WO2001001448A1

WO2001001448A1 - Method and device for excitation and maintenance of a discharge in electrodeless lamp

Info

Publication number: WO2001001448A1
Application number: PCT/RU2000/000248
Authority: WO
Inventors: Jury Vladimirovich Korchagin
Original assignee: Jury Vladimirovich Korchagin
Priority date: 1999-06-25
Filing date: 2000-06-23
Publication date: 2001-01-04
Also published as: AU5581600A; RU2156517C1

Abstract

An electrodeless lamp is excited at resonant frequency by means of a high quality cavity resonator having a strong link with a microwave source. After the lamp is switched on, the cavity resonator quality factor drops sharply, leading the microwave source to retune to working frequency at which the discharge is maintained. In case when the discharge dies out the excitation process repeats automatically.

Description

METHOD AND DEVICE FOR EXCITATION AND MAINTE¬

NANCE OF A DISCHARGE IN ELECTRODELESS LAMP

TECHNICAL FIELD

The invention relates to the discharge excitation in the gas-filled

electrodeless lamps using electromagnetic microwaves

PRIOR ART

The main tiling to effectively use electrodeless lamps is the sta¬

bility of the discharge excitation in impulse as well as continuous

modes of operation Usually they use a device containing microwave

magnetron, a chamber or resonator in which an electrodeless lamp lt-

self is disposed The electrodeless lamp is a glass or quartz volume

filled up with a gas under certain pressure [Patent US 5905342, MKI

HOlj 65/04, appl 02 12.1996, publ 18.05.1999]. Normally a gas with

a low breakdown threshold is used, e.g argon with a sufficiently low

pressure (much lower than atmospheric one) with addition of small

amounts of mercury' or like substance, which are in a condensed state under normal temperature. After the discharge is excited essentially in

a pure argon the temperature increases and the mercury or the like

substance vaporizes thus increasing the pressure inside the lamp to a

working level. It take several seconds for the lamp to heat up and

reach its working mode.

The main drawback of this method of exciting and maintaining

the discharge is that it is impossible to restart the discharge if it

extinguishes due to some reason until the lamp cools down. This is

because the microwave field strength required for the argon

breakdown under low pressure is not enough to excite the discharge

under high pressure when the mercury has vaporized. This makes an

electrodeless lamp to be sufficiently sluggish and unstable device

capable of working with a limited number of gases under low

pressure.

In some devices [Patent US 4359668, MKI⁶ H01J 007/46,

appl.15.07.1981 , publ.16.1 1.1982; Patent US 5886480, MKI H05B

41/16, appl.08.04.1998, publ.23.03.1999] where the problem is solved

of creating an electrodeless lamp with multiple inertia-free

initialization which can work not only with rare gases under low pressure but also with molecular and electrically negative gases under

the pressure of the order of higher then that of the atmosphere

For this purpose special additional means are used to excite the

discharge in a gas under pressure which is higher than a certain critical

pressure with depends upon the gas composition As such auxiliary

ultraviolet source or additional electrical impulse high-voltage

discharger are used or separate resonant modes are used to excite and

maintain the discharge [Patent US 5786667, MKI H01J 65/04, appl

09 08 1996, publ 28 07 1998]

All these means allow to excite the discharge but at the same

time they make the device complicated and if the discharge

extinguishes a new external initiation is required, which makes it

necessary to use special monitoring and ignition control devices

The method is known to excite and maintain the electrodeless

lamp discharge [Patent US 5767626, MKI H01 J 65/04,

appl 06 12 1995, publ 16 06 1998] using microwave powei, utilizing

the cavity resonator, exciting oscillations at the frequency which is

resonant to the system comprising microwave source, resonator and an

electrodeless lamp, increasing the electromagnetic field strength to the

breakdown level of the gas filling the electrodeless lamp, exciting the discharge and maintaining the discharge by means of microwave

power. The electrodeless lamp is disposed in the resonator cavity

having one resonant frequency when the lamp is in an unexcited state

(this frequency is used to excite the discharge) and the second

frequency (which is higher than the first one) is used to maintain the

discharge in the electrodeless lamp.

