Apparatus for conveying electrostatic charges, in particular for very high voltage electrostatic generators

In a very high voltage electrostatic generator, the electrostatic charges are conveyed by a belt which rotates in a closed circuit between the earthed zone and the high voltage zone of the generator. According to the invention, a structure (6) offering a flat surface (60) to the belt section (5) is provided opposite the belt section (5). A gap (50) containing sulphur hexafluoride is thus formed between the structure (6) and the belt section (5). The structure (6) is made of a dielectric material (61) such as an epoxy resin which preferably contains conductive elements (62). The structure (6) is in addition traversed by tubules (63) opening on to the surface (60) so as to enable a cushion of sulphur hexafluoride to be formed in the space (50). This arrangement can operate under higher electric charge outputs than has hitherto been possible.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION 
This invention, made by the Institute of Nuclear Physics and Particle 
Physics at the Centre of Nuclear Research in Strasbourg,relates to the 
transport of electrostatic charges by mechanical means. 
It is known that very high voltages can be obtained by means of 
electrostatic generators. To operate these generators, it is necessary to 
incorporate in them a device for conveying the electrostatic charges 
between a zone of low voltage (or earth) of the generator and the high 
voltage zone of the generator. This is known as the feed device of the 
electrostatic generator. 
Electrostatic generator feed devices used at the present time are of two 
types: 
The system of the Felici type using an insulating cylinder, which provides 
powerful outputs of (currents of) electrostatic charges without, however, 
being able to reach very high voltages; and 
the system of the Van de Graaff type using a flat insulating belt, which 
can operate up to very high voltages but provides only limited outputs or 
charge currents. 
The second system is used for supplying electrostatic accelerators emloyed 
mainly for research applications in nuclear physics. For other, more 
recent applications, it is desirable to be able to obtain both a high 
output or charge current and a very high voltage. This problem has not so 
far been resolved satisfactorily in Van de Graaff generators. 
In these electrostatic accelerators, the distribution of electric field 
between the zone of very high voltage and the zone of very low voltage or 
earth voltage is controlled by separate devices such as gradient bars. The 
feed device, as well as other parts of the accelerator, are located in a 
gaseous atmosphere consisting mainly of sulphur hexafluoride. These 
gradient bars are placed on either side of each section of the belt, 
namely the ascending and descending section. The position of equilibrium 
of each belt section between the gradient bars surrounding them is, 
however, unstable. When attempts are made to increase the density of 
electric charges or, more precisely, the net balance of electric charges 
transported by each section of the belt, the belt is subject to mechanical 
instabilities which give rise to disturbances such as vibrations, 
premature wear and even breakdown. 
The present invention seeks to provide a solution to the problem of 
increasing the charge output. 
The proposed device for conveying the electrostatic charges is of the type 
comprising, inside a gaseous atmosphere: 
A flat insulating belt turning in a closed circuit between an earthed zone 
and a high voltage zone which are spaced apart, 
means for depositing electric charges on the belt in the earthed zone, 
means for extracting electric charges from the belt in the high voltage 
zone and 
two structures placed close to each section of the belt between the high 
voltage zone and the earthed zone to control the gradient of the electric 
field along the belt between these two zones. 
According to a first feature of the invention, each of the structures 
placed close to each section of the belt extends continuously along the 
whole length of its particular section of belt and is non-conductive in 
the direction of displacement of the belt. This structure comprises, on 
the side facing the belt, a flat surface where it is designed to produce a 
cushion of ambient gas between itself and the belt. 
In practice, the distance between the belt and the said flat surface is 
less than a millimeter. 
According to another feature of the invention, each structure consists 
mainly of an epoxy resin equipped on the side of the belt with orifices 
placed at more or less regular intervals apart and supplied with ambient 
gas to produce a substantially uniform pressure at the orifice outlets. 
This enables the aforesaid air cushion to be produced. 
The mean distance between adjacent orifices is advantageously several 
centimeters while the diameter of each orifice is of the order of a 
millimeter. 
In a first embodiment of the invention the structure contains elongated 
conductive elements placed perpendicularly to the direction of movement of 
the belt and arranged at regular intervals along the belt and parallel to 
the plane of the belt. 
These conductive elements may end flush with the surface of the insulating 
structure opposite the belt. 
