PLASMA LIGHT SOURCE
The present invention relates to a plasma light source.
High Frequency (HF) Plasma is a term often applied to mean both Radio
Frequency, RF (~ 1 - 300 MHz) and Microwave (~ 0.3 - 300 GHz) excited plasmas.
Most HF Plasmas used as light sources are fully localised inside the HF field
applicator, that is the discharges are sustained in capacitive or inductive circuits and in
resonant cavities, coaxial lines and waveguides.
A drawback of an air filled resonant cavity device is that the size of the cavity
is determined by the frequency of operation. Technically successful cavity systems
have been designed for operation at 2.4GHz. At suitable frequencies (ISM -
Industrial, Scientific and Medical - bands) below this frequency the size of the cavity
and the associated waveguides is liable to become physically too large for use in
commercial lighting systems. It also becomes difficult to design high pressure plasma
chambers for such cavities which operate plasmas at combinations of high radiation
efficiency and usefully low power, i.e. less than 400 watts, required for most
commercial applications. Indeed even at 2.45GHz obtaining system powers of less
than 400 watts with plasmas of the required radiation efficiency can be difficult.
In order to provide plasmas with a high radiation efficiency and operation at
powers less than 400 watts it is known to operate plasma chambers within a dielectric
filled resonant cavity. While this latter configuration is suitable as a light source for
applications such as projection where small source size is the primary benefit being
sought, the first configurations had serious limitations for general lighting situations
because of the obstruction of a high percentage of light from the source by the opaque
dielectric structure. In this configuration less than 50% of the surface area of a bulb is
able to emit light into a limited solid angle, 2steradian, of free space. This surface
area is usually maximised by designing a portion of the bulb volume to be external to
the cavity.
As shown in our International Application No PCT/GB2008/003829, we have
overcome this drawback. In that application, we describe a light source to be powered
by microwave energy, the source having:
• a body having a sealed void therein,
• a microwave-enclosing Faraday cage surrounding the body,
• the body within the Faraday cage being a resonant waveguide,
• a fill in the void of material excitable by microwave energy to form a light
emitting plasma therein, and
• an antenna arranged within the body for transmitting plasma-inducing,
microwave energy to the fill the antenna having:
• a connection extending outside the body for coupling to a source of
microwave energy;
wherein:
• the body is a solid plasma crucible of material which is lucent for exit of light
therefrom, and
• the Faraday cage is at least partially light transmitting for light exit from the
plasma crucible,
the arrangement being such that light from a plasma in the void can pass through the
plasma crucible and radiate from it via the cage.
As used in that application:
• "lucent" means that the material, of which an item described as lucent is
comprised, is transparent or translucent;
• "plasma crucible" means a closed body enclosing a plasma, the latter being in the
void when the void's fill is excited by microwave energy from the antenna;
• "Faraday cage" means an electrically conductive enclosure of electromagnetic
radiation, which is at least substantially impermeable to electromagnetic waves at
the operating, i.e. microwave, frequencies.
In this application we use "Faraday cage" in analogous manner, but not
restricted to enclosing microwaves but extended to enclosing the electromagnetic
waves at the operating frequency whatever that may be in the HF band as defined
above. We do not use the term "plasma crucible" in this application.
Plasmas can be created by travelling waves in waveguides and slow wave
structures, so called Travelling Wave Discharges (TWD). For lighting purposes one
member of this class of discharges, the Surface Wave Discharge (SWD), has in the
past been widely assessed as being particularly promising; this is the propagative
Surface Wave Discharge SWD. This type of discharge is well known in the literature,
electromagnetic energy forms the plasma and the plasma itself is the structure along
which the wave is propagated. A practical field applicator for a SWD is a surfatron.
Surfatrons are wide band structures that may be used over a frequency range of
200MHz to 2.45GHz and have the property that very high energy coupling
efficiencies can be achieved. Greater than 90% of the HF energy can be coupled into
the plasma. Although SWD's launched by surfatrons have been proposed for lighting
applications, these have been aimed at low pressure discharges. The major
application for SWD's is large volume sub-atmospheric to atmospheric pressure
plasmas for various processes in microcircuit fabrication. For high pressure lighting
applications there is a drawback. The volume of the plasma is very dependant on the
plasma pressure and plasma power. At powers of less than 400 watts and pressures of
a few atmospheres the vast bulk of the plasma is contained within the launching
structure, so that given the opaque nature of the known surfatron devices very little of
the light produced by the plasma can be harvested.
A typical surfatron structure is shown in diagrammatically in Figure 1. The
surfatron 1 has an HF structure consisting of two metal cylinders 2,3 forming a
section of coaxial transmission line 4 terminated by a short circuit 5 at one end and by
a circular gap 6 at the other. A HF electric field extending through the gap can excite
an azimuthally symmetric surface wave to sustain a plasma column 7 of excitable
material in a dielectric tube 8 arranged co-axially within the cylinders. A coaxial,
cylindrical, capacitative coupler 9 is positioned between the cylinders, with a
connection 0 extending out through outer cylinder. There it is connected to an input
transmission line. A plate is attached to the inner conductor to form a capacitance
between this plate and the inner metal cylinder.
