Colors in a plasma globe

[Orthoceras]

TL;DR

Why are the streamers blue while the pads at both ends are orange, in a plasma globe?

I have a common plasma globe with blue streamers and orange pads at both ends. The orange light is emitted by neon and the blue light is presumably emitted by argon and xenon.
Why are the streamers blue while the pads at both ends are orange? A plasma globe's electric field is strong near the central electrode, decreasing with distance, so I would not expect the orange color at both ends.

 

[renormalize]

TL;DR: Why are the streamers blue while the pads at both ends are orange, in a plasma globe?

How much research did you attempt before posting your question? I ask because at or near the top of a simple google search for "plasma globe colors" is the Wikipedia link https://en.wikipedia.org/wiki/Plasma_globe which states:

"Plasma balls are driven by high-frequency (approximately 35 kHz) alternating current at 2–5 kV... The radio-frequency energy from the transformer is transmitted into the gas within the ball through an electrode at its center... Plasma filaments extend from the inner electrode to the outer glass insulator, giving the appearance of moving tendrils of colored light within the volume of the ball (see corona discharge and electric glow discharge)."

"Some balls have a control knob that varies the amount of power going to the center electrode. At the very lowest setting that will light or "strike" the ball, a single tendril is made. This single tendril's plasma channel engages enough space to transmit this lowest striking energy to the outside world through the glass of the ball. As the power is increased, this single channel's capacity is overwhelmed and a second channel forms, then a third, and so on. The tendrils each compete for a footprint on the inner orb as well. The energies flowing through these are all of the same polarity, so they repel each other as like charges: a thin dark boundary surrounds each footprint on the inner electrode."

"The ball is prepared by pumping out as much air as is practical. The ball is then backfilled with neon to a pressure similar to one atmosphere. If the radio-frequency power is turned on, if the ball is "struck" or "lit", now, the whole ball will glow a diffuse red. If a little argon is added, the filaments will form. If a very small amount of xenon is added, the "flowers" will bloom at the ends of the filaments."

So ultimately it is the energy variations within the currents flowing through the plasma in different regions that excite the various noble gasses to glow with their characteristic colors in those regions.

 

[Charles Link]

The OP may find it of interest that as the degree of ionization in a plasma increases, basically as the current density rises, the electrical resistance decreases. That way, once a path, i.e. an arc, through the plasma is established, it tends to stay there with some stability. This is what makes up the filaments.

Edit: Note that it can take a couple hundred volts or more to start an arc in a plasma, e.g. a fluorescent light bulb, but once the arc is established, it can run at ten or twenty volts or thereabouts. If I'm not mistaken, you do need to exceed the ionization potential of the gas that is employed to sustain the arc. The ionization potentials are typically in the ten volt range.

 

[Orthoceras]

How much research did you attempt before posting your question? I ask because at or near the top of a simple google search for "plasma globe colors" is the Wikipedia link ...

I'm sorry I annoyed you with my question. However, your quote from Wikipedia does not really answer my question about the different colors. What I meant with the question is more in line with the Franck-Hertz experiment. The first excitation energy for neon is 16.6 eV, for argon it is 11.6 eV, and for xenon it is 8.3 eV. The orange color at both pads means that electrons reach a kinetic energy of at least 16.6 eV. The blue color in the streamer means that electrons do not reach 16.6 eV, instead they collide inelastically with argon or xenon. I don't understand why electrons reach a higher kinetic energy at the pads than in the streamer.

 

[Charles Link]

I'm sorry I annoyed you with my question. However, your quote from Wikipedia does not really answer my question about the different colors. What I meant with the question is more in line with the Franck-Hertz experiment. The first excitation energy for neon is 16.6 eV, for argon it is 11.6 eV, and for krypton it is 9.9 eV. The orange color at both pads means that electrons reach a kinetic energy of at least 16.6 eV. The blue color in the streamer means that electrons do not reach 16.6 eV, instead they collide inelastically with argon or krypton. I don't understand why electrons reach a higher kinetic energy at the pads than in the streamer.

one micron=1.24 eV so that 620 nm=orange is 2.48 eV. The energies you have there are all in the UV if I'm not mistaken. I don't know the answer though to why the streamer is blue and the pads are orange. The orange would be of lower energy than the blue.

The ionization potential for argon (15.8 volts) is lower than neon (21.6 volts), so the free electrons might be coming mostly from the argon, but the visible bound electron transition energies are lower for the orange=neon.

If it takes more energy to start to excite the neon, perhaps the free electrons pick up speed as they traverse the streamers. That's just a guess, but I think a possibility.

 

[Orthoceras]

one micron=1.24 eV so that 620 nm=orange is 2.48 eV. The energies you have there are all in the UV if I'm not mistaken. I don't know the answer though to why the streamer is blue and the pads are orange. The orange would be of lower energy than the blue.

The orange of neon, and the blue of argon/xenon, are fluorescence colors. De-excitation occurs in a sequence of smaller energy steps, whereas excitation occurs in a single step..

The ionization potential for argon (15.8 volts) is lower than neon (21.6 volts), so the free electrons might be coming mostly from the argon, but the visible bound electron transition energies are lower for the orange=neon.

