Note: Descriptions are shown in the official language in which they were submitted.
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Docket ~lo. GCD 86-50
RA~I~ FRFQ~ENCY EXCITE~ RING LAS~R GYROSCOPE :-
BAC~GROUN~ OF TIIE INVENTIO~I
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1. Field of the Invention
This invention relates to optical rotation sensors;
and, particularly, this invention relates to ring laser
gyros, having an active medium gain that is excited by a
radio fre~uency signal emitted from a helical resonator
within an enclosed cavity.
2. Description of the Related Art
-
Ring laser gyroscopes are a class of optical rotation
sensors that have been developed to provide an alternative
form of rotational measurement to the mechanical gyroscope.
A ring laser gyroscope employs the Sagnac effect to detect
rotation. A basic two mode ring laser gyroscope has two
independent counter rotating light beams which propagate in ~,
an optical ring cavity. These two light beams propagate in a
closed loop with transit times that differ in direct
proportion to the rotation rate of the loop abou~ an axis
perpendicular to the plane of the loop. Besides the planar
ring laser gyro, other path geometries have been used; for
example, a non-planar yyroscope has been disclosed in United
States Patent 4,482,249 to Dorschner, which teaches an
out-of-plane light path that provides the
reciprocal splitting of two pairS of counter rotating beams.
This out-of-plane gyroscope has been known in the literature
as the mu~tioscillator ring laser gyroscope.
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Yet anotl~er alternative to the multioscillator ring
laser gyroscope is a split gain multi-mode ring laser
gyroscope. Both the
multioscillator ring laser gyroscope and the Split Gain
~lulti-Mode Ring Laser provide out of-plane geometry and at
least two pairs of counter propogating mocles of light l)eams
to measure rotation with respect to an inertial frame of
reference. These non-planar gryos have been developed to
avoid the need for mechanical dithering~ Dithering is needed
in planar gyroscopes to prevent counter rotating travelling
waves from locking at low rotation rates.
lleretofore, ring laser gyros have operated using at
least a dome-like configured metallic or glass covered
metallic cathode and at least two anodes, which extend
outward from the monolithic glass body of the ring laser gyro
to excite the gas medium contained within the gyroscope.
DC discharge has been used which excites gas contained in the
ring laser gyro pathway between the cathodes and each of the
anodes.
Both the prior art and ~he present invention will be
described in conjunction with the accompanying drawings in
which:
Figure 1 shows a prior art configuration of a
DC-excited planar ring laser gyro.
Figure 2 shows a top plan view of a planar ring laser
gyro that is radio frequency excited in accordance with the
teachings of this invention.
Figure 3 shows a side elevational view of the planar
radio frequency excited ring laser gyroscope.
Figure 4 shows a perspective view of a radio
frequency excited planar ring laser gyroscope.
Figure 5 shows an alternate embodiment of this
invention wherein a multioscillator ring laser gyro is radio
frequency excited in accordance with the teachings of this
invention.
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~ igure 6 is a perspective view of a multi-oscillator
ring laser gyroscope showing, in partial section, the
configuration of the radio frequency excitation mechanism
used ~y a split gain multi-mode ring laser gyroscope in
accordance with this invention.
Figure 7 is a schematic diagram showing the control
electronics for the radio frequency excitation drive system
for driving the resonator means in accordance with this
invention.
Figure 8A and 8~ show experimental test results
derived from experiments conducted using a radio frequency
excited ring laser gyroscope.
Figure 1 shows a prior art planar DC-excited ring
laser gyro. The ring laser gyroscope 1~ is formed from a
monolithic glass body such as Zerodur, which is a tra~emark
of the Schott Glass Works Co. of West Germany. A similar
glass that may be used as the ring laser gyro 12 is
manufactured under the trademark "CERVIT" sold by
Owens-Illinois. Both materials are mixtures o~ glass and
ceramic that have opposite temperature expansion
coefficients, thus providing an overall minimal dimensional
changes over a wide range of temperatures.