The object of the present invention is to provide an improved light source.
According to the invention there is provided a light source to be powered by
High Frequency energy, the source having:
• an enclosure of lucent material, the enclosure having:
• a sealed void therein,
• a fill in the void of material excitable by High Frequency energy to form a
light emitting plasma therein,
• a High Frequency energy-enclosing Faraday cage surrounding the enclosure,
the Faraday cage being:
• at least partially light transmissive for light exit from the plasma crucible
and the Faraday cage having:
• two end portions and an outer sleeve between the end portions, and
• an antenna arranged within the Faraday cage for transmitting plasma-inducing,
High Frequency energy to the fill, the antenna having:
• a connection extending outside the Faraday cage for coupling to a source of
High Frequency energy;
wherein:
• a High Frequency energy-barrier cylindrical inner sleeve is arranged within
the outer sleeve, the inner sleeve being:
• at least partially light-transmissive for light passage therethrough and
being,
• connected electrically at one end to one end portion of the Faraday cage
and
• defining a launching gap at the other end with the other end portion of the
Faraday cage,
• the enclosure is arranged within the inner sleeve and
• the antenna is arranged between the inner and the outer sleeves;
whereby High Frequency energy introduced between the sleeves via the antenna can
be launched via the gap into the inner sleeve for excitation of the plasma and radiation
of light through the sleeves and out of the source.
Whilst it can be envisaged that the space between the sleeves could be empty
of solid material; preferably the space between the sleeves is at least partially filled
with lucent, solid dielectric material. In the preferred embodiment, the space is
substantially filled with quartz.
Further, it can be envisaged that the inner sleeve is of greater cross-section
than the void enclosure, the intervening space being empty of solid material.
However, the intervening space is preferably filled with lucent, solid dielectric
material. A number of configurations are possible:
• the inner sleeve being of greater cross-section than the void enclosure, the
intervening space being filled with lucent, solid dielectric material;
• the void enclosure being a bulb containing the fill, the bulb being housed in a bore
in a lucent, solid dielectric material body within the inner sleeve. Preferably the
bulb fills the bore in the body and is fused thereto. Alternatively, the bulb is
radially spaced from the bore in the body and is fused thereto;
• the inner sleeve being of substantially the same cross-section as the void
enclosure, the void being a bore in the enclosure, sealed at both ends thereof.
Preferably, the void is at the launching gap end of the inner sleeve.
In the preferred embodiment:
• the lucent, solid dielectric material within the inner sleeve and between the sleeves
are separated by the thickness of the inner sleeve only at the launching gap;
• the inner and the outer sleeves are reticular and metallic; and
• the outer sleeve has an imperforate rim via which the light source is clamped to a
metallic carrier providing one end portion of the Faraday cage.
To help understanding of the invention, a specific embodiment thereof will
now be described by way of example and with reference to the accompanying
drawings, in which:
Figure 1 is a diagrammatic cross-sectional side view of a known surfatron;
Figure 2 is a diagrammatic cross-sectional side view of a light source in
accordance with the invention; and
Figure 3 is a view similar to Figure 2 of a variant of the light source of Figure
2.
Referring to Figure 2, there is shown diagrammatically a light source 11 to be
powered by High Frequency energy, in particular 433MHz energy. It comprises:
• a central body 12 of fused quartz, the body being circularly cylindrical, 32mm
long and 16mm in diameter;
• a void 14 in the central body, the void being formed as a 4mm bore in the body,
10mm long and sealed via the vestige 15 of a tube fused to the body and through
which the void was evacuated and filled;
• a fill 16 in the void of material excitable by High Frequency energy to form a light
emitting plasma therein, typical the fill is of metal halide material in an inert gas
atmosphere;
• an inner sleeve 17 of perforate metal shim extending along the length of the
central body to within 2.5mm of its void end to provide a launching gap 18. The
sleeve has a transverse end portion 19 extending across the other, inner end of the
central body;
• an outer cylinder of fused quartz 20, also 32mm in length, with an internal bore 2 1
such as to be a sliding fit with the inner sleeve, itself a sliding fit on the central
body. The result is a thin gap between the two quartz elements 12,20 at the
launching gap, which is negligible in electromagnetic terms. The outer cylinder is
81mm in outside diameter;
• an outer sleeve 22 of perforate metal, enclosing the outer cylinder and having an
end portion 23 extending across the flush, void ends of the quartz body and
cylinder 12,20, with an aperture 24 for the tube vestige 15. The outer sleeve has a
skirt 25 extending past the flush other ends of the quartz elements over an
aluminium carrier 26, where it is clamped, by known shown means, holding the
quartz elements against the carrier. Thus the sleeve forms, with its end 22 and the
carrier 26, a Faraday cage around the quartz and the plasma void 14;
• an antenna 27 insulated from and extending from the carrier into a bore 28 in the
quartz cylinder 20 for introducing HF radiation into the coaxial wave guide
formed by the perforate inner and outer sleeves 17,2 1. Their perforation is such as
to make them opaque and enclosing to the HF radiation yet light transmissive,
whereby light from the plasma can pass through them. The portion of the antenna
in the carrier provides a connection to a non-shown source of HF energy.
