Intro: Simple TC Powered Plasma Globe
A plasma globe is a must-have item for any aspiring weekend physicist. A variety of interesting demos are possible using the swarming streams of ionized particles produced by conducting a high voltage spark through a partial vacuum. There are many informative web sites about plasma physics (see sample links below), so this instructable just provides construction details with a minimal focus on theory.
The most challenging step when building a plasma globe is the evacuation of a glass container on the cheap -- vacuum pumps can be pricey. However, a good quality glass jar w/a narrow neck, a rubber sink stopper, a threaded bolt, a suitable high voltage power source (such as a Tesla Coil), tap water, some hardware and epoxy as well as access to a freezer are what you will need to assemble this simple project. If you can boil water, this project will be will be a breeze. :>D
Of course, use common sense and always keep safety in mind while operating the globe. This project involves electricity and water! If you accidentally connect with the high voltage circuit, the resulting shock can really ruin your weekend plans.
Step 1: Prepare Stopper & Assemble Discharge Electrode
First, locate a glass container with a volume anywhere from .25 to 1.00 liter. If you can find one, a round bottom, boiling flask makes the best container. Next, locate a sink stopper that's the correct size. Make sure that the stopper fits tightly into the neck of the jar or flask. I used a 1.00 liter flask found in a box of discarded university lab supplies and a 1-1/2 inch stopper from a local hardware store.
Drill or punch a hole in the stopper's center just large enough to accept a threaded bolt. Secure the bolt in place w/a washer and nut on each side as shown. The bolt, which will be the discharge electrode, should be long enough so the head is positioned near the center of the flask.
Step 2: Complete Discharge Electrode
Fill inside of stopper w/epoxy to make an airtight seal between the bolt & the stopper hole. Allow electrode assembly to dry completely; otherwise, the vapor pressure produced as the epoxy dries can reduce your vacuum.
Step 3: Creating a Partial Vacuum
If you are using a boiling flask, fill about half way w/water and bring to a boil. Continue boiling over a low flame to remove the dissolved air for about 15 min or until about 1/4 of the water remains. Turn off the flame and immediately stopper the flask. If you don't have a boiling flask, just pour hot tap water into the jar and stopper it. Caution! Don't pour boiling water into untempered glassware, the glass may shatter! Place flask (or jar) in a freezer and leave it there until water turns to slush.
Step 4: Confirm Partial Vacuum
A stopper that's difficult to remove from the neck of the flask is an indication of a strong vacuum. Carefully swirl remaining water around the inside of the flask to remove condensation droplets. The inside of the flash should be clear. Your plasma globe is now finished. If the stopper doesn't resist when gently pulled, there may be a leak. Try another stopper.
Step 5: Finding a Power Source
The high voltages at radio frequencies that are generated by a Tesla Coil create the best visual FX. My TC was temporarily down for repairs, so I tried the retro route and experimented w/an original Model T spark coil purchased through Ebay. Spark coils are electro-mechanical, step-up transformers used to make the sparks that ignited gasoline vapors in early model car engines. Many Old School coilers actually made their first Tesla coils w/these things.
I attached an insulated lead between the discharge electrode and a contact plate of the coil. I taped the return lead to the side of the flask.
Step 6: Test Run Using a TC
The results w/the spark coil weren't worth a digital picture. So, when I finished the repairs on the TC (there's a an i'ble describing the construction of my TC here) I placed the globe on the toroid.
BTW, if you don't have a TC, there's a popular driver circuit for plasma globes made from a TV flyback transformer and a single 3055 switching transistor. This site has construction details. Also, mail order places such as Information Unlimited sell high voltage, high frequency power supplies similar to this one if you don't want to do the build.
Step 7: Final Results
Although the plasma streams were visible in the light, the display was better in a darkened room. Here are some pictures of a basic spark, secondary arcs drawn by a pair of pliers, a corona discharge as well as a mean looking power arc.
Good luck & have fun!