any D2O (20) will be pumped out by the turbo.

Helium plasma

The teenagers have produced a helium plasma.
Their experiment started with a bang (18 bar of helium on a duct-tape hose-connection) and ended with sparks.
This is the video of their plasma, the focus in the center of the grid is clearly visible:

Standard fusor experiment

Two teenagers confiscated my equipment to do the standard fusor experiment.
This is their grid, polished aluminum ground plate, feedthrough with viton ring, fused quartz isolation, hard soldered stainless steel grid.

Octavio Part Three

So, in octavio part two the conclusion was that the pinch coil configuration (current in the outer coils opposite to the current in the core) cannot create a virtual cathode.
So the only configuration left now is the core with focus coils:

The focus coils with the core will create a magnetic field which is zero in the center of the core and with straight field lines between the core and the focus coils.
To understand the idea, take a look at the following 2D analogy (not a cross section, just an analogy, you can click the image to see it enlarged):

Outside the core the electrons will spiral in the magnetic field, near the center they will widen their spiraling path and they will be repelled outwards by the electric field of the virtual cathode.
As explained in octavio part two the electrons will slow down because they loose energy when they are accelerated/de-accelerated by the electric field between the focus coils and the core, and when spiraling in the magnetic field, bremsstrahlung and synchrotron radiation losses.
As explained in part two, to compensate for the losses an AC voltage can be superimposed on the DC potential of the focus coils, this will also introduce extra losses (bunch induction, explained in part two), which can be compensated by the same AC power.
This will create an oscillating virtual cathode, the idea is to create a situation where the DC current on the high voltage positive core is almost zero.
The voltage between the wehnelt cylinders and the focus coils should be negative most of the time, only when the AC power is at it´s positive peak the electric field between the wehnelt cylinders and the focus coils should allow some electrons to be injected into the system, the AC power maximum positive peak is the moment when most of the electrons are near the focus coils, the negative space charge in the center of the focus coils at that moment can prevent too much electron injection (in other words, the input current is self regulating when the focus coils have the right negative DC bias).
Some preliminary calculations show that the resonant frequency can easily get higher than two gigahertz, but it will be lower when the magnetic field is stronger. Feeding AC power to such a construction at such high frequencies is another challenge.
I've been thinking about a self-oscillating octavio, but naturally every oscillation will get it's AC power by slowing down electrons, keeping the electrons at a high speed will always dissipate AC power. Any self oscillation (some kind of buncher/catcher or reflex-klystron-like construction) should be between the the electron guns and the focus coils, not between the focus coils and the core.
Still lots of calculations and simulation to do, and I have to find a way to make it self-oscillating.

Polishing the ground plate

Did some polishing last weekend, here is picture of the ground plate, nice and shiny:

The ground plate will have two 1-pole, two 8-pole and one 4-pole feedthrough.
Also tried to fix the mass spectrometer, still have the problem that after bake-out all the QMG064 sensors fail.
I tried to clean the grid with fine quartz powder and ultrasound cleaning, no effect at all..... to be continued.

Octavio Part Two

So, how would it be possible to make a virtual cathode with the spherical octahedron configuration?
What we need is a volume where the magnetic field strength is low or zero, confined within a stronger magnetic field.
Take a look at the following picture:

Still the same core, but now it's surrounded by 8 coils, this picture also shows the 8 electron guns.
The core is at a high positive electric potential (e.g. +50kV), the outer coils (pinch-coils) are at a medium positive potential (e.g. +500...1000V) and the wehnelt cylinders are at a low positive potential (e.g. +50V), the hot cathodes of the electron guns are slightly more positive (e.g. +55V) than the wehnelt cylinders. (to prevent electrons flowing from the electron gun cathodes to the grounded reactor wall the electron gun cathodes are not at ground potential.)
The current through the outer coils creates a magnetic field in the opposite direction of the magnetic field created by the sphere triangle it is facing, cancelling the magnetic field in the center of the sphere triangle.
They will "pinch holes" in the magnetic field of the core, that's why I call them pinch-coils. (originally I thought these coils would have to have their magnetic field in the opposite direction, making the field lines outside the core straight in stead of diverging, creating a strong focus effect for the electron beam, it might still be interesting to do that experiment also, in that case I would call them focus-coils).
The volume where the magnetic field is low or zero takes the shape of a "stretched cube with long points", as if you would take a cube and pull the corners until they are long enough to just touch the centre of the outer coils.
This is exactly the shape needed to confine the electrons in the centre and be able to inject the electron beams.
Just as in the polywell, this could create a cloud of electrons in the centre of the core.
But that still means that electrons loose energy and slow down, the fastest electrons passing through the centre of the core and the slower electrons drifting towards the grid and hitting the grid.
As in the polywell this process will "blur" the virtual cathode.
When the electrons are leaving the core, they slow down, turn around, get accelerated in the opposite direction and enter the core again. In this process they loose energy (brehmsstralung again), so they will slow down and blur the virtual cathode.
This can be prevented by superimposing an AC voltage on the pinch coil potential. This AC voltage should have the same period as the time it takes for an electron to travel from one pinch coil through the core to the pinch coil on the other side of the core.
The idea is to create an oscillating virtual cathode (in resonance), where the electrons don't slow down, in other words, the losses are compensated by the AC power applied to the pinch coil potential. (The load on the AC power supply behaves  as a capacitor with a high dielectric loss, the "dielectric loss" is the power needed to prevent the electrons from slowing down).
Not only the brehmstrahlung losses have to be compensated, when we have an oscillating virtual cathode the electrons form "bunches", just like they do in a klystron, these bunches induce currents in the core, this energy is also lost and can be compensated by the AC power on the pinch coils.
This is completely different compared to the polywell, in the polywell the electrons have a wide energy distribution and the virtual cathode is a statistical effect, a cloud of electrons, in the resonating octavio the electrons have a narrow energy distribution and the virtual cathode is oscillating and sharp.
Next step would be to see if it's possible to apply POPS, maybe by superimposing an extra AC voltage of a lower frequency on the potential of the pinch coils, maybe it's enough if the the octavio resonance frequency is a multiple of the POPS frequency.

This sounds great, if it was true.....

However, if an electron would be kept on it's track by the "magnetic tunnel" created by the octavio core and the pinch coils, then it's path would be bent in the wrong way when the electron travels the other way.
So we have to use focus coils, not pinch coils, where the electron path outside the core will be a spiral.
I will explane more about the focus-coil configuration in octavio "part three", in part one I explaned that the core alone would not work, the core with focus coils might create a virtual cathode, probably a spherical "shell like" virtual cathode.
The experiment will be about the core with focus coils and an AC voltage on the focus coils to compensate bremsstrahlung, synchrotron radiation and bunch-induction losses.