Electrostatic accelerator and free electron beam laser using the accelerator

Electrostatic accelerator includes an accelerating column (20), a high voltage terminal (18) located at one end of said accelerating column and electric charge transport means, said transport means incorporating a high frequency accelerator such as a high frequency electron accelerator (54) able to supply an electron beam and means (56) for supplying the electron beam to the high voltage terminal, said electric charges being constituted by the electrons supplied by said high frequency accelerator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electrostatic accelerator incorporating 
an accelerating column, a high voltage terminal at one end of said 
accelerating column and electric charge transport means. 
2. Discussion of the Background 
Such electrostatic accelerators of the Van de Graaf type are already known 
and have been used for accelerating ions in various nuclear physics 
research projects. Electrostatic accelerators also have other 
applications, particularly in the field of free electron lasers. 
Electrostatic accelerators have numerous advantages in the latter field. 
They supply pulses, whose durations are very long compared with those of 
the pulses supplied by pulse-type or high frequency accelerators. The 
spectral width of the line emitted by a free electron laser associated 
with an electrostatic accelerator is very small. The quality of the 
electron beam accelerated by an electrostatic accelerator is excellent and 
well adapted to the requirements of free electron lasers. Such an 
accelerator makes it possible to recover with high efficiency the energy 
of the electron beam, following the passage of said beam through the 
cavity of said free electron laser associated with said accelerator. Due 
to the possibility of recovering the energy of the electron beam, the 
overall efficiency of the free electron laser is very high. 
However, a Van de Graaf-type electrostatic accelerator still suffers from a 
disadvantage. The electric charge transport means incorporated in such an 
accelerator are in the form of a belt or some other mechanical transport 
device, such as e.g. a Pelletron or Laddertron. Therefore, in such an 
accelerator, the value of the charging current (current corresponding to 
said electric charges) is low. 
SUMMARY OF THE INVENTION 
The object of the present invention is to obviate this disadvantage and 
improve the performance characteristics of a Van de Graaf-type 
electrostatic accelerator by increasing its charging current to well 
beyond the threshold permitted by the known transport means such as e.g. 
belts or Pelletrons. 
To do this, the present invention uses a beam of electric charges (such as 
electrons or negative ions or positive ions) in place of a mechanical 
charge transport means. 
Specifically, the electrostatic accelerator according to the invention 
comprising an accelerating column, a high voltage terminal located at one 
end of said accelerating column and means for the transport of the 
electric charges is characterized in that the transport means incorporate 
a high frequency accelerator able to supply a beam of electric charges in 
order to form the charging current of the electrostatic accelerator. 
According to a preferred embodiment of the invention, the high frequency 
accelerator is a high frequency electron accelerator able to supply an 
electron beam and the electric charge transport means also incorporate 
means for supplying the electron beam to the high voltage terminal in 
which said electrons accumulate, said electric charges being constituted 
by the electrons supplied by said high frequency accelerator. 
By using such electric charge transport means, the present invention makes 
it possible to multiply by at least ten the threshold of the charging 
current permitted by known electrostatic accelerators. 
An electrostatic accelerator according to the invention is able to produce 
an accelerated electron beam (main beam), whose current is very high, e.g. 
approximately 1 to 20 A, by recycling said electron beam and storing 
charges in the capacitance of the high voltage terminal of said 
accelerator. 
When using same with a free electron laser, recycling the main electron 
beam is provided a very high efficiency exceeding 90%, following the 
passage of said beam into the laser, allowing the latter to operate with a 
high intensity main electron beam (1 to 20 A), whilst maintaining the 
charging current at a very low value of approximately 0.5 to a few mA in 
an exemplified manner. 
As it is possible to compensate part of the losses of the main electron 
beam by a low voltage electric generator placed in the high voltage 
terminal of the electrostatic accelerator, it is sufficient for the 
charging current of the latter to compensate the electrons lost in the 
magnetic wiggler of the free electron laser and in the structures in which 
the main electron beam is propagated. Therefore the ratio of the intensity 
of the charging current to the intensity of the main electron beam 
supplied by the electrostatic accelerator can have a very low value of 
approximately 2.times.10.sup.-4 to 3.times.10.sup.-4. 
A high frequency accelerator having conventional performance 
characteristics is able to supply a charging current of this type to an 
electrostatic accelerator, whose performance characteristics are far 
higher then those of the high frequency accelerator. 
