17 07 2012

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During the first trial, our langmuir probe told us that our electron beam intensity was fluctuating at 60 Hz.

This is a problem, because one of the main things we are trying to study is the way changes in beam intensity affect potential well depth, so we want a steady intensity. The frequency of the fluctuations suggest that the AC-powered hot cathode is to blame.  I don’t totally understand the details of how a hot cathode running on AC 120v 60Hz translates to this waveform:

data from the langmuir probe displayed on the oscilloscop

The important thing is to prevent it. To do that, I put a full wave bridge rectifier in the power supply. It converts the AC coming from the wall to DC

It has three essential components.

1) The bridge rectifier

This change the AC sine wave into a waveform expressed by the function abs(sin(x)):

Better, but still not steady DC.

2) The filter capacitor

This gets rid of the ripple. you could compare the capacitor to a bucket with a hole in the bottom. Even if I vary the rate at which putting water into the bucket, the rate at which it come out is always going to be more or less the same, provided that it is sufficiently large compared to the volume of water going in.

However, its impossible to get an absolutely perfect DC output with this setup, because the ammount of charge on the capacitor does affect the voltage at which the current comes out.

This 680 uF capacitor takes away enough of the ripple for our purposes:

the output of the power supply when hooked up to a light bulb

3) An isolation transformer

Usually, the diode bridge and the capacitor would be enough, but our AC isn’t coming from the wall, its coming from a grounded auto transformer. this is a problem because the rectifier only works if the AC input is floating. A transformer with an equal number of primary and secondary wingdings accomplishes this without stepping the voltage up or down.

Nest step is to test it in the chamber.

Domenick Bauer

## New High Voltage Probe

18 10 2011

Just got this Tektronix P6015 high voltage probe. It goes up to 40 kV!

Well actually it only goes to 27 kV without the fluorocarbon 114 dielectric.

It’s huge! Shown next to a normal probe:

Just in time for the next run.

This is the same probe used in the Sydney experiment.

## Arduino Controls 30,000 Volts

10 09 2011

Today I made arduino control 30,000 volts.

My arduino has 3 channels of analog output 0 to 5 volt.

For testing I used this sin wave generator sketch:

int pwmPin = 9; // output pin supporting PWM

void setup(){

pinMode(pwmPin, OUTPUT); // sets the pin as output

}

void loop(){

float something = millis() / 1000.0;

int value = 127.5 + 127.5 * sin( something * 2.0 * PI );

analogWrite(pwmPin,value);

}

This generates a lazy 2 Hz sin wave.

But the output is not really analog, it’s pulse width modulation(PWM):

This tutorial shows how to smooth out  PWM using a low pass filter. My low pass filter used 6kΩ resistor and 4.7 µF @ 45V capacitor.

Here we have the raw PWM output superimposed with the filtered output:

Looks good!

Now we just add the voltage doubling op-amp circuit I made previously, and BOOM:

This shows the source signal and the voltage doubled signal.

Sweet! Now we can control the 30,000 volt glassman power supply.

Here the arduino is sending a slow sin wave to the glassman’s voltage control:

 From 2011-09-10

The Glassman’s slew rate is really slow without a load.

Here is the setup:

## Emergency Stop

14 02 2010

I put together an emergency stop button for the reactor:

This will be positioned a safe distance from the reactor, so you can kill the high voltage from a distance. I’ll wire it up and test it today.

## -30kV / 10mA

24 10 2009

If you’ve been following me on twitter, you know I received a -30kV / 10mA Glassman a few days ago. Now it’s online and it kicks ass. Current limiting, remote controllable… it’s the second unit from the top:

Here is an air plasma it produced:

With current limiting and good air metering, we can get a stable plasma. I notice you get a sense for the plasma just by _listening_ to the glassman. When the plasma is unstable the glassman softly clicks along with the plasma burst.

## Grounding

29 08 2009

Spent the last two days throughly grounding the entire system.

We start by attaching a ground line directly to the overhead sprinkler system. First we scrape off the paint:

Then we attach this purpose made ground clip:

Here it is installed:

We are using 12 AWG insulated stranded copper. In addition to this pipe ground, we are also using outlet ground from two different circuits:

Ground each piece of rack equipment, and the rack itself:

Ground several points on the chamber, sled, and pump:

Here is the high voltage connection point with high voltage divider inside glass insulation:

## Standoff for Superconducting Magrid

29 08 2009

I’ve been doing some brainstorming on the standoff for the superconducting magrid. This is a messy problem! You need a cryogenic feedthrough that is also a high voltage standoff. Then you need to pass in the YBCO superconducting cables, and wiring for the persistent switch.

Yesterday I realized that we can get most of the way there by welding together two off the shelf components:

This diagram refrences parts 9812107 and 9611005 from insulatorseal which is a subsidiary of MDC Vacuum. I’ve sent this drawing to insulatorseal for a quote. One problem is that part 9611005 is only rated up to 6kV so we will need a custom variant to get to the 10kV to 40kV range.

The idea with this setup is that the high voltage could come through a standard HV feedthrough and connect to the insulated tip of this feedthrough via a connecting wire: