Plasma is considered the 4th state of matter and comprises more than 99% of the visible universe. Most often, it is defined in terms of a gas. That "gas" may be super hot yet tenuous, capable of existing only in the vacuum of space. Or it may be dense, lying at enormous depths at the core of stars. Plasmas may be completely ionized, but that is not always the case. There are low-temperature plasmas as well as plasmas that are only weakly dissociated into ions. Some are neutral overall, but others may possess unbalanced charge. Plasmas can conduct electricity and are often produced using high voltages or tremendous heat. Sparks, often just lightning bolts in miniature, are plasmas. (Not included are sparks produced by the incandescence of metal particles being ground on a wheel or such.) Plasmas are capable of generating magnetic fields as well as responding to them.

The glow of neon and fluorescent tubes, carbon arcs, the radiance of our sun, the blue-white streak of lightning, globes of ball lightning, comet tails, glowing nebulae, single-bubble sonoluminescence, the ever-present solar wind along with accompanying aurorae, and the fireballs of nuclear explosions are all examples of plasma in action. Many plasmas are pristine, but others may be dusty. Left to their own devices, with no external energy sources and without huge gravitational interactions, plasmas coalesce into neutral gases.

First, a quick look at several of plasma's many faces.

The sun

The solar system's biggest plasma globe, our sun, is all plasma and 1.53 billion yards wide! Its atmosphere is constantly being driven outwards in the form of the solar wind that continuously bathes our earth. The solar wind is comprised of electrons and protons plus ions of the heavier elements. Most of the time, our earth is shielded from the deadly solar radiation thanks to our magnetosphere.

At right, those images tinted yellow are standard white light photos of the sun taken with an aluminized- glass filter. Those tinted red were acquired with a temperature-tuned, hydrogen-alpha filter. The pictures were taken on January 13 & 14, 2000, using a black&white CCD camera mounted on a 10" Meade LX-200 Schmidt-Cassegrain telescope. Two of the photos (both with longer frame times) show one of the most extensive sunspot groups ever...uh... spotted on our sun. I took these pictures during the most recent peak of solar activity. Currently, we are at a sunspot minimum.

Also, notice the limb darkening effect so characteristic of the sun and other stars whose disks have been successfully viewed by astronomers. The dramatic drop of light intensity towards the sun's limb is partially due to optical depth effects as well as to temperature differences of solar depths that we can visually explore at varying angles. When we look at the center of the solar disk, we are looking deep within the photosphere—perhaps the lowest 100 km. Deep translates into hot. However, if you try to determine the solar temperature based on comparisons to blackbody radiation, you won't get measurements any higher than around 5800 Kelvin. The sun, a sphere of plasma, grows more and more opaque with depth. Calculations based on estimates of the solar mass and the resulting gravitational compression reveal a core temperature of approximately 13,000,000 K.

Lightning

Another spectacular plasma display much closer to home here in Florida, nature's electrical fireworks greet us each year during the summer months. Typically, lightning bolts carry tens of thousands of amps at tens of millions of volts. Based on spectroscopic analysis, calculated temperatures of approximately 30,000 Kelvin are developed during lightning's cloud-to-ground discharges.

In many ways, what is true for lightning holds true for laboratory generated electrical discharges, though on a much smaller scale of course. As with lightning, the temperature of brilliant sparks can be determined spectroscopically. Other studies can be carried out as well. The photo below is a montage of five pictures that I took of my handy little lightning generator in action. For these pictures, approximately 100 kv DC from a Cockcroft-Walton power supply was applied to the stranded copper electrodes.

If you happen to be standing next to a lightning flash as it hits the ground or a nearby structure, you may hear a loud bang. Those standing much further away will hear the deep, resonant sound of thunder initiated by lightning's shock waves. This is due in part to the strong attenuation of high frequencies in the atmosphere because of scattering, along with reverberation and echoes from surrounding structures, mountains, clouds, etc. Low-frequency sound waves will arrive later than surviving high-frequency waves.

