Name: Niraj Sheth (Sponsor: Andrew
Post-Zwicker)
Institution: Princeton
Plasma Physics Laboratory
Address: C/o
James Morgan
PPPL
P.O.
Box 451
Princeton,
NJ 08543
Apparatus Title:
DC Glow Discharge Tube
Abstract (50-75 words)
Plasma
physics exposure for most students does not come until the graduate level. This setup was designed as a safe approach
to bringing plasmas into more classrooms and lecture halls while, at the same
time, maintaining variability with the parameters. This DC glow discharge tube aims to remedy this educational gap
as both an apparatus for demonstration and limited experimentation.
Description:
Equipment list:
Plastic
hose: 5/8” OD
(2)
“Quick Disconnect” couplings
(2)
PVC Short Tees, 1/2” NPT size
Cast
Acrylic Hollow Rod: 2” ID, 2.5” OD, min. 15” length
(1)
1/4” OD Stainless steel rod
(2)
2” diameter, 3/8” thickness Stainless steel disks
(2)
2.5” OD, 2” ID O-rings
Cast
Acrylic Sheet: at least 6.5” x 13”, 1” thickness
(2)
1/2” NPT PVC threaded pipe
(2)
1/2” NPT male to pipe (barbed) adapters
Small
hose pipe clamps
Swagelok
Needle valve
High
voltage variable power supply: min 1500 V
Resistor:
min 100,000 Ω
Necessary
wires
Vacuum
pump: at least 25 microns max vacuum
Pressure
gauge
Convectron
Description:
Plasma
physics exposure for most students does not come until the graduate level. This setup was designed to be a safe
approach to bringing plasmas into more classrooms and lecture halls, and this
attention to safety is evident in its construction. At the same time, several of the parameters are variable,
allowing for much operational use. This
DC glow discharge tube aims to remedy this as both an apparatus for
demonstration and limited experimentation.
Its function as a tool for
demonstration, without manipulation of the plasma parameters, introduces the
concept of a plasma as an ionized gas.
The different characteristic regions of a DC glow discharge plasma can
be observed, including the Faraday dark space, the positive column, and the
anode glow. Also, moving and standing
striations in the positive column can be observed under certain conditions,
along with electromagnetic waves with the proper accessories. The electromagnetic nature of the plasma’s
subatomic components can also be demonstrated relatively easily with the aid of
a simple bar magnet. The response in
the plasma in proximity to the magnet indicate that the plasma behaves like an
ionized gas.
As assembled, it allows for variability in gas type,
pressure, voltage, and electrode separation distance. Because the tube connecting the two blocks requires no special
machining, a variable set of tube sizes can also be used easily. With these parameters, students can perform
several basic plasma physics experiments.
In a breakdown voltage lab, students can observe the effects of
electrode separation distance and pressure on the necessary voltage to ignite a
glow discharge plasma, known as “breakdown voltage.” By means of graphing their data, students can discover Paschen’s
curve, a graph characteristic of glow discharge plasmas. By manipulating the pressure and voltage, the
changes of the charged subatomic particles that comprise the plasma can be
observed macroscopically. This allows
for the introduction of concepts such as mean free path, electron and ion
temperatures, and shielding. The needle
valve in the vacuum system permits the infusion of different gases, such as
neon and argon. By taking the spectra
of the positive and negative columns, which are the sections of illumination
associated with the anode and cathode respectively, the spectral properties of
these gases can be investigated.
Construction:
The
set-up consists of two 6.5 x 6.5” blocks with 2” grooves on the insides and
tapped holes through the middle. An
acrylic tube fits between the two grooves on the blocks, and two O-rings
between the tube and the blocks in the grooves help to make a vacuum-tight
seal. By means of a PVC threaded pipe,
a PVC Tee is attached to the threaded holes in each block. On the opposite end of the tee, a “Quick
Disconnect” coupling allows for a rod attached to the electrode to be inserted
with an air-tight seal. The middle
extension of the tee attaches either to the vacuum pump or to the Swagelok
needle valve. The sections of the rods
extending out beyond the Quick Disconnect coupling are connected to the
electrical setup, one to the high voltage power source and one to ground. The “hot” rod, the one at a high voltage, is
covered with an insulator for safety reasons.
Figure
1 Figure
2
Conclusion:
The
varied applications and uses of plasmas occur in many areas, from scientific
research to manufacturing to consumer products. Students in many fields will eventually encounter plasmas in one
form or another because of the wide range of their applicability. Undoubtedly, the best place for this initial
exposure is in the classroom. Yet,
according to Elizer et al, plasma
physics is “inscrutable” and thus taught sporadically at universities and not
at all in high school physics classes. This apparatus aims to provide a tool
for that exposure and bring plasmas more into students’ awareness.
References:
Eliezer, Y., Eliezer, S. (1989). The Fourth State of Matter – An Introduction to Plasma Physics, Adam Hilger, Bristol and Philadelphia. Pp. 1-20.