In order to implement this method two magnetrons are used

which are independently coupled by waveguides to the resonator. The

first magnetron provides the microwave power at the frequency which

is resonant to the resonator with an unexcited lamp and is used to start

the discharge. After the lamp has ignited, the second magnetron is

turned on and the first magnetron is turned off. This is accomplished

by a timing circuit or by a photocell sensing the output of the lamp,

which is connected to switching electronics. After the lamp is ignited,

it becomes more conductive, thus effectively making the electrical

dimensions of the cavity smaller. The frequency of the second

magnetron is selected to be higher than the frequency of the first

magnetron to compensate for the change in electrical dimensions after

ignition, so that the cavity with the ignited lamp is resonant or near

resonant at the frequency of the second magnetron. 80 This method allows to increase the field strength mside the

electrodeless lamp at the time immediately before the ignition

provided precise tuning of the magnetron and the system comprising

the cavity resonator and the electrodeless lamp to the same frequency.

However in the invention discussed this problem is not solved.

85 Even for the cavity resonator with a not so high quality factor Q =

1000 for the microwave band, the relative precision of coincidence

between the magnetron frequency and the system resonant frequency

must be ~1/Q = 0.001 .

Since it is impossible to fine tune the magnetron and the system

90 onto the same frequency, the main problem is the temperature and

time instability of the system resonant frequency and the magnetron

frequency, due to geometrical expansion of the resonator, magnetron,

waveguides, and the electrodeless lamp itself in the course of

operation.

95 Thus the temperature of the magnetron anode block after the

power is supplied increases by several hundreds of degrees and the

temperature of the electrodeless lamp itself changes from that of a

liquid nitrogen to -500-600 C and higher. The same time the resonator

remaining essentially under room temperature, the magnetron 100 frequency and the resonator frequency change differently which leads

to resonance being broken.

The suggested embodiment is rather complicated and contains

two magnetrons, lamp ignition monitoring system, magnetron

switching system, and need fine frequency tuning.

105 Besides it is doubtful that it is necessary to maintain the

discharge in the electrodeless lamp at a higher frequency and that this

higher frequency will be a resonant or near- resonant for the resonator

with an ignited lamp.

But first, after the discharge is started there is no need to

no resonantly increase the field strength which is done by a cavity

resonator, since to maintain the gas discharge much lower field

strength is required than that for a breakdown (normally 10-1000 time

lower) [Riser Yu. P. Gas discharge physics. Moscow, Nauka, 1987].

And second, after the discharge start the electrodeless lamp

115 becomes an effective absorber of the microwave power, thus

decreasing the system quality factor to Q ~ 10 - 20 at its best. At such

a low Q factor the width of the system resonant curve is large enough

and there is no need for fine tuning. The higher frequency of the

second magnetron is not due to the change in the system resonant 120 frequency after the lamp is ignited, but because it is necessary to

coordinate the impedances of the magnetron as a microwave generator

on one side and the electrodeless lamp as this generator load on the

other side.

If the impedances are coordinated well enough the microwave

125 frequency for maintaining the discharge can be both higher and lower

than the first resonant frequency.

INVENTION DISCLOSURE

130 The main goal of the present invention is to ensure stable

ignition and maintenance of the discharge in an electrodeless lamp

without any kind of control systems, to ensure the possibility of the

lamp functioning under higher gas pressure or when using the gas

with a higher breakdown threshold; to ensure inertia-free and multiple

135 ignition and extinguishing of the discharge, and to exclude

temperature and time instability when magnetron is tuned to resonant

frequency of the cavity resonator.

This is achieved by excitation and maintaining the discharge in

an electrodeless lamp by means of an electromagnetic microwave, 140 which entails using a cavity resonator, exciting oscillations at the

frequency corresponding to the resonant frequency of the system

containing microwave source, resonator, electrodeless lamp,

increasing the electromagnetic field strength to a breakdown level for

the gas filling the electrodeless lamp, thus exciting a discharge and

145 subsequent maintaining the discharge by applying a microwave

radiation. The method is characterized that the field strength is

increased by way of establishing a strong link between the microwave

source and the resonator, the oscillations are excited at the resonant

frequency of the system foπned by the microwave source and the

150 resonator. The antmodes of the standing wave formed in the resonator

are identified as well as the electric field vector oscillation direction.