I wanted to have a little more fun with lightning, so I recorded two loud snaps or bangs produced by electrical discharges in my lightning generator. Next, I applied commercial sound editing software to the recording to add echoes, reverb, low-frequency boosting along with high-frequency attenuation, waveform stretching, and pitch shifting (just like Mother Nature). The results are two peals of thunder that can be played back using the control panel below the lightning photos. The second peal of thunder is preceded by a brief snap (the thunderclap) and then the low-frequency rumbles come rolling in. In nature, these rumbles are chiefly a result of different sections of the miles-long lightning channel being situated at different distances from the listener. The sound waves arrive at different times and, coupling this with high-frequency attenuation, low-frequency rumbles are heard.

To try to duplicate the distance effect when the microphone is only a few feet from the entire channel, I applied an amplitude-modulation envelope to the recorded waveform. Because of the presence of numerous low-frequency rumbles, the thunder definitely sounds better on some computer audio systems than others. Those with a good bass response will do a good job; otherwise, the thunder will sound tinny and awful. (Sure. Blame the computer.)

Finally, take a look at the Gallery section of my Web site for a video of an electrical discharge that appears to behave like miniature ball lightning. Though the phenomenon seen there is almost certainly corona discharge—a form of plasma—the behavior is really amazing!

Spectral lamps

Common place in laboratories, glass tubes filled with various gases and vapors serve as convenient sources of the spectrum of the elements. These spectra may be used for chemical analysis through direct comparison with an unknown spectrum (emission spectroscopy) or by absorption of particular spectral lines by an unknown sample (atomic absorption spectroscopy). In the latter instance, the analyst already suspects the presence of a particular element or group of elements and is attempting to perform a quantitative analysis of the unknown. Often the tubes function as lamps illuminating optical setups such as interferometers and microscopes. Cadmium and krypton have convenient spectral lines that have been used as laboratory calibration standards of length.

These tubes are typically excited by voltages high enough to ionize the gas or vapor contained within. Geissler tubes are early examples of spectral lamps, and they produce particularly colorful arrays. In modern labs, hollow cathode lamps (HCLs) with cathodes composed of the element whose lines are to be displayed are used. While their shelf life is limited, they do produce steady, constant, characteristic emission lines. To do this, the HCLs are hooked up to power supplies capable of supplying 100 to 300 volts DC at currents of up to 30 ma or so. Small quantities of either neon or argon sealed in the HCLs are ionized by the moderate voltages and then begin sputtering the cathode. After having some of their electrons knocked off by the noble-gas bombardment, the vaporized, and now positively charged, sputtered ions of the elements continue colliding with other ions and electrons. The ions are quickly neutralized by collisions with electrons, and energy in the form of light (visible and invisible) is released. This maelstrom of activity constitutes the plasma in the HCL.

For some elements, such as iodine, sulfur, selenium, phosphorus, and rubidium, tubes without self-contained electrodes that would be subjected to chemical attack often produce better spectra. Radio frequency power, typically operating at 27.12 MHz, is applied to the sealed tube by coupling through a tuned inductor wrapped around the glass or quartz housing. The inductively coupled plasma spectra produced are brighter, sharper, and often show more numerous lines. Furthermore, the tubes themselves are much longer lasting than standard HCLs. Due to the absence of internal electrodes, these tubes are called electrodeless discharge lamps (EDLs).

Incidentally, microwave-excited sulfur EDLs (cost around $100) are currently being reevaluated as possible replacements for tungsten light bulbs ($0.50). Though the price difference is horrendous, the microwave gadgetry will be a great boon to scientists and should be viewed in that light.

The spectra in the following three photos were produced with a homemade, 2"x2" holographic, low-dispersion, transmission grating. The nicest thing about making your own gratings is that you can tailor-make them to produce all kinds of optical effects. I used a silver halide sensitized gelatin process (SHSG yields holograms with very high diffraction efficiency and low noise) to generate a clear-as-glass final product. Using UV-activated optical cement, I placed a 2"x2" clear, thin glass plate over the emulsion side to protect it for general use in optical filter mounts. The apparent differences in dispersion seen in the spectra are due to the different magnifications used in recording them.