The inner sleeve 17, at its end portion 1 , is earthed to the carrier, in the same
way as the outer sleeve and its end portion 23. Thus the gap 18 between the end of
the inner sleeve and the end portion of the Faraday cage forms a launching gap for the
HF energy to radiate to the plasma void and establish and maintain the plasma therein.
Light from the plasma passes through the quartz and through the perforations in the
sleeves and the end portion 19, and thus out of the light source.
In the variant of Figure 3, the inner sleeve 17 is shorter and the launching gap
is wider, typically 10mm, such that the bulk of the light passes out of the source via
the outer sleeve 22 only of the Faraday cage.
CLAIMS:
1. A light source to be powered by High Frequency energy, the source having:
• an enclosure of lucent material, the enclosure having:
• a sealed void therein,
• a fill in the void of material excitable by High Frequency energy to form a
light emitting plasma therein,
• a High Frequency energy-enclosing Faraday cage surrounding the enclosure,
the Faraday cage being:
• at least partially light transmissive for light exit from the plasma crucible
and the Faraday cage having:
• two end portions and an outer sleeve between the end portions, and
• an antenna arranged within the Faraday cage for transmitting plasma-inducing,
High Frequency energy to the fill, the antenna having:
• a connection extending outside the Faraday cage for coupling to a source
of High Frequency energy;
wherein:
• a High Frequency energy-barrier cylindrical inner sleeve is arranged within
the outer sleeve, the inner sleeve being:
• at least partially light-transmissive for light passage therethrough and
being,
• connected electrically at one end to one end portion of the Faraday cage
and
• defining a launching gap at the other end with the other end portion of the
Faraday cage,
• the enclosure is arranged within the inner sleeve and/or the launching gap and
• the antenna is arranged between the inner and the outer sleeves;
whereby High Frequency energy introduced between the sleeves via the antenna can
be launched via the gap into the inner sleeve for excitation of the plasma and radiation
of light through the sleeves and out of the source.
2. A light source as claimed in claim , wherein the space between the sleeves is
empty of solid material, except that of the void enclosure.
3. A light source as claimed in claim 1, wherein the space between the sleeves is at
least partially filled with lucent, solid dielectric material.
4. A light source as claimed in any preceding claim, wherein the inner sleeve is of
greater cross-section than the void enclosure, the intervening space being empty of
solid material.
5. A light source as claimed in any one of claims 1 to 3, wherein the inner sleeve is
of greater cross-section than the void enclosure, the intervening space being filled
with lucent, solid dielectric material.
6. A light source as claimed in claim 5, wherein the void enclosure is a bulb
containing the fill, the bulb being housed in a bore in a lucent, solid dielectric material
body within the inner sleeve.
7. A light source as claimed in claim 6, wherein the bulb fills the bore in the body
and is fused thereto.
8. A light source as claimed in claim 6, wherein the bulb is radially spaced from the
bore in the body and is fused thereto.
9. A light source as claimed in any one of claims 1 to 3, wherein the inner sleeve is
of substantially the same cross-section as the void enclosure, the void being a bore in
the enclosure, sealed at both ends thereof.
10. A light source as claimed in any preceding claim, wherein the void is at the
launching gap end of the inner sleeve.
1 . A light source as claimed in any one of claims 5 to 10 as appendant to claim 3,
wherein the lucent, solid dielectric material within the inner sleeve and between the
sleeves are separately by the thickness of the inner sleeve only at the launching gap.
12. A light source as claimed in any one of claims 5 to 11, wherein the lucent, solid
dielectric material is fused quartz.
13. A light source as claimed in any preceding claim, the inner and the outer sleeves
are reticular and metallic.
14. A light source as claimed in claim 13, wherein the outer sleeve has an imperforate
rim via which the light source is clamped to a metallic carrier providing one end
portion of the Faraday cage.
15. A light source as claimed in any preceding claim wherein the void is arranged
axially of the light source at least partially over-lapping with the inner sleeve.
16. A light source as claimed in any preceding claim wherein the void is arranged
axially of the light source so as not to over-lap with the inner sleeve.

cited in search report date member(s) date
US 5028847 A 02-07-1991 EP 0357451 Al 07-03-1990
P 2192607 A 30-07-1990
US 4792725 A 20-12-1988 CA 1260531 Al 26-09-1989
EP 0225753 A2 16-06-1987
J P 62140355 A 23-06-1987