The electrodeless lamp is disposed in an antinode area and the

discharge is maintained by a microwave radiation at the frequency

corresponding to the working frequency of the microwave source.

155 The well known device for excitation and maintenance of the

discharge contains the microwave source, cavity resonator and the

gas-filled electrodeless lamp. The microwave source used is the

microwave magnetron having a working frequency which is by no

more than 20% different from the resonator frequency. A strong link 160 is established between the microwave source and the resonator To do

this, the emitting element of the microwave magnetron is disposed

mside the cavity resonator and the electrodeless lamp is disposed in

the antinode of the standing wave formed mside the resonator

The cavity resonator can be made in such a way so that at least

165 part of its surface is punched by openings so that it will be relatively

transparent to electrodeless lamp optical emission while at the same

time have a high reflectivity coefficient for the microwave radiation

thus not decreasing the cavity resonator quality factor Q

To bring out the electrodeless lamp optical emission the cavity

170 resonator can be partially made of optically transparent dielectric or

semiconductor

Also for this purpose the electrodeless lamp can be partially

brought out beyond the resonator through an opemng or openings in

the resonator wall

175 Besides a light guide can also be used to brmg the electrodeless

lamp optical emission to the outside of the resonator

To facilitate the breakdown m the case a high breakdown

threshold gas is used or when the gas pressure is high, a conductive

breakdown initiator can be place mside the lamp This allows to 180 reduce the field strength required for the breakdown to occur by

several times (up to a factor of 10) The initiator can be made straight

or curved, as for example an open inductive loop In this in order to

achieve the best possible result the electrodeless lamp with a

breakdown initiator must oriented in a certain way with respect to the

185 microwave radiation electric vector

To separate the discharge for the lamp walls the lamp can be

place such that the lamp wall are in the node of the standing wave,

while the mside of the lamp is in the standing wave antinode

Comparative analysis have shown that the solution proposed is

190 differs from the prototype in that the electromagnetic field strength of

the microwave source is increased, to achieve this a strong link is

established between the microwave source and the resonator,

oscillations are excited at the frequency corresponding to the resonant

frequency of the system comprising the microwave source and the

195 resonator, the antmodes of the standing wave which foπns in the

resonator are identified along with the direction of the electric vector

oscillations, the electrodeless lamp is dispose in an antinode

Contrary to the prototype the present embodiment contains only

one magnetron The working frequency of the magnetron is by no 200 more than 20% different from that of the resonator. The emitting

element of the microwave magnetron is disposed in the cavity

resonator and the electrodeless lamp is placed in the antinode of the

standing wave which foπns in the resonator.

The essence of the invention is as follows:

205 As is known, a far greater electric field strength is required to

excite the discharge than that maintain it. Thus fairly powerful

microwave sources are used. The other way is to introduce additional

elements to initiate the discharge, such as spark dischargers,

ultraviolet lamps, cryogenic, vacuum and other systems.

210 If a low power microwave source is used such as a microwave

magnetron with power of 100 Wt to several kWt, then at the stage

preceding the initiation, it is necessary to increase the field strength to

a required level. The devices is thus must be supplemented by a

microwave energy accumulator, such as a cavity high quality factor Q

215 resonator.

However there is problem to tune the microwave source to the

resonant frequency of the cavity resonator as well as the problem

associated with the temperature and time instability of the microwave

source and the resonant frequency of the resonator. These problems 220 can be easily solved by using a magnetron (or similar magnetron-like

device) as a microwave source and establishing a strong link between

the magnetron and the resonator The system thus formed which

includes the magnetron, the cavity resonator and the electrodeless

lamp is a united resonator with its own resonant frequency and quality

225 factor Q The factor Q of this resonator is high when the lamp is not

ignited and low when it is ignited

It is know how to tune and stabilize a magnetron frequency by a

cavity resonator In this case additional resonant states appear in the

magnetron anode resonant system regardless of the type of the link

230 between the magnetron anode block and the resonator [D V

Samsonov, Osnovu rascheta I konstruirovania magnetronov, Moscow,

Sovetskoye Radio, 1974, pp 167-194]