A hydrogen spectrum tube photographed through a homemade grating. These Geissler-type tubes are often used as low-cost spectral sources in school labs.

This view of a manganese hollow cathode lamp was photographed through the business end of the tube. The flat, crystal-clear viewport is made of either quartz or borosilicate depending on the importance of the UV spectrum of the particular element.

A cadmium electrodeless discharge lamp plasma seen in action. This lamp operates on 9 watts of RF power. The EDL housing is made of quartz. The tube contains argon along with a few milligrams of the element or one of its volatile salts.

Inertial electrostatic confinement plasma

The device used for producing this type of plasma is called a Farnsworth-Hirsch or Hirsch-Meeks fusor. Philo T. Farnsworth, of early television development fame, laid the groundwork for the IEC fusor concept back in the 1930's. He picked up that work again starting in the 50's. Robert Hirsch, then part of Farnsworth's research group, introduced substantial improvements in the 60's. Many amateur scientists and hobbyists have constructed fusors. Some have gone on to investigate the fusor's neutron-generation capabilities using deuterium gas. Hunt around on the Internet and you'll be treated to tantalizing glimpses of stainless-steel artistry sculpted by these highly motivated and skilled enthusiasts of plasma science.

At right, like a glowing submersible probing the murky ocean depths with a lone spotlight, is an inertially confined plasma sphere called a poissor. In reality, the spotlight is a projecting trumpet of plasma acting as an escape valve to free trapped negative ions and electrons from their confinement at the fusor's center. Where the ion beam strikes the containing Pyrex bell jar, the glass exhibits a blue cathodoluminescence and can get quite hot. The chamber is hooked up to a constantly running vacuum pump. The vacuum gauge read 14 millitorr (microns) when I took the picture. Only air at low pressure filled the bell jar. Early in the pumping process and at some repeatable point in the application of high voltage, the direction of the trumpet could be seen quickly darting around, first pointing at one location in the bell jar and then another. When the vacuum deepened, the trumpet stabilized both in shape and direction as seen here.

My demonstration fusor is powered by a variable 20kv DC power supply. As far as grid alignments and the degree of vacuum are concerned, conditions for producing ion-channeling trumpets are much less stringent than those needed for achieving the fusor's star mode. If one is interested in studying ion motion, magnetic field interactions wtih charged particles, and Doppler-shift spectroscopy, then a demo fusor is probably a pretty good place to start.

Laser discharge

Here, my trusty old Laser Photonics Printfinder Nd:Yag laser was focused through a small plano-convex lens (9.5 mm diameter x 13.5 mm focal length) onto the surface of a graphite rod. When new, the laser boasted 150 mJ infrared (@ 1.064 microns) and 35 mJ green (@ 532 nm) light energy combined. It was capable of producing a spark in midair through any short focal length lens. Today, the laser can still produce a spark in air but it requires a microscope objective to do it. However, after only a few flashes, the microscope objective is destroyed!

To produce a reasonable plasma without sacrificing any more lenses, I used a graphite rod as a target. Under the intense pulsing, a plume of carbon vapor is produced that further enables a spark breakdown. (Carbon vapor forms and exists at temperatures in excess of 4200 K. It is quite possible that extremely small quantities of Fullerenes and carbon nanotubes were formed in the nanosecond pulses.)

The technique of bombarding targets with high-energy pulsed laser radiation is used for laser-induced breakdown (LIB). If desired, the graphite rod could be soaked in a solution of an unknown substance. Small samples of minerals or metals can be targeted as well. LIB creates a plasma displaying many of the spectral lines characteristic of the target.

Dusty plasma

Of great interest to plasma physicists and astrophysicists, the presence of micro- and nanoparticles within plasmas produces remarkable effects. Comet tails and the radial spokes on saturn's rings are assumed to be examples of dusty plasmas.