According to a special embodiment of the electrostatic accelerator 
according to the invention, the high frequency accelerator thereof is 
constituted by a structure having a cavity formed by an external 
cylindrical conductor and an internal cylindrical conductor, which are 
coaxial to one another, a high frequency source supplying the cavity with 
an electromagnetic field at a resonant frequency of the cavity, the radial 
component of the field having a maximum in at least one plane 
perpendicular to the axis common to the external conductor and the 
internal conductor, said external and internal conductors of the cavity 
having diametrically opposite openings located in the plane for the 
introduction of the electron beam into the cavity and its extraction in 
said plane, whereby said high frequency accelerator also comprises at 
least one electron deflector able to deflect the electron beam having 
traversed the cavity along a diameter, whilst keeping it in the plane and 
it is then reinjected into the cavity along another diameter. 
A high frequency accelerator with this structure is called the Rhodotron 
(registered trademark). 
Such an accelerator is described in documents (1) to (3) which, like the 
other documents cited hereinafter, are referred to at the end of the 
present description. 
In an electrostatic accelerator according to the invention, the electron 
beam supply means can comprise a column for decelerating the electrons 
from the high frequency accelerator. 
The electrostatic accelerator according to the invention can accelerate 
electrons and also have means for recovering electrons which it has 
accelerated and which have then been used, said recovery means 
incorporating a column for decelerating the electrons which have been 
used. 
In this case, according to an advantageous embodiment of said electrostatic 
accelerator it is possible to simplify the structure of the latter and 
therefore reduce its cost. The three columns, namely the accelerating 
column, the decelerating column of the supply means and the decelerating 
column of the recovery means have the same structure. The electrostatic 
accelerator comprises a single tube in which the three columns are grouped 
and which serves to accelerate electrons to be supplied by said 
electrostatic accelerator for the deceleration of electrons from the high 
frequency accelerator and for the deceleration of electrons which have 
been used. 
The electrostatic accelerator according to the invention is in particular 
used for producing a free electron laser. 
The present invention also relates to a free electron laser incorporating 
an electron electrostatic accelerator able to supply an electron beam and 
a magnetic wiggler which is traversed by said electron beam, said free 
electron laser being characterized in that the electrostatic accelerator 
is that forming the object of the present invention and which incorporates 
a high frequency electron accelerator. 
Finally, when the electrostatic accelerator according to the invention 
comprises the aforementioned recovery means, the latter can recover the 
electrons which have traversed the magnetic wiggler, the decelerating 
column of the recovery means being linked with said magnetic wiggler.

The free electron laser system diagrammatically shown in FIG. 1 and which 
has a known Van de Graaf-type electrostatic accelerator is installed at 
the University of California - Santa Barbara (UCSB). 
FIG. 2 is a diagrammatic view of said known electrostatic accelerator. 
With regards to electrostatic accelerators for free electron lasers and in 
particular with regards to the free electron laser system installed at the 
University of California - Santa Barbara, reference should be made to 
documents (4) to (7). 
The free electron laser system diagrammatically shown in FIG. 1 comprises a 
Van de Graaf-type electrostatic accelerator 2, a magnetic wiggler 4 
forming part of the free electron laser, mirrors 6 and 8, which are 
parallel to one another and on either side of the wiggler 4 and which form 
a resonant cavity, a duct 10 in which is formed a vacuum and in which is 
propagated the electron beam produced by the electrostatic accelerator 2, 
part of said duct 10 emanating from said accelerator 2 and extends to one 
side of the wiggler, whilst the other part of said duct 10 emanates from 
the other side of the wiggler and returns to the electrostatic accelerator 
2, as well as various transport and matching means placed along the duct 
10. Certain of the said means 12 are provided for transporting the 
electron beam in the duct 10 and for matching said beam to the 
wiggler-resonator system of the free electron laser. The remainder 14 of 
said means transports up to the accelerator 2 the electron beam from the 
wiggler 4 and also serve to match said beam to the decelerating tube of 
the electrostatic accelerator 2. 
Thus, the electron beam produced by the electrostatic accelerator 2 
traverses the wiggler 4, where it is able to produce a coherent light beam 
and then returns to the electrostatic accelerator 2. The latter is placed 
in a sealed enclosure 16 (cf. FIG. 2) filled with gaseous SF.sub.6. 