According to the "minimum dissipation" principle, the

"magnetron-resonator" system oscillations are excited at the frequency

235 where the ratio of the energy conserved to the total loss energy over a

period reaches its maximum This is a maximum quality factor

frequency Since the magnetron resonance system quality factor

(nonnally < 100) is much less than that of the cavity resonatoi

(noπnally 1000 - 10000 and more), the oscillations are excited at the 240 resonant frequency of the magnetron-resonator system. Naturally this

frequency should be fairly "close" to the magnetron working

frequency. The degree of closeness depends on the magnetron type

and how it is connected with the resonator. Other magnetron like

microwave sources can also self-tune to the frequency of a specific

245 cavity resonator provided there is a strong link. The oscillation

frequency is set with very short period of time, usually -10 ^" s.

This means that if the magnetron is strongly linked with the

cavity resonator and their frequencies coincide to an accuracy of not

more than 20 percent, the oscillations will be excited at the resonant

250 frequency of the system foπned by the magnetron and the resonator

without any mechanical or electrical frequency tuning elements. Of

course, the absolute value of this resonant frequency depends on time

and temperature, but the mutual tuning of the magnetron and the

resonator conserves. Such a strong link is provided due to the

255 radiating microwave magnetron element being located in the

resonator.

This working mode can be sustained if there is a high quality

factor Q of the whole system which includes the magnetron, the cavity

resonator, and the electrodeless lamp. To do this requires exclusion of 260 all the energy losses except those due to a finite conductivity of the

resonator walls.

All these condition being met, the energy starts to build up

within the cavity resonator and a standing wave is being formed with

the maxima and minima of electric field spaced depending on the

265 specific cavity resonator type (cylindrical, coaxial, rectangular, etc.)

and the type of the oscillation excited.

In this situation, the electric field intensity in the standing wave

antmodes is much higher than the magnetron field intensity in a free

space. For high quality Q factor resonators such an intensity increase

270 can be very high, up to 100 times and more.

Usually when a cavity resonator is used to achieve a microwave

breakdown, a gas discharge which forms when intensity reaches a

breakdown level, spreads over the whole resonator volume. But if an

electrodeless lamp containing the gas with a breakdown level

275 substantially lower than the gas m resonator is disposed inside the

resonator, the discharge starts only within the electrodeless lamp and

does not spread to other areas of the resonator. This is because field

strength increases mside the resonator only up to the moment when

the breakdown occurs in the most electrically weak element, the 280 electrodeless lamp in our case. Immediately after the discharge is

excited the field strength drops sharply due substantial decrease of the

system quality factor because of the absorption of the microwave

energy in gas discharge.

The electrodeless lamp being present changes the resonant

285 frequency of the magnetron-cavity resonator system. But since there is

virtually no dissipation the energy into the outer space at the pre-

discharge stage, the quality factor of the system formed by the

magnetron, resonator and the electrodeless lamp remains high. In this

case the microwave magnetron tunes itself to the resonant frequency

290 of the system formed by the magnetron, resonator and the

electrodeless lamp. When there is a big difference between the

fundamental working frequency of the magnetron and the system

resonant frequency exceeding several percent, the magnetron

oscillation can excite at the parasitic oscillation mode. This can

295 decrease the effective radiation power. But since the magnetron works

at this mode very short time, there is no overheating and failure of the

magentron.

After the discharge is excited the electrodeless lamp begins to

absorb the energy and the whole system quality factor drops sharply. 300 It is then energetically advantageous for the magnetron to oscillate at

its proper frequency, and the magnetron retunes to this frequency and

the energy is emitted at this new frequency. The process of the

discharge ignition and magnetron retuning takes very short period of

time about -10^"6 s. This makes the electrodeless lamp to be a virtually

305 inertia-free device (provided a proper gas filling is used, which for

example does contain mercury).

