Plasma Striations Demonstrations
When lighting fluorescent tubes with induction (Tesla) coils, I often
have seen plasma striations, patterns of light and dark, often flowing
along portions of the tube, These patterns are occasionally seen in
conventionally powered fluorescent tubes, usually in cold weather or
end-of-life conditions. Striations are also reported in fluorescent
being run off of a dimmer. The consensus is that these striations are
an artifact of low current density in the tube under observation.
How often do you get to work
with ionized low pressure (2.5 Torr) argon,
ultra-violet light, RF coils and high voltage for under $100?
[In the images below, the playback rate is too high by a ratio of
F15T8/CW Tube Driven by Induction Coil
This investigation uses two Tesla coils:
- The ‘Acrylic’ made of 434 turns of 30 gauge wire (200 feet) wound
on an acrylic tube 1.758 inch in diameter. This coil measures 23 Ohms.
2.63mH and is driven with a 28Vpp sinewave around 1.628 MHz.
- The ‘PVC’ is made from 585 turns of 30 gauge wire (also 200
feet) on a 1 inch PVC form. This coil measures 23 Ohms. 2.63mH and is
driven with a 28Vpp sinewave around 1.742 MHz.
The bottom of the winding is driven by a function generator at or just
below resonance, while the top is open. By adjusting the frequency, I
can get the tubes to light automatically when power is present.
However, for greater insight into the physics of the coil and plasma
interactions, I prefer to use a setup that requires human interaction
to start and latch the light. I set the frequency of the function
generator to be about 3% below resonance. To strike the arc in the
tube, I move my hand near the tube. My body capacitance lowers the
resonant frequency of the coil. At the proper separation, resonance is
achieved. The resulting high voltage ionizes the tube, resulting in a
transformer coupling to the plasma, as well as the plasma providing the
of a capacitor (the coil windings being the other), keeping the circuit
at resonance as my hand is withdrawn.
Initially, the tube was hand-held near the coil. Other variations
included a T mount, initially using a pasteboard cradle seen below,
later using a groove ground in the top of the form, and vertical
mounts, sometimes using the plastic tube of the winding, other times
using a spent spool former as a base.
My initial work used a F15T8/CW tube. This is about 15 inches long, and
is a convienent length to use for public displays of Tesla coils. This
also happens to be a common desk lamp tube, as noticed in the some of
the background pictures. This tube has a clamping breakdown voltage
around 55V, and is usually run at 300 mA current. These are low
pressure tubes, about 2.5 torr for the Argon, with mercury partial
pressures much lower, around 50 mTorr. I am driving
these coils with RF level signals around 1.7 MHz and voltages on the
order of 1-2kV. Electrical wavelengths (lambda = c/f) at these
correspond to 170 meters or so, Consequently, the striations are likely
not due to standing waves in the RF excitation. (They may be related to
frequency of the mercury vapor.) I wanted to see the striations better,
and the phosphour coating was in the way. So, I changed to
germicidal tubes, G15T8, which uses a quartz tube (transparent to UV)
and no phosphour. This is a definite eye hazard. I
recommend using video recordings of the plasma glow, rather than
repeated optical observations with your last two good eyes. However,
the clear tube lets us see important details.
G15T8 Tube Driven by Induction Coil
- We don't directly excite the phosphour from electric fields. We
really have plasma bubbles.
- The bluish glow is Hg at 430 nm and 560 nm. Argon does not seem
to contribute in the visible range. (Edmund Scientific
- The plasma bubble do not touch the walls. (No surprise, as the
wall cools the plasma and causes recombination.)
- The bubble diameter is close to the tube diameter.
- The bubbles flow in the direction of reduced electric field.
- The plasma originates closes to the coil, rather than forming in
the center of the bore.
Details of Plasma Generation Zone
Next are series of vertical mounted tubes. We see the neutral flow
point corresponds to a point about 2/3 up the winding for the closest
tube/winding position. We also see some downward flow in the
Vertical Closest to Coil
Vertical Further Out
Vertical Still Further Out
Vertical Furthest Out, Some
The electric field to drive a fluorescent tube is, by design, very low
at 55V/15inch, 150 V/m, or 1.5V/cm. The mean
path for Argon (71pm radius assumed) at pressures of 2.5T is
quite small compared to the tube, at 0.14 mm. The
excitation energy (but not ionization) for Argon is about 11V. The
excited Argon efficiently transfers energy to the mercury (10.4V) which
ionizes. Upon recombination, the mercury radiates with the bulk of the
energy concentrated in UV around 254 nm and 185 nm. Clearly multiple
collisions are required to get electrons up to activation energy for
the Argon. I am having confusion reconciling the scale and location of
the ionized bubbles with my expected field around the coil. I am
comfortable with field levels of 60V/mm, as witnessed by NE-2 bulb
ionizations. I need to find a few more numbers, such as the carrier
density, recombination time, and plasma frequency before I feel I fully
understand this. I am also pondering the cathode fall regions of an
electrode, versus electrodeless operation of the plasma here.
Links to Large MOV files