In this photo of a special tube containing minute amounts of mercury vapor, two rings (one complete and one partial with cusped ends) have formed after a few minutes of operation. The bright, greenish-white outermost ring is due to the intersection of a laser lightsheet with the glass tube. Without that lightsheet illuminating a thin slice of plasma, the rings would not be visible.

More details about this phenomenon plus a video of a ring forming and its stunning collapse, along with a magnified view of particles moving in the cusp, will be seen on the page dealing with my dusty plasma experiments (work in progress).

Canal rays

I want to conclude this introductory page with a look back—way back. In 1886, the German physicist Eugen Goldstein developed a partially evacuated vacuum tube equipped with a perforated cathode. The tube was powered by a high-voltage DC supply. The remarkable expedient of drilling holes in the cathode allowed scientists to separately view and manipulate positive ions that were heading in one direction and the negative ions and electrons going in the other. The canal-ray tube designation may be a result of the resemblance of the positive-ion stream to water flowing through canals, or maybe it was due to Goldstein's referring to the holes as canals rather than channels.

Interestingly, the colors emitted by the collective ion streams differ. Early on it was found that hydrogen nuclei passing through the cathode could be "weighed" by measuring their deflection using powerful magnetic fields. This permitted determining the proton's mass relative to that of the electron. Goldstein's deceptively simple yet ingenious device was a very early forerunner of the mass spectrometer.

As seen in the photo of my vintage canal ray tube, the glowing gases of ions both positive and negative are plasmas. This is an instance of charged-particle plasmas that are not locally neutral but are so overall. The anode is the small disc at lower right. The bright spot along the wire leading upwards from the base to the cathode is due to a break in the glass tube insulating the wire.

(If you're curious as to what happens when the leads from the power source to the tube are reversed, you might want to build or buy a canal-ray tube. Check out Physical Optics by Robert W. Wood and look for Doppler effect in the index. His drawing and experiments should whet the appetite. In fact, the entire book is a superb classic!)

In the pages to follow, we'll take a more in-depth look at some of the plasmas discussed above. I will also report on my experiments with both sonoluminescence and dense plasma focus.

Coming soon, a closer look at dusty plasmas.