Since there is a strong link between the microwave magnetron

and the resonator on one side and the resonator and the electrodeless

lamp on the other side, the energy emitted is effectively transferred

310 mto the burning zone. Also there is no need to coordinate the

impedances, it can be made by slight moving of the lamp in the

antinode area or choosing the form and dimension of the lamp or by

other known methods, for example by selecting the foπn and

dimension of the magnetron emitting element to adjust the emission

315 phase [US patent US5525865, MKI⁶ H01J 65/04, appl.25.02.1994,

publ. l 1.06.1996].

If due to some reasons the discharge dies out, the whole system

quality factor reinstates very fast, and the discharge appears again.

The time interval for the discharge to restore is very small (~10^~6 sec) 320 and equals the recombination time of the free charges in the lamp The

magnetron tunes again to the resonant frequency, energy is

accumulated again and a new discharge takes place This ensures a

stable operation of the lamp There is no need to wait until the lamp

cools down for the discharge to reappear This magnetron and other

325 magnetron like device operation mode can be implemented m a

continuous generation modes and in both impulse and impulse-

periodic modes In this cases the impulse duration must exceed the

time for energy accumulation in the resonator in order to efficiently

increase the field strength

330 Using cavity resonators with a quality factor Q - 1000 - 10000

and implemented the method suggested it is possible to increase the

filed in the electrodeless lamp location by a factor of 30 - 100, which

is enough for the majority of cases Sometimes it necessary to obtain

the discharge under very high breakdown threshold This can take

335 place when for example the electrodeless lamp is filled up with the gas

under piessure substantially higher than atmospheric or when

molecular and/or electrically negative gases are used It is then

possible to decrease the breakdown threshold usmg a conductive

breakdown initiator placed mside the gas filling of the lamp and 340 properly oriented with respect to the microwave radiation electric

vector. Here by virtue of the local heterogeneity of the field in the

initiator vicinity the breakdown threshold can be decreased by several

times (up to 10 times) [patent RU 2046559, MKI⁶ H05H 1/46,

appl.30.12.92, publ.20.10.95] .

345 To isolate the discharge from the lamp walls the form and the

dimension of the lamp walls can be made to correspond the standing

wave field distribution. If the lamp walls are in the minimum of the

standing wave field, the discharge inside the lamp does not touch the

lamp walls thus allowing to lower the thermal loading on the lamp

350 walls.

To effectively bring out the electrodeless lamp optical emission

the lamp can be partially placed inside the resonator and partially

brought out through and opening in the resonator wall. A light guide

can also be used.

355

BRIEF DESCRIPTION OF THE DRAWINGS

360

The apparatus work is illustrated by the Figure. Fig.l shows the

apparatus version where the electrodeless lamp is fully disposed

within the resonator and contain a straight line breakdown initiator.

The apparatus contains a microwave magnetron 1, a radiating

365 element 2 placed inside a cavity resonator 3, and an electrodeless lamp

4 also placed mside a cavity resonator. The electrodeless lamp can

optionally contain the breakdown initiator 5.

The apparatus works in the following way.

After the voltage is supplied to the magnetron 1 a frequency

370 corresponding to the resonant frequency of the magnetron 1 - resonator

3 - electrodeless lamp 4 system is set within a very short time (-10^" s).

Then energy builds up and within -10^" s reaches a breakdown

intensity level. A discharge sets in the electrodeless lamp, the

magnetron retunes to the working frequency and the electrodeless

375 lamp absorbs the energy radiated by the radiating source 2 of the

magnetron 1. The breakdown initiator is an optional element and can

be built m the electrodeless lamp if necessary. If the discharge

extinguishes the initiating process repeats automatically. 380

INDUSTRIAL APPLICABILITY

385 For typical cylinder resonators excited at the TM010 mode the

field strength at the center of the resonator is, according to

[McDonald, Microwave breakdown is gases]

390 where

PQ is the magnetron microwave radiation power,

Q is the resonator quality factor,

coo is the microwave radiation angular frequency,

η = 0.27-εo-V (2)

395 where

ε₍₎ = 8.85- 10^" F/m is the pennittivity of free space,

V is the resonator volume, chosen to be V - λ .

