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Re: Langmuir Probe Creation and Design

Posted: Sun Apr 25, 2010 5:00 am
by Mike Beauford
Hi Brian,

My bet is it's the impedance of the power supplies that is causing the curves to be different. I'm wondering if there is a way to massage the data to remove any type of impedance mis-match?

My 2 cents!

Mike Beauford

Re: Langmuir Probe Creation and Design

Posted: Mon Apr 26, 2010 2:09 pm
by Richard Hull
I'll go along with Brian on the impedance issue. Like Chris noted, put a large external capacitor across the supply output and do the work. I am with you that the old Kepco might be the correct supply to use here.

Richard Hull

Re: Langmuir Probe Creation and Design

Posted: Mon Apr 26, 2010 9:31 pm
by bk8509a
Center probing. I'll tell you guys how it goes in a few days.
We're in the process of taking our finals.

Re: Langmuir Probe Creation and Design

Posted: Tue Apr 27, 2010 7:38 pm
by DaveC
Brian -

My question was not so much about what "floating" or "isolated" meant , but how you will or are making this measurement. In other words, How are YOU isolating your power supplies? (That's was behind the schematic request.)

Most low voltage power supplies will bring into the circuit the internal impedances to ground of the supplies. At low plasma potentials, this is rarely a problem. But at multi-kilovolt potentials, a low voltage power supply's insulation resistance will be broken down.

At times this issue is circumvented by using batteries in some sort of insulating housing, long range potential adjustments with plastic rods, voltmeters and ammeters surrounded by corona shields and possibly suspended by monofilament fishing lines, or supported on plasctic cups, ... you know the "good" experimental procedures that Feynmann seemed to appreciate most. (See his comments about experimental work at Princeton vs MIT in "Surely You're Joking Mr Feynmann")

Regarding your latest adventure with measurements at the center of fusor, these will bring the isolation issue into sharp focus, since the ion potentials would be expected to approach HV supply voltages.

So be careful about floating low voltage supplies here, you may breakdown the isolating resistance. A good HV isolating transformer (see one of the earlier threads) would work well to preserve the hardware.

Dave Cooper

Re: Langmuir Probe Creation and Design

Posted: Thu Apr 29, 2010 7:50 am
by bk8509a
I will have a bigger post on this later. I'm writing my senior thesis right now. Best data yet. I'll let you have your opinions.
Yes, it is repeatable.