Plasma is considered the 4th state of matter and comprises more than 99% of the visible universe. Most often, it is defined in terms of a gas. That "gas" may be super hot yet tenuous, capable of existing only in the vacuum of space. Or it may be dense, lying at enormous depths at the core of stars. Plasmas may be completely ionized, but that is not always the case. There are low-temperature plasmas as well as plasmas that are only weakly dissociated into ions. Some are neutral overall, but others may possess unbalanced charge. Plasmas can conduct electricity and are often produced using high voltages or tremendous heat. Sparks, often just lightning bolts in miniature, are plasmas. (Not included are sparks produced by the incandescence of metal particles being ground on a wheel or such.) Plasmas are capable of generating magnetic fields as well as responding to them. The glow of neon and fluorescent tubes, carbon arcs, the radiance of our sun, the blue-white streak of lightning, globes of ball lightning, comet tails, glowing nebulae, single-bubble sonoluminescence, the ever-present solar wind along with accompanying aurorae, and the fireballs of nuclear explosions are all examples of plasma in action. Many plasmas are pristine, but others may be dusty. Left to their own devices, with no external energy sources and without huge gravitational interactions, plasmas coalesce into neutral gases. First, a quick look at several of plasma's many faces.
The sun The solar system's biggest plasma globe, our sun, is all plasma and 1.53 billion yards wide! Its atmosphere is constantly being driven outwards in the form of the solar wind that continuously bathes our earth. The solar wind is comprised of electrons and protons plus ions of the heavier elements. Most of the time, our earth is shielded from the deadly solar radiation thanks to our magnetosphere. At right, those images tinted yellow are standard white light photos of the sun taken with an aluminized- glass filter. Those tinted red were acquired with a temperature-tuned, hydrogen-alpha filter. The pictures were taken on January 13 & 14, 2000, using a black&white CCD camera mounted on a 10" Meade LX-200 Schmidt-Cassegrain telescope. Two of the photos (both with longer frame times) show one of the most extensive sunspot groups ever...uh... spotted on our sun. I took these pictures during the most recent peak of solar activity. Currently, we are at a sunspot minimum.
Also, notice the limb darkening effect so characteristic of the sun and other stars whose disks have been successfully viewed by astronomers. The dramatic drop of light intensity towards the sun's limb is partially due to optical depth effects as well as to temperature differences of solar depths that we can visually explore at varying angles. When we look at the center of the solar disk, we are looking deep within the photosphere—perhaps the lowest 100 km. Deep translates into hot. However, if you try to determine the solar temperature based on comparisons to blackbody radiation, you won't get measurements any higher than around 5800 Kelvin. The sun, a sphere of plasma, grows more and more opaque with depth. Calculations based on estimates of the solar mass and the resulting gravitational compression reveal a core temperature of approximately 13,000,000 K.
Lightning Another spectacular plasma display much closer to home here in Florida, nature's electrical fireworks greet us each year during the summer months. Typically, lightning bolts carry tens of thousands of amps at tens of millions of volts. Based on spectroscopic analysis, calculated temperatures of approximately 30,000 Kelvin are developed during lightning's cloud-to-ground discharges. In many ways, what is true for lightning holds true for laboratory generated electrical discharges, though on a much smaller scale of course. As with lightning, the temperature of brilliant sparks can be determined spectroscopically. Other studies can be carried out as well. The photo below is a montage of five pictures that I took of my handy little lightning generator in action. For these pictures, approximately 100 kv DC from a Cockcroft-Walton power supply was applied to the stranded copper electrodes. If you happen to be standing next to a lightning flash as it hits the ground or a nearby structure, you may hear a loud bang. Those standing much further away will hear the deep, resonant sound of thunder initiated by lightning's shock waves. This is due in part to the strong attenuation of high frequencies in the atmosphere because of scattering, along with reverberation and echoes from surrounding structures, mountains, clouds, etc. Low-frequency sound waves will arrive later than surviving high-frequency waves. I wanted to have a little more fun with lightning, so I recorded two loud snaps or bangs produced by electrical discharges in my lightning generator. Next, I applied commercial sound editing software to the recording to add echoes, reverb, low-frequency boosting along with high-frequency attenuation, waveform stretching, and pitch shifting (just like Mother Nature). The results are two peals of thunder that can be played back using the control panel below the lightning photos. The second peal of thunder is preceded by a brief snap (the thunderclap) and then the low-frequency rumbles come rolling in. In nature, these rumbles are chiefly a result of different sections of the miles-long lightning channel being situated at different distances from the listener. The sound waves arrive at different times and, coupling this with high-frequency attenuation, low-frequency rumbles are heard. To try to duplicate the distance effect when the microphone is only a few feet from the entire channel, I applied an amplitude-modulation envelope to the recorded waveform. Because of the presence of numerous low-frequency rumbles, the thunder definitely sounds better on some computer audio systems than others. Those with a good bass response will do a good job; otherwise, the thunder will sound tinny and awful. (Sure. Blame the computer.)