A square optical pathway 19 is defined within the
gyroscope 10 by 4 legs, 16, 18, 2~, and 22. Leg portion 18A,
22, and 20A ~orm a segment of the optical pathway which glows
due to the DC discharge between the cathode 24 and the
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respective anodes ~6 and 28. During manuEacture, a gaseous
mixture of helium and two isotopes of neon provide an active
medium that is excited along the DC discharge path defined by
segments 1~, 22, and 2~.
Gas is provided to the cavity during manufacture by
tl)e fill stem 30 through the anode 26. The cathode is
generally grounded, while the anode potentials are each
brought up to 15~0 volts, through use of a balast resistor
32. ~t each corner of the pathway 14, a mirror is positioned
to reflect light around the ring laser gyro. The mirrors,
34, 36, 38, and 4~ are mounted to the frame 12. ~ more
detailed description Or the operation of the planar ring
laser gyro together with the particular manncr of DC
excitation is also described in US Patents 9,115,004 to
15 Hutchings and US Patent 4,612,647 to Norvell, each patent
being assigned to the common assignee of this application.
In addition to the high voltage and high current
regulation require~ents needed for DC excitation, à number of
problems have been associaced with the manufacture and
reliability of DC discharge ring laser gyroscopes. A prime
problem is that of Langmuir flow which can cause a biased and
therefore inaccuracies in the rotational sensing capabilities
of the ring laser gyro, unless the gyroscope is provided with
two balanced current discharge paths. A discharge between a
single anode and cathode causes the molecules in a gas laser
cavity to flow in a preferred direction. This flow gives
rise to a bias or inaccuracy in the rotational sensing
capability of a ring laser gyroscope, since each of the
clockwise and anti-clockwise modes of light beams propogating
in the cavity will be influenced differently by this flow
phenomena. In a DC discharge excitation mechanism, as
illustrated in Figure 1, the only manner of offsetting the
bias problem is to exactly balance the currents and lengths
of the two discharge legs 18A and 20A in each half of the
discharge region. This is a difficult and costly process.
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The power supplies associated with DC excitation are
expensive and bulky. A 3-4,0~0 volt potential is necessary
to start the discharge processs, and continued operation of
the discharge requires a high voltage source of 1500 volts.
The cathodes and anodcs themselves have associated pLoblems
including leaks at the seals and shortened lifetimes.
~ lso, a phenomenom known as cathode sputtering arises
and limits the lifetime oE the discharge system. Cathode
sputtering is characterized by the degradation of a
protective oxide coating on the outside o~ the cathode for a
good part of its life. The discharge process eventually eats
through the oxide coating, exposing the underlying aluminum
of the cathode. Once this aluminum is exposed, cathode life
deteriorates very quickly and results in an inoperative or
non-usable laser structure. This cathode sputtering is a
severe limitation on the life of a gas riny laser gyroscope.
Also there are instabilities in the discharge when the DC
discharge is initially activated after filling the ring
lasers with gas during manuEacture.
In certain ranges of current operation, instabilities
in the current and voltage discharge arise. These
instabilities limit the range of current and gas pressure
that can be used with a DC discharge ring laser gyroscope.
Also, the DC discharge process is relatively inefficient in
providing high energy electrons to pump the gas laser atomic
energy level. Some of the problëms associated with DC
discharge in a ring laser gyroscope are also described in
Laser_~pplications, edited by Monte ~oss, pages 133-200
(Academic Press r 1971) .
In addition to the operation of the planar ring laser
gyro through DC discharge as shown in Figure l, the operation
of the multioscillator laser gyroscopes, as described in an
article by Chow, et. al., at pages 918-936, IEEE Jo~rnal of
Quantum Electronics, vol. QE-16, No. 9, Sept. 198~ is
discussed in this article. In both the out-of-plane and
Zeeman effect multioscillator ring laser gyroscopes, it is
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preferred that the active medium not interfere with certain
axially uniform fields needed for the operation of these
types of ring laser gyroscopes. As with the planar r-ing
laser gyroscope, DC discharge methods have created similar
problems for multioscillator ring laser gyroscopes.