In one variation, the conductive elements are embedded at a selected depth 
within the volume of the insulating structure. 
The feed tubules of the orifices advantageously extend at least in part to 
the inside of the said conductive elements. 
In cross-section, the conductive elements are preferably so designed that 
they minimize the distortions of the local electric field created by their 
presence in the vicinity of the belt. 
In another embodiment of the invention, the said structure is made of an 
insulating material which is homogeneous but rendered slightly conductive 
within its volume. 
Inside the structure, the feed tubules of the orifices are advantageously 
inclined at an angle to the direction of movement of the belt. In the 
preceding embodiment, the portion of tubules extending through the 
insulating material is also advantageously set at an angle to the 
direction of movement. 
According to another feature of the invention, the dielectric constant of 
the epoxy resin is approximately 5 to 8. 
The two structures are preferably arranged each on the outside of its 
respective section of the belt. 
The invention applies in particular to cases in which the ambient gas used 
is sulphur hexafluoride. 
Other features and advantages of the invention will be apparent from the 
detailed description given below with reference to the attached drawings, 
as briefly described below.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows an electrostatic generator 1 having a part at earth potential 
1M and a part at very high potential 1H at its other end. The generator 
may be mounted vertically or horizontally. The internal structure (not 
shown) of the electrostatic generator 1 may be similar, for example, to 
that described in the French Patent Application published under the number 
2 498 040. 
To produce a high voltage, it is necessary to pass electric charges from 
the earthed zone 1M of the generator to the very high voltage zone 1H. 
For very high voltages, electric charges are mainly conveyed mechanically 
by means of a belt indicated by the reference 3. This belt 3, which is 
normally flat and insulating, moves in a closed cycle (circuit) between 
the lower roller 4M and the upper roller 4H. 
A device 2M placed between the earthed zone 1M of the generator and the 
lower part of the belt provides for the deposition of electric charges on 
one of the sections of the belt. Another device 2H in the upper part 
extracts electric charges from the belt to transfer them to the very high 
voltage part 1H of the generator. 
Substantially the same electric field then exists between the upper part 
and the lower part of the belt as between the very high voltage zone 1H of 
the generator and its earthed zone 1M. The whole arrangement of devices 
described above is located in an atmosphere of a gas such as sulphur 
hexafluoride which has good disruptive characteristics. 
It is known to use so-called gradient bars to control the distribution of 
the electric field along the belt. 
FIG. 1 shows gradient bars 6A and 6B on either side of the ascending 
section 5 of the belt to ensure the transfer of charges between the device 
2M and the device 2H. Similar gradient bars 8A and 8B are provided on 
either side of the descending section 7 of the belt 3. 
This arrangement has hitherto proved satisfactory in electrostatic 
generators where it is generally not necessary to transmit a high output 
of electric charges, that is to say a powerful electric current between 
the earthed zone 1M and the very high voltage 1H of the accelerator. It 
should be remembered, however, that the feed belt is frequently a source 
of breakdown of an electrostatic accelerator. 
It may be noted that each section of the belt travels between gradient bars 
situated on either side of the belt. If one assumes that the overall 
charge of this section of the belt is positive then these charges will 
create lines of force which will close up again either on the right or 
left of the gradient bars. When the belt is exactly equidistant between 
the bars on the righthand side and the bars on the lefthand side (assuming 
a symmetrical structure), then the electric field is identical on the 
right and left. A position of equilibrium will then prevail. However, the 
belt, like any mechanical device, is subject to minor displacements about 
its central position. When a displacement of the section of belt causes 
this section to move away from its central position and closer to the 
gradient bars situated on one side, an instability will become apparent 
since the electric field then tends to increase the distance between the 
belt and its central position. 
It is then found that the belt tends to place itself flat up against the 
gradient bars either on the right or on the left. The greater the charges 
carried by the belt, the more strongly will the belt rub up against these 
bars. The man of the art knows well that this gives rise to all sorts of 
difficulties, not the least of which is the wear and tear on the belt due 
to friction against the gradient bars. 
As a result of these phenomena, the transport of charges by belt has 
hitherto been considered to be very difficult to apply for systems of high 
outputs or currents although the use of the belt is a necessary measure 
for producing very high voltages. 