Finally, take a look at the Gallery section of my Web site for a video of an electrical discharge that appears to behave like miniature ball lightning. Though the phenomenon seen there is almost certainly corona discharge—a form of plasma—the behavior is really amazing!
Spectral lamps Common place in laboratories, glass tubes filled with various gases and vapors serve as convenient sources of the spectrum of the elements. These spectra may be used for chemical analysis through direct comparison with an unknown spectrum (emission spectroscopy) or by absorption of particular spectral lines by an unknown sample (atomic absorption spectroscopy). In the latter instance, the analyst already suspects the presence of a particular element or group of elements and is attempting to perform a quantitative analysis of the unknown. Often the tubes function as lamps illuminating optical setups such as interferometers and microscopes. Cadmium and krypton have convenient spectral lines that have been used as laboratory calibration standards of length. These tubes are typically excited by voltages high enough to ionize the gas or vapor contained within. Geissler tubes are early examples of spectral lamps, and they produce particularly colorful arrays. In modern labs, hollow cathode lamps (HCLs) with cathodes composed of the element whose lines are to be displayed are used. While their shelf life is limited, they do produce steady, constant, characteristic emission lines. To do this, the HCLs are hooked up to power supplies capable of supplying 100 to 300 volts DC at currents of up to 30 ma or so. Small quantities of either neon or argon sealed in the HCLs are ionized by the moderate voltages and then begin sputtering the cathode. After having some of their electrons knocked off by the noble-gas bombardment, the vaporized, and now positively charged, sputtered ions of the elements continue colliding with other ions and electrons. The ions are quickly neutralized by collisions with electrons, and energy in the form of light (visible and invisible) is released. This maelstrom of activity constitutes the plasma in the HCL. For some elements, such as iodine, sulfur, selenium, phosphorus, and rubidium, tubes without self-contained electrodes that would be subjected to chemical attack often produce better spectra. Radio frequency power, typically operating at 27.12 MHz, is applied to the sealed tube by coupling through a tuned inductor wrapped around the glass or quartz housing. The inductively coupled plasma spectra produced are brighter, sharper, and often show more numerous lines. Furthermore, the tubes themselves are much longer lasting than standard HCLs. Due to the absence of internal electrodes, these tubes are called electrodeless discharge lamps (EDLs). Incidentally, microwave-excited sulfur EDLs (cost around $100) are currently being reevaluated as possible replacements for tungsten light bulbs ($0.50). Though the price difference is horrendous, the microwave gadgetry will be a great boon to scientists and should be viewed in that light. The spectra in the following three photos were produced with a homemade, 2"x2" holographic, low-dispersion, transmission grating. The nicest thing about making your own gratings is that you can tailor-make them to produce all kinds of optical effects. I used a silver halide sensitized gelatin process (SHSG yields holograms with very high diffraction efficiency and low noise) to generate a clear-as-glass final product. Using UV-activated optical cement, I placed a 2"x2" clear, thin glass plate over the emulsion side to protect it for general use in optical filter mounts. The apparent differences in dispersion seen in the spectra are due to the different magnifications used in recording them.

A hydrogen spectrum tube photographed through a homemade grating. These Geissler-type tubes are often used as low-cost spectral sources in school labs.	This view of a manganese hollow cathode lamp was photographed through the business end of the tube. The flat, crystal-clear viewport is made of either quartz or borosilicate depending on the importance of the UV spectrum of the particular element.	A cadmium electrodeless discharge lamp plasma seen in action. This lamp operates on 9 watts of RF power. The EDL housing is made of quartz. The tube contains argon along with a few milligrams of the element or one of its volatile salts.