Likewise, the Split Gain Multi-Mode Ring Laser Gyro
would preferably confine the active medium to the area where
a uniform magnetic field is also applied. This is not easily
accomplish~d with DC excitation.
For all the fo~egoing reasons, an alternative method
of excitation of the gain ;nedium of a ring laser gyroscope is
desirable.
In the past, alternative methods of excitation of a
laser gas medium have ~een attempted with varying degrees of
success. ~F excitation of a helium neon mixture has been
reported as early as 1961 in Physical Review Letters, vol. 6,
No. 3, pages 1~6-110, in an article by A. Javan. J.P.
Goldsborough has described an RF induction excitation of a
continuous wave visible laser in vol. 8, No. 6 of Applied
Physics Letters, (15 March 1966), pages 137-139.
US Patent 3,772,611 to Smith, assigned to Bell
Telephone Laboratories, issued November 13, 1973, describes
an RF excited ring type capallary tube 11 (Figure 1) which
may have utility as a rotation rate sensor. The '611 patent,
however, does not teach an efEicient design for utilizing the
RF excitation. This '611 patent also referred to "A Wave
Guide Gas Laser" in an article dated Sept. 1, 1971, in vol.
19, Applied Physics Letters, No. 5t pages 132-134. In this
article, Smith described a combined RF and DC voltage excited
capillary wave guide containing a mix of helium neon gas.
These inductive coupled RF excitation methods were by
necessity high power and created substantial electrical
interference and noise which disturbed other instrumentation
associated with rotation sensing.
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UK patent application published July 19, 1987 (G~
21858~6~) discloses a ring laser which is excited by
transverse electrical discharge operating at a high ~requency
alternating voltage. Although this disclosure claims a low
voltage excitation range, it operates through capacitive
coupling to the gaseous medium in a transverse direction to
the passageway between the mirrors. Use of this transverse
direction-excited, high frequency, alternating voltage,
capacitively coupled to the active medium would result in
contamination of the passageways of the ring laser gyroscope
cavity due to the constant bombardment of the gaseous media
against the walls ancl the high RF powers needed to drive the
discharge. This is counter-productive to a long life
operation of a ring laser gyroscope.
Thus, although the prior art referenced have
disclosed alternative methods of excitation other than DC
discharge for gaseous laser and ring laser gyroscopes, these
alternative solutions to the problem of DC dischar~e have
been inadequate.
2~ SUMMARY 0~ Tl~E INVEN~10~l
Disclosed herein is a radio frequency excited ring
laser gyroscope including a resonant closed cavity defining a
closed optical path which is filled with a gain medium. The
ring laser gyroscope of this invention includes a means for
exciting the gas medium which comprises a resonant cavity
formed around a helical coil for applying a radio frequency
signal to excite the discharge of a gain medium. The high
frequency signal that is imparted by the resonant cavity is
in the range of 5 Mhz to 550 Mhz. ~ resonant means for
applying a radio frequel)cy signal for exciting a discharge of
the gain medium should include a helical coil surrounding a
portion of the closed optical path where the helical coil is
contained within a closed conductive resonator shield. Along
at least one leg of the monolithic dielectric body of the
ring laser gyroscope a portion of the monolithic body is
carved out to allow one of the tubular bores to be surrounded
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by ti~e helical coil and enclosed in a conductive resonator
shield. An RF oscillator is coupled and connected to the
resonant means to supply a radio frequency signal to the
resonant means for excitation of the active gain medium.
S The advantages of the invention disclosed herein will
become more apparent from a review of the accompanying
drawings and detailed description of the preEerred embodiment
of this invention which follows.
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DETAILED DE~CRIPTION OF THE PREFERRED EI~BODIMENT
With reference to Figures ~, 3 and 4, a radio
frequency excited planar ring laser gyroscope 50 is shown.
The ring laser gyroscope is comprised of a low thermal
expansion glass body 52 into which an optical pathway defined
by legs 54, 56, 58 and 60. Positioned at each corner ar=e
mirrors 62, 64, 66 and 68. During manufacture, gas is
introduced to the cavity defined by the legs through the
passageway 7~.
