Capacitive bridge for Debye length measurements

. dinsdag 15 oktober 2013
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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

. woensdag 29 mei 2013
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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

. zaterdag 22 december 2012
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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

. vrijdag 4 mei 2012
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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

. zondag 29 april 2012
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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

. dinsdag 10 april 2012
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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

. zaterdag 7 april 2012
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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 (