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.