At least one of the legs 60 has been carved to
accomodate the positioning of a helical coil 72 which is
surrounded by a resonant cavity shield 74. The coil 72 is
driven by an RF frequency oscillator 76 (Figure 2) operating
at a voltage of approximately 28 volts. The radio frequency
oscillator 76 may be a modified Colpitts Oscillator.
Generally, a signal in the range of 5 to 55~ megahertz is
imparted to the coil. By completely enclosing the coil 72
with a resonator cavity shield 74, a closed resonant cavity
is formed which serves to amplify the signal produced within
2B the cavity. In the preferred embodiment the resonant shield
74 should be of conductive material with low resistivity.
The helical coil 72 is preferrably formed from copper wire
and the resonator shield made from copper tube to form the
radio frequency resonant cavity. The shield could be formed
from other metals coated on a tubular substrate s~ch as, but
not limited to, gold, silver or aluminum~ The resonant
cavity 75 encloses the portion of the gain medium 8~ that is
excited by the radio frequency discharge. By operating at i
the proper radio frequencies, the cavity may be run as a
fullwave, halfwave, or quarter wave, length resonator.
Readings of the electro magnetic field outside the resonator
reveal that the cavity has localized the electro magnetic
field. Any spurious signals have been reduced in order to
minimize any undesirable effects of the electro magnetic far
field on the operation of the ring laser gyroscope or related
measuring electronic components. Thus, unlike radio
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frequency induced excitation as disclose~ in the prior art,
the use of a resonant cavity allows the optimi~ation of a
strong excitation with low power and minimal radio frequency
interference (RFI)~
An enclosed helical resonator, such as that shown in
Figures 2-4, may provide high Q. The helical resonator~
assembly is comprised of a coil 72 within the resonator
shield 74, where one end of the coil 72 is solidly connected
to the shield 74~ The other end of the coil 72 is an open
circuit, except for a possible trimming capacitor (not
shown). In this configuration, the resonator assembly
resembles an L-C circuit; but, instead of being a
lumped-constant device, lts operation can be described in
terms of distributed inductance, capacitance, and resistance.
The resonator must be properly aligned for optimum
performance. ~ccording to techniques ~;nown in the art (See
W. W. MacAlpine, et. al., "Coaxial Resonators with Helical
Inner Conductor, Proceedings of the IRE, December, 1959, pp. I
2099 - 2105 at 2100), alignment of the resonator coil can be
achieved according to the following equations:
) Qu 50D fo /2' where
Qu = unloaded Q;
D = inside diameter of the resonator shield;
and r
fO - Resonant frequency lMhz).
(2) N = 1900/(f D) turns
where:
N = total number of windings
fO = Resonant frequency <MHz>; and,
D = inside diameter of the resonator shield.
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Additional considerations of parameters needed to
achieve optimum RF coil operation have been treated in the
conventional arts, as indicated by the MACALPINE Article,
SUPR~.
Figure 5 shows an alternative embodiment of the radio
frequency excitation system of this invention used in
conjunction with a multioscillator ring laser gyroscope 82.
As with the planar ring laser gyroscope, the frame 84 is made
from a monolithic dielectric material having a low thermal
expansion over a wide temperature range (between -50 deg. C -
150 deg. C). As has been disclosed in the art, this form oE
multioscillator ring laser gyroscope is positioned in an
out-of-plane configuration in order to provide reciprocal
splitting between sets of let and right circular pola~ized
beams of light. (A more detailed explanation may be ~ound in
the Laser Handbook Vol. g, edited by M.L. Stitch, published
by North Holland, 1985, pages 230-332). Heretofor, the
multioscillator ring laser gyroscope has had its active
medium excited by a DC discharge between a cathode and at
least 2 anodes. An alternative form of excitation of the
multioscillator ring laser gyroscope is shown in Figure 5.
