PRIOR ART The main tliing 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 it- 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. 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 HOIJ 007/46, appl. 15.07.1981, publ. 16.11.1982; 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 HO l J 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 HOIJ 65/04, appl. 06.12.1995, publ. 16.06.1998] using microwave power, 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 coinected 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.

This method allows to increase the field strength inside 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.

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-I/Q = 0.001.

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

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

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.

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 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 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

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 frequency for maintaining the discharge can be both higher and lower than the first resonant frequency.

INVENTION DISCLOSURE 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 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,

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 system formed by the microwave source and the resonator. The antinodes 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.

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

is established between the microwave source and the resonator. To do this, the emitting element of the microwave magnetron is disposed inside the cavity resonator and the electrodeless lamp is disposed in the antinode of the standing wave formed inside the resonator.

The cavity resonator can be made in such a way so that at least 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 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 opening or openings in the resonator wall.

Besides a light guide can also be used to bring the electrodeless lamp optical emission to the outside of the resonator.

To facilitate the breakdown in the case a high breakdown threshold gas is used or when the gas pressure is high, a conductive breakdown initiator can be place inside the lamp. This allows to

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 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 inside of the lamp is in the standing wave antinode.

Comparative analysis have shown that the solution proposed is 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 resonator, the antinodes of the standing wave which fonns 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

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 forms in the resonator.

The essence of the invention is as follows: 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.

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 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

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 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 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 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 (normally < 100) is much less than that of the cavity resonator (normally 1000-10000 and more), the oscillations are excited at the

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 cavity resonator provided there is a strong link. The oscillation frequency is set with very short period of time, usually ~10 ~8 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 frequency of the system formed 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 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

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 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 antinodes is much higher than the magnetron field intensity in a free space. For high quality Q factor resonators such an intensity increase 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 substantially lower than the gas in 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 inside the resonator only up to the moment when the breakdown occurs in the most electrically weak element, the

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 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 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 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.

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 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 into 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 form and dimension of the magnetron emitting element to adjust the emission phase [US patent US5525865, MKI6 HOIJ 65/04, appl. publ.11.06.1996].

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)

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 magnetron like device operation mode can be implemented in 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.

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 place when for example the electrodeless lamp is filled up with the gas under pressure substantially higher than atmospheric or when molecular and/or electrically negative gases are used. It is then possible to decrease the breakdown threshold using a conductive breakdown initiator placed inside the gas filling of the lamp and

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].

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 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.

BRIEF DESCRIPTION OF THE DRAWINGS 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 element 2 placed inside a cavity resonator 3, and an electrodeless lamp 4 also placed inside 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 corresponding to the resonant frequency of the magnetron 1-resonator 3-electrodeless lamp 4 system is set within a very short time (-10-gus).

Then energy builds up and within reaches a breakdown intensity level. A discharge sets in the electrodeless lamp, the magnetron retunes to the working frequency and the electrodeless 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 in the electrodeless lamp if necessary. If the discharge extinguishes the initiating process repeats automatically.

INDUSTRIAL APPLICABILITY 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] where Po is the magnetron microwave radiation power, Q is the resonator quality factor, is is the microwave radiation angular frequency, T1 = 0.27 (2) where So = 8.85 10-l2 F/m is the pennittivity of free space, V is the resonator volume, chosen to be V # #3, where X 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, Mo/27r = 2.45 GHz, V = 11, Q = 103 the electric field strength will be Eo-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 = 800Wt and the frequency f= xo/27 : = 2.45 GHz. It has been thus possible to use resonator made of copper with a volume V = 0.71 for the wavelength 12cl. 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.

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 mains.

Besides, the apparatus is very compact and simple, there are no mechanically moving parts and no controlling or switching circuits inside. 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 in various fields such as in medicine for ultraviolet sterilization. Also this device being 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.