Inertial electrostatic confinement plasma The device used for producing this type of plasma is called a Farnsworth-Hirsch or Hirsch-Meeks fusor. Philo T. Farnsworth, of early television development fame, laid the groundwork for the IEC fusor concept back in the 1930's. He picked up that work again starting in the 50's. Robert Hirsch, then part of Farnsworth's research group, introduced substantial improvements in the 60's. Many amateur scientists and hobbyists have constructed fusors. Some have gone on to investigate the fusor's neutron-generation capabilities using deuterium gas. Hunt around on the Internet and you'll be treated to tantalizing glimpses of stainless-steel artistry sculpted by these highly motivated and skilled enthusiasts of plasma science. At right, like a glowing submersible probing the murky ocean depths with a lone spotlight, is an inertially confined plasma sphere called a poissor. In reality, the spotlight is a projecting trumpet of plasma acting as an escape valve to free trapped negative ions and electrons from their confinement at the fusor's center. Where the ion beam strikes the containing Pyrex bell jar, the glass exhibits a blue cathodoluminescence and can get quite hot. The chamber is hooked up to a constantly running vacuum pump. The vacuum gauge read 14 millitorr (microns) when I took the picture. Only air at low pressure filled the bell jar. Early in the pumping process and at some repeatable point in the application of high voltage, the direction of the trumpet could be seen quickly darting around, first pointing at one location in the bell jar and then another. When the vacuum deepened, the trumpet stabilized both in shape and direction as seen here.
My demonstration fusor is powered by a variable 20kv DC power supply. As far as grid alignments and the degree of vacuum are concerned, conditions for producing ion-channeling trumpets are much less stringent than those needed for achieving the fusor's star mode. If one is interested in studying ion motion, magnetic field interactions wtih charged particles, and Doppler-shift spectroscopy, then a demo fusor is probably a pretty good place to start.
Laser discharge Here, my trusty old Laser Photonics Printfinder Nd:Yag laser was focused through a small plano-convex lens (9.5 mm diameter x 13.5 mm focal length) onto the surface of a graphite rod. When new, the laser boasted 150 mJ infrared (@ 1.064 microns) and 35 mJ green (@ 532 nm) light energy combined. It was capable of producing a spark in midair through any short focal length lens. Today, the laser can still produce a spark in air but it requires a microscope objective to do it. However, after only a few flashes, the microscope objective is destroyed! To produce a reasonable plasma without sacrificing any more lenses, I used a graphite rod as a target. Under the intense pulsing, a plume of carbon vapor is produced that further enables a spark breakdown. (Carbon vapor forms and exists at temperatures in excess of 4200 K. It is quite possible that extremely small quantities of Fullerenes and carbon nanotubes were formed in the nanosecond pulses.) The technique of bombarding targets with high-energy pulsed laser radiation is used for laser-induced breakdown (LIB). If desired, the graphite rod could be soaked in a solution of an unknown substance. Small samples of minerals or metals can be targeted as well. LIB creates a plasma displaying many of the spectral lines characteristic of the target.
Dusty plasma Of great interest to plasma physicists and astrophysicists, the presence of micro- and nanoparticles within plasmas produces remarkable effects. Comet tails and the radial spokes on saturn's rings are assumed to be examples of dusty plasmas. In this photo of a special tube containing minute amounts of mercury vapor, two rings (one complete and one partial with cusped ends) have formed after a few minutes of operation. The bright, greenish-white outermost ring is due to the intersection of a laser lightsheet with the glass tube. Without that lightsheet illuminating a thin slice of plasma, the rings would not be visible. More details about this phenomenon plus a video of a ring forming and its stunning collapse, along with a magnified view of particles moving in the cusp, will be seen on the page dealing with my dusty plasma experiments (work in progress).
Canal rays I want to conclude this introductory page with a look back—way back. In 1886, the German physicist Eugen Goldstein developed a partially evacuated vacuum tube equipped with a perforated cathode. The tube was powered by a high-voltage DC supply. The remarkable expedient of drilling holes in the cathode allowed scientists to separately view and manipulate positive ions that were heading in one direction and the negative ions and electrons going in the other. The canal-ray tube designation may be a result of the resemblance of the positive-ion stream to water flowing through canals, or maybe it was due to Goldstein's referring to the holes as canals rather than channels. Interestingly, the colors emitted by the collective ion streams differ. Early on it was found that hydrogen nuclei passing through the cathode could be "weighed" by measuring their deflection using powerful magnetic fields. This permitted determining the proton's mass relative to that of the electron. Goldstein's deceptively simple yet ingenious device was a very early forerunner of the mass spectrometer. As seen in the photo of my vintage canal ray tube, the glowing gases of ions both positive and negative are plasmas. This is an instance of charged-particle plasmas that are not locally neutral but are so overall. The anode is the small disc at lower right. The bright spot along the wire leading upwards from the base to the cathode is due to a break in the glass tube insulating the wire. (If you're curious as to what happens when the leads from the power source to the tube are reversed, you might want to build or buy a canal-ray tube. Check out Physical Optics by Robert W. Wood and look for Doppler effect in the index. His drawing and experiments should whet the appetite. In fact, the entire book is a superb classic!)
In the pages to follow, we'll take a more in-depth look at some of the plasmas discussed above. I will also report on my experiments with both sonoluminescence and dense plasma focus.
Coming soon, a closer look at dusty plasmas.

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