One of the four legs, positioned between mirrors 86 and 88,
of the out-of-plane multioscillator ring laser gyroscope 82
is carved out of the body. The passageway 91 defined by the
25 leg between 86 and 8B is integral with the frame 84 and is
preferrably a cylindrical pathway, Wound around this pathway
91 is a Radio Frequency helical coil 94. Surrounding the
coil 94 is preferrably a cylindrical radio frequency
resonator shield 96. Positioned on the leg opposite 91
30 between mirrors 90 and 92 is the optical rotator 97 ~such as
a Faraday rotator~ which provides non-reciprocal splitting
between clockwis~ and counter-clockwise beams of light within
two sets of left and right circularly polarized beams in the
presence of a uniform magnetic field.
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It will be noted that the radio frequency resonator
shield 96 provides good isolation for all signals generated
within the helical coil 94 and the resonator cavity 95. ~y
shiel~ing the resonator cavity 95, the radio frequency
resonator shield 96 prevents any radio frequency interference
from affecting the detection circuitry or any other
electronics.
With reference to Figure 6, a Split Gain Multi-Mode
Ring Laser Gyroscope 100 is shown. The frame 102 is a mono-
lithic dielectric material at a low coefficient of thermalexpansion. The split gain multi-mode ring laser gyroscope is
also config~red in an out-of-plane configuration. A strong
permanent magnet 116 is used to cause the split gain effect
need~d to operate this form of ring laser gyroscope. When
the split gain gyroscope is operated by use of conventional
DC discharge, a permanent magnet (not shown) must be
positioned on a leg opposite the gain medium and the leg
where the cathode is positioned. This is not, however, the
most desirable positioning of the permanent magnet in the
split gain multi-mode ring laser gyroscope.
Preferrably, the split gain multi-mode ring laser
gyroscope operates best when the active medium is contained
within the permanent magnet 116. This is difficult to
achieve when using a conventional DC discharge manner of
- Z5 medium excitation. The radio frequency excitation system of
this invention provides an optimum alternative to the
conventional DC discharge.
~gain with reference to Figure 6, the active medium
discharge pathway 104 is positioned between mirrors 106 and
108. Within the pathway 104 gaseous medium (preferrably a
helium-neon mixture) is excited by use of a radio frequency
helical coil which applies a radio frequency signal from an
RF oscillator (not shown) to a resonant cavity 110 formed
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between the radio frequency helical coil 112 and the radio
frequency resonator shield 114. The radio frequency
resonator shield 114 may be a copper shield which surrounds
copper wire forming the radio frequency helical coil 112. In
order to provide compactness and optimum design, the radio
frequency sllield 114 may be positioned along the inner
diameter of the permanent magnet 116 which also surrounds the
medium passageway 110.
By configuring the radio frequency excitatio~ system
within the permanent magnet 116 as shown in Figure 6, one is
able to achieve optimum results with regard ~o both the
excitation of the active medium 118 within the passageway 110
as well as the con~inelnent of that medium 110 within the
permanent magnet length 116, a necessary factor in achieving
optimum split gain multi- mode ring laser gyroscope
operation. Each signal then plays a different role. The
high radio frequency signal imparted on the helical coil 112
is used to excite the active medium 118. The low DC si~nal
applied simultaneously ~o the coil 112 is used to fine tune
the magnetic field arising from the permanent magnet 116.
Since the permanent magnet 116 is preferrably a cylindrical
configuration, the radio frequency resonator shield (119)
should also be a concentric cylindrical shape within the
inner surface of the permanent magnet 116.
~5 In this manner, a radio frequency excited Split Gain
Multi-Mode ~ing Laser Gyroscope is disclosed which does not
need independent DC magnets positioned on a ley away from the
active medium 118, as is required in a DC excited split gain
multi-mode ring laser gyroscope.
Figure 7 shows the control electronics for the radio
frequency drive system. In order for the radio frequency
excitation taught in this invention to operate most
efficiently, the radio frequency resonator and coil system
150 must be regulated so that the excited medium is
maintained at a relatively constant power level while the
radio frequency resonator signal is maintained at a
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relatively constant frequency. ~he combination of electronic
components shown generally in power control loop 152 controls
the power provided to the radio frequency resonator 150,
while the frequency control loop 154 controls the frequency
of the signal drive in the radio frequency resonator 150.