According to the Wikipedia quote in the post by renormalize, the pressure in the globe is similar to one atmosphere. At that pressure, the mean free path might be too small for electrons to reach a kinetic energy greater than the ionization energy of neon/argon/xenon.?

 

[Charles Link]

Just an additional comment or two: What you @Orthoceras are asking seems simple in a way, but plasma physics can get very complicated, and the best we might be able to do is get a reasonably accurate idea of what is going on inside the lamp.

Years ago, as an undergraduate student, I did an independent study project on plasma physics with a professor who specialized in plasma physics, and when the subject came up on the Jacob's ladder, he really could IMO have given a better explanation than he did, but seemed to skirt around trying to explain it by saying it was just a toy. The way I see that one is the arc starts at the bottom where the electrodes are closer together and some convection occurs as the arc works its way to the top where it then gets unstable from convection and breaks apart, and then another one starts near the bottom.

 

[sophiecentaur]

TL;DR: Why are the streamers blue while the pads at both ends are orange, in a plasma globe?

I would not expect the orange color at both ends.

Why not? We are not dealing with an anode and cathode as with DC . Both ends of the streams are under much the same (AC) conditions once every cycle of the 35kHz supply voltage. Because the curvature of the inner sphere and outer sphere there will be an overall difference in field but I would expect the gas atoms to emit the same spectral wavelengths everywhere along the stream.

Local fields will alternate and have a continuum of values per cycle. The ions will accelerate along the stream until colliding and emitting the photon. The two visible colours could be due to a mixture of elements or two different transitions in the atoms of just one element and a shorter delay for one energy transition (giving the red) and for the longer delay for the blue.

 

[Charles Link]

Unless we see some data from the manufacturer for a particular bulb, I don't know that we have a good handle on the voltage, current, or frequency.

Edit: It could even be very good if the manufacturer would supply the buyer with the details of the physics that are going on in his particular bulb. :)

 

[sophiecentaur]

I don't know that we have a good handle on the voltage, current, or frequency.

I bet they don't differ much between designs - cheap and short lived. That Wiki article says most of what we need to know, I think.

 

[Charles Link]

I bet they don't differ much between designs - cheap and short lived. That Wiki article says most of what we need to know, I think.

I'm more familiar with arc lamps running at higher currents and low voltages. When they operate at high voltage, the currents will be very small. The lamps that operate at low voltages normally need a higher voltage to start the arc, unless thermionic emission is employed with a heated cathode. In any case, the subject covers a fairly broad range, and can get a little complicated.

The old fluorescent lamps worked with mercury, but typically used xenon to start them, until the lamp got warm enough to vaporize the mercury.

 

[SredniVashtar]

I happen to own one such ball where a potentiometer varies one parameter (voltage?) through a sliding potentiometer.

In this video the slider is slowly moved from its central position to the far left, and then from there to the far right (and then back to minimum).



At minimum (I think) settings there is a blue haze that subsequently turns into a purple haze and only after the slider has reached a certain position the blue tendrils form. They increase in number until the maximum setting is reached.

The phone used to make the video picks up sound that is not there in reality.

 

[Charles Link]

I think the potentiometer might be a ballast resistor (in series with the supply voltage and plasma material) with a resistance that can be varied. If that is the case, characterizing the voltage could be difficult, because the plasma behaves so non-linearly, where the voltage through the plasma drops as the current increases.

Interesting video in any case, but we still don't have much reliable data for these bulbs. The google of post 2 IMO is only the tip of the iceberg.

 

[Orthoceras]

Why not? We are not dealing with an anode and cathode as with DC . Both ends of the streams are under much the same (AC) conditions once every cycle of the 35kHz supply voltage. Because the curvature of the inner sphere and outer sphere there will be an overall difference in field but I would expect the gas atoms to emit the same spectral wavelengths everywhere along the stream.

Local fields will alternate and have a continuum of values per cycle. The ions will accelerate along the stream until colliding and emitting the photon. The two visible colours could be due to a mixture of elements or two different transitions in the atoms of just one element and a shorter delay for one energy transition (giving the red) and for the longer delay for the blue.

The blue/red color change occurs in some globes halfway the streamer, so not always at the border between streamer and pad. A paper, by the Princeton Plasma Physics Laboratory, on Measurements of the motion of filaments in a plasma ball says the color change is presumably caused by the lower electron energy or electric field strength as the filaments propagate away from the central electrode.

Measurements of the motion of filaments in a plasma ball, M. D. Campanell, et al., Phys. Plasmas 17, 053507, 2010 (behind a paywall).

It is an interesting paper, the propagation of the streamer from the inner to the outer sphere was filmed at 0.5 million frames per second. The globe operates at 22 kHz, and five cycles are shown. The streamer channel with its free electrons is presumed to persist the whole time, uninterrupted, but light is emitted only briefly (6 μs per half cycle, where a half cycle is 22 μs). The front of the light emitting part of the streamer is seen to propagate from the inner to the outer sphere at a speed of 10^6 m/s.

Blue/red color change halfway the streamer in my plasma globe

 

[Charles Link]

It would be interesting to know the voltages in the bulb in this Princeton paper. I don't have access to the paper.