where λ is the length of the microwave in free space. Using the equation (1) it is possible to choose a cavity resonator

with a relatively low quality factor (Q) reaching a sufficiently high

field strength at the center of the resonator at the same time

For example, when P₀ - 1 kWt, ω₀/2π = 2 45 GHz, V = 11, Q =

10 the electric field strength will be E₀ - 160 kV/cm For example,

the breakdown intensity for the air under atmospheric pressure is

about 30 kV/cm

EXAMPLE OF USE

We used a magnetron with a power supply from a conventional

microwave oven with a power P = 800 Wt and the frequency f = ω₀/2π

= 2 45 GHz It has been thus possible to use resonator made of copper

with a volume V = 0 71 for the wavelength λ = 12cm The resonator

has a partially punched surface

The electrodeless lamp was made as a quartz bulb with a

volume of 0 11 Industrial argon was used as a filling gas with a small

addition of mercury The electrodeless lamp optical emission power

was 100 Wt The discharge was starting and extinguishing with a 50Hz frequency corresponding to single-period power supply of the

magnetron

420 The apparatus works very stable, and in case of the discharge

extinguishing the excitation takes place without waiting for the lamp

to cool down Such a device can operate from a normal mams

Besides, the apparatus is very compact and simple, there are no

mechanically moving parts and no controlling or switching circuits

425 mside There is no problem to tune the magnetron and the resonator

onto the same frequency and thus there is no time or thermal

instability due to imprecise frequency tuning

This makes it possible to use the apparatus m various fields

such as m medicine for ultraviolet sterilization Also this device being

430 relatively small and simple makes it possible to use it in all the fields

where there is a need for a simple and reliable source of light

Claims

1 The method of excitation and maintaining the discharge in an electrodeless

lamp by means of an electromagnetic microwave, which entails using a cavity

resonator, exciting oscillations at the frequency corresponding to the resonant

frequency of the system containing microwave source, resonator, electrodeless

lamp, increasing the electromagnetic field strength to a breakdown level for the

gas filling the electrodeless lamp, thus exciting a discharge and subsequent

maintaining the discharge by applying a microwave radiation The method is

characterized that the field strength is increased by way of establishing a strong

link between the microwave source and the resonator, the oscillations are

excited at the resonant frequency of the s\stem formed by the microwave

source and the resonator The antmodes of the standing wave formed in the

resonator are identified as well as the electric field vector oscillation direction

The electrodeless lamp is disposed in an antinode area and the discharge is

maintained by a microwave radiation at the frequency corresponding to the

working frequency of the microwave source

2 The apparatus to implement the method 1 Contain the microwave source,

the cavity resonator and the gas-filled electrodeless lamp, which differs in that

the electrodeless lamp is disposed in the antinode of the standing wave formed

in the resonator and the microwaves are generated by a magnetron The

working frequency of the magnetron differs by no more than 20% from the

3AMEHHK)mHH IHCT (IIPABHJIO 26) resonant frequency of the cavity resonator in which the emitting element of the

magnetron is placed

3 The apparatus according to claim 2 wherein the cavity resonator has got a

partially punched surface, relativel transparent for an optical radiation of the

electrodeless lamp and having a high reflectivity coefficient for the microwave

radiation

4 The apparatus according to claim 2 wherein the cavity resonator is made

partially of transparent dielectric or semiconductor

5 The apparatus according to claim 2 wherein the electrodeless lamp contains

a conductive breakdown initiator in its gas volume The initiator is directed

parallel to the electric standing wave vector

6 The apparatus according to claim 2 wherein the electrodeless lamp contains

a conductive breakdown initiator in its gas volume The initiator is an

inductive open loop directed perpendicularly to the magnetic standing wave

vector

7 The apparatus according to claim 2 wherein the electrodeless lamp is

disposed in the antinode of the standing wave in such a wave that the bulb of

the lamp is located in the standing wave minima

8 The apparatus according to claim 2 wherein the electrodeless lamp is only

partially disposed in the resonator and partially is brought out to the outside of

the resonator through the opening or openings in the resonator wall

flIO H JIHCT ϋPABHJIO 26

9. The apparatus according to claim 2 wherein the light guide is used to bring

out the optical electrodeless lamp radiation to the outside of the resonator.

3AMEHΛK>mHH JIHCT ϋPABHJIO 26)