The power control loop 152 receives an input signal
through the photo-diode 15G, in the form of light intensity
of the gain medium 155. This signal is then amplified in the
pre-amplifier 158 and provided as an output voltage signal to
the differential amplifier 160. The diEferential amplier 160
compares the voltage power signal to a reference voltage. If
the operating power voltage signal is higher than the
reference, or lower than the reference, an output signal
resulting from common mode rejection by the differential
amplifier 160 provides an error signal input to the
integrator 162. The integratcr 162 then provides an output
signal which controls the electronic attenuator 169 to
adjust the voltage power supply provided to the radio
frequency resonator.
The electronic attenuator 164 provides an output
signal to the radio frequency amplifier 166 which couples its
output signal to the radio frequency coupler 168. The
resulting output at 170 will either raise or lower the power
provided to the RF frequency coil 172 within the radio
frequency resonator 150. Additionally, the frequency of the
RF resonator 150 is monitored by the radio frequency coupler
168 and is detected by an amplitude modulator detector 17~.
The output from the detector 174 of the frequency control
loop 154 is then amplified by the pre-amplifier 176.
The output of the pre-amplifier 176 is then provided
to the phase lock loop 180. The phase lock loop 180 is
comprised of synchronous detector 178, an integrator 179, and
a local oscillator 177. (This is a known configuration for an
analog phase lock loop). The phase lock loop 180 locks in on
a frequency determined by the local oscillator 177 and the
output of the phase lock loop 180 i 5 then provided to the
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summing amplifier 1~2. The summing amplifier 182 then
adjusts the voltage to the voltage controlled oscillator 184.
The voltage controlled oscillator 184 provides an
output adjustment to the electronic attenuator 169 which in
S turn, as previously described, provides frequency control
through the radio frequency coupler 168 to the frequency of
the signal applied to the coil 172 of the ~F resonator 15C.
In this manner, both power and frequency control and
consistency are maintained.
If it is desired to operate the ring laser gyroscope
at full wave resonance the gain medium 155 may be captured
within the coil 172 of the RF resonator 150. The positioning
of this gain medium 155 may be controlled by the electronics
shown in Figure 7; but, this is only one embodiment for
accomplishing this goal. It would be known generally that a
digital or other hybrid analog digital servo system may also
be used to control power and frequency of the RF resonator
15~. ~
Figures 8~ and 8~ show the results of radio frequency
excitation experiments for a planar ring laser gyroscope.
When radio frequency excitation is applied to the ring laser
gyroscope, curves 190 and 192 of Figure 8A show that the
optimum power is provided at the center of the radio
frequency resonator tuning frequency at 189 and 191
respectively. Curve 190 is shown at 29 decibels referred to
one milliwatt (dBm) or .75 watts, while curve 192 shows a 30
dBm input or 1 watt. In either case it can be shown that as
one detunes the resonator from the central frequency only 3
Mhz lower or 6 Mhz higher, there is a significant drop off in
excitation. ~rhus the fine tunability of the radio freguency
excited gyroscope is shown in figure 8A. Figure 8B shows
that the power increases relatively linearly. As input power
and voltage is increased from a 27 volt input to a 32 volt
input, curves 194 and 196 show that the wattage output oE the
linear laser increases in a substantially linear fashion.
Curve 194 exhibits the 200 Mhz signal with 31 turns while
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curve 196 shows a 112 Mhæ signal with 55 turns. It thus can
be seen from Figure 8B that there is a linear relationship
between output and input power that is smooth and provides a
high optimum efficiency for excitation of the gain medium in
the ring laser gyroscope.
While a preferred embodiment of the radio frequency
excitation system for a variety of embodiments of ring laser
gyroscopes have been shown, it is clear that alternative
resonator configurations may be used. These configurat;ons
could also provide a low power, high efficiency output that
is simple and avoids all the disadvantages of the traditional
DC discharge medium excitation devices of the prior art.
While preferred embodiments have been shown, alternative
e~uivalent embodiments are intended to be covered in the
appended claims which follow this disclosure.
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