Instead of using gradient bars arranged on either side of each belt 
section, the invention provides that a continuous structure which is 
non-conductive in the direction of the belt be provided on one side of 
each belt section. Moreover, this structure has a flat surface on the side 
of the belt, where it is designed to produce a cushion of ambient gas such 
as sulphur hexafluoride between itself and the belt. 
The possibility of obtaining a very high voltage is preserved by using a 
linear element such as the belt. The belt is maintained at a very small 
distance from the aforesaid surface of the continuous structure to enable 
charges to be conveyed at a high density, which is all the higher the 
thinner the layer of film of gas, according to the law of disruption of 
the particular gas. 
The composition and form of the structure are designed to minimize the 
distortions of the local electric field which may occur in the vicinity of 
the belt due to the geometrical configuration of the structure so that a 
more favourable law of disruption may be obtained since this will be all 
the better the more the distortions are reduced. 
Three embodiments of the invention are illustrated in FIGS. 2 to 4. These 
figures are partial representations of a belt section and of the structure 
facing one side of the belt, which would normally be the outside of the 
belt. 
The section illustrated is the ascending section 5 and it is assumed to 
carry a density of positive charges. The potential thus increases from 
left to right. 
In FIG. 2, the structure 6 situated opposite the section 5 has a flat 
surface 60 in the immediate vicinity of the belt section. The distance 
between the belt and the flat surface 60 is of the order of a millimeter 
or several millimeters. 
The structure 6 consists of an epoxy resin having a dielectric constant 
preferably in the region of about 5 to 8. It contains conductive bars 
62-1, 62-2, 62-3 embedded in the matrix of the epoxy resin 61 and 
extending transversely to the direction of movement of the belt 5. These 
bars extend beyond the matrix 61 on either side (FIGS. 2A and 2B). They 
are placed with their large dimension parallel to the surface 60. 
Gas distribution tubes 63-1 to 63-3 pass transversely through the structure 
6, preferably extending through the conductive bars 62-1 to 62-3 in their 
major dimension. Each tube 63 distributes gas to side tubules 66 opening 
on to the surface 60 by orifices having a diameter of the order of a 
millimeter while the average distance between adjacent orifices may be 
several centimeters. The distribution of orifices on the surface 60 need 
not be strictly regular since a statistically balanced distribution of 
orifices may be sufficient to produce a cushion of gas. 
The tubes 63-1 to 63-3 are under an excess pressure of ambient gas which 
enables the gas cushion 50 to form in the space between the surface 60 and 
the upper surface of the belt section 5. 
One arrangement for obtaining the gas cushion is illustrated in FIGS. 2A 
and 2B which in this respect supplement FIG. 2. 
Each tube 63 is closed at one end, for example by a plug 64. A disc 65 
screwed to the other end of the tube is perforated in the axial direction 
to enable the tube 63 to communicate at 67 with a pipe 9 (for example a 
pipe made of Rilsan, Registered Trade Mark) which conducts SF.sub.6 under 
pressure to various tubes 63. The tubes 63 and discs 65 are made of metal, 
for example stainless steel. 
The discs of form spark gaps between themselves in the direction of 
movement of the belt 5, as shown between 65-1 and 65-2 (FIG. 2B). The pipe 
9 extends along the side of the structure 6 in the direction of movement 
of the belt 5. 
In one exemplary embodiment (50 kV between bars), the bars 62 which have a 
width of 1 cm are placed at intervals of 2.5 cm with a space of 1.5 cm 
between bars. The spark devices 65 are discs having a thickness of 6 mm 
with an external diameter of 22 mm on their rounded periphery (for the 
sake of clarity, the figures are not drawn to this scale). 
The arrangement according to the invention enables the dielectric belt 5 to 
be placed at a very small distance from the structure 6 according to the 
invention in the gas. This is an advantage since the smaller this distance 
the more easily can the appearance of disruptive fields in this gap be 
avoided. 
When the charge conveyed by the belt section 5 tends to increase, a force 
of attraction is produced between this section 5 and the structure 6 
facing it, as already mentioned above. However, since the pressure in the 
cushion of gas in the space 50 increases as the aforesaid distance 
decreases, a force of repulsion will be produced which tends to compensate 
for the force of attraction between the belt 5 and the structure 6. 
Consequently and contrary to what has been possible in the prior art, a 
position of stable equilibrium can now be obtained for the belt section 5 
in relation to the surface 60 of the structure 6 with a very small space 
between these parts. 
FIG. 3 illustrates a second embodiment of the invention, again showing the 
belt section 5 and the space 50. 
The structure facing the belt section 5, now marked by the reference 
numeral 16, has a surface 160 on the side facing the section 5. The bars 
162-1 to 162-3 are again embedded in the dielectric resin 161 but at some 
distance from the surface 160. 
The distributor tubes, now marked by the reference numerals 163-1 to 163-3, 
extend through the conductive bars 162. The gas flows to the surface 160 
through tubules 164 which are preferably set at a slope, as illustrated. 
The sloping arrangement of the tubules inside the resin provides for a 
better functioning of the whole apparatus with regard to the establishment 
of lines of force (electric field) between the belt section 5 and the 
conductive elements 162. The air cushion may alternatively be obtained as 
in the first embodiment (FIGS. 2A and 2B). 
In the two embodiments described above, the structures 6 and 16 are 
heterogeneous. Distortions may therefore occur in the electric field at 
the level of the interval 50 between the structure and the belt section 5. 
It is found possible to choose the form of the bars 62 or 162 on the side 
facing the belt section so as to minimize these distortions of the 
electric field. This also helps to improve the characteristics of 
disruption in the gap 50. 
With this in view, and particularly in their illustration of the shapes, 
the appended drawings are incorporated with the description not only to 
enable the invention to be more easily understood but also to contribute 
to the definition of the invention. 
An inspection of FIG. 2 will show that in profile the elements 62 are 
rounded off on the upstream side of the belt until they meet the surface 
60. From then on, the profile moves progressively further away from the 
surface 60. 
Furthermore, the elements 62 are in the form of prisms based on the contour 
illustrated in FIG. 2. Opposite the belt, these elements may have a 
contour similar to that currently in use for gradient bars. 
In FIG. 3, the end of each element 162 on the side facing the belt is 
similar to that of the elements 62 of FIG. 2 but more rounded off. This is 
possible in this case because the distance between the elements 162 and 
the surface 160 is sufficiently large so that the distortions induced in 
the electric field in the gap 50 by the existence of these elements are 
less marked. 
FIG. 4 shows another embodiment of the invention, in which the structure, 
now indicated by the reference numeral 26, is homogeneous, at least on a 
macroscopic scale. 
This structure is composed of an epoxy resin 261 which is insulating but 
rendered partially conductive within its volume by a suitable charge in 
the epoxy resin, which is conventional. 
Care must be taken, however, to ensure that this conductive charge in the 
resin 261 does not produce too great a conductivity in the structure 26 in 
the direction of movement of the belt 5. 
Distributors 265-1 to 265-n are again provided in the structure 26 to 
supply tubules 266-1 to 266-n which open on to the surface 260 opposite 
the belt 5 to produce a cushion of gas in the space 50. In this case it is 
again preferable to arrange the tubules 266 at an angle to the direction 
of movement of the belt 5 to prevent difficulties caused by the presence 
of gas tubes extending the space 50 to the interior of the structure 26. 
The supply of gas may be arranged as illustrated in FIGS. 2A and 2B but 
without the metal sparking devices. One variation consists of supplying 
the tubes 265 with gas from a laterally placed tank put under a pressure 
of SF6. This variation may be applied to the preceding embodiments. 
The apparatus according to the invention may be used to supply an 
electrostatic generator to enable it to deliver high outputs or electric 
currents under very high voltages. 
The invention may be used for all industrial applications of electrostatic 
generators, both as such and as components of more complex systems such as 
electrostatic accelerators and more particularly for applications 
requiring high powers. 
The invention in particular enables the output of existing electrostatic 
accelerators to be increased. 
It also enables electrostatic generators to be constructed for various 
applications, such as the purification of water. 
It is to be understood that the invention is not limited to the embodiments 
described. Means equivalent to those mentioned above may be employed. In 
particular, the term "insulating flat belt" does not exclude the 
possibility of the belt containing conductive inclusions, provided the 
belt on the whole remains insulating along its length.