This electrostatic accelerator 2 comprises a high voltage terminal 18 
raised to a potential of -3 MV, an accelerating tube 20 or accelerating 
column placed at the end of the high voltage terminal and which 
accelerates the electron beam produced by the accelerator 2 and which is 
called the main beam and a decelerating tube 22 or decelerating column, 
which recovers the main beam after the passage of the latter into the 
wiggler 4 and which decelerates said main electron beam. This decelerating 
tube 22 is also placed outside the high voltage terminal. 
The electrostatic accelerator also comprises within the high voltage 
terminal 18 an electron gun 24 and means for the electric power supply of 
the latter which are not shown, said electron gun being ale to produce a 
50 keV electron beam, said electrons then being accelerated in the 
accelerating tube 20, an electron collector 26, together with not shown 
means for polarizing the electrodes of said collector 26 to appropriate 
voltages of respectively -40, -43.3, -46.7 and -50 kV, said electron 
accelerator collecting the electrons of the main beam when the latter has 
passed through the decelerating tube and a 10 kW electric generator 28 for 
supplying the various equipments within the high voltage terminal 18. 
The electrostatic accelerator 2 also comprises a rotary shaft 30 made from 
an electrically insulating material and which is able to withstand a 3 MV 
potential difference between said ends, said shaft mechanically driving 
the generator 28 raised to the high voltage of 3 MV and a Pelletron chain 
32 supplying to the high voltage terminal 18 a current of approximately 
500 microamperes, so as to compensate the electron losses of the main beam 
in the loop traversed by the latter between the accelerating column 20 and 
the decelerating column 22, together with a motor 34 driving said 
Pelletron chain 32. 
It is pointed out that the diagram of FIG. 2 is extracted from document 
(4). 
As has been stated, it is known to use a belt for transporting electric 
charges from the earth of ground potential to the high voltage in a Van de 
Graaf-type electrostatic accelerator. Reference should be made in this 
connection to document (8). 
In order to transport the electric charges, such a belt can be replaced by 
a Pelletron chain, as is the case in the system used by the University of 
California, Santa Barbara. 
Other Pelletron chains are known, e.g. that of the National Electrostatic 
Corporation/U.S.A. (N.E.C.) and in which connection reference can be made 
to document (9) and that used in quantities in the tandem accelerator of 
the University of Yale (cf. document (10)). 
A mechanical device for transporting electric charges comparable to the 
Pelletron and which is called the Laddertron is also known. The latter is 
used in the Daresbury tandem accelerator (cf. document (11)). 
The circuit diagram of a Van de Graaf-type electrostatic accelerator with 
electron beam recovery of the type used in the free electron laser system 
installed at the University of California - Santa Barbara is shown in FIG. 
3, wherein the high voltage terminal 18 is raised to an electric potential 
U compared with earth or ground. The potential U is e.g. -3 MV, as stated 
hereinbefore. 
The high voltage terminal 18 contains the electron gun 24, the beam 
collector 26, the power generator 28 (10 kW generator in the embodiment 
shown in FIG. 2) together with a first generator 36 and a second generator 
38 for adjusting the polarization voltages of the electrodes of the 
electron gun 24. 
In the diagram of FIG. 3, it is also possible to see outside the high 
voltage terminal 18, the accelerating tube 20, the decelerating tube 22, a 
charging generator 40 which, in the case of FIG. 2, is constituted by the 
Pelletron chain 32, a leakage circuit 42 corresponding to a discharge by 
the corona effect, as well as the polarization resistors of the 
accelerating tube 20 and the decelerating tube 22 and the wiggler 4 of the 
free electron laser using the electrostatic accelerator. 
The electron gun 24 comprises an electron emitting cathode 44 and an 
electrode 46 for accelerating the electrons emitted by the cathode 44. 
The generator 28 keeps constant the electrical potential of each of the 
collecting plates of the collector 26 compared with the potential of the 
electron gun cathode 44. 
FIG. 3 also shows an electrically conductive plate 48, which closes the 
high voltage terminal 18 and which is raised to the potential U. The 
electrons emitted by the cathode 44 are accelerated by the electrode 46 
and then pass through an outlet hole 50 of the plate 48. The electrons 
then successively traverse the accelerating tube 20, the wiggler 4 and the 
decelerating tubes 22. The electrons then traverse an intake hole 52 of 
the plate 48 and are recovered by the collector 26. 
As can be seen in FIG. 3, the branch of the circuit diagram on which is 
located the charge generator 40 is grounded at one side and at potential U 
on the other. The intensity of the current supplied by the charging 
generator 40 is designated Ich. The intensity of the current of the 
electron beam from the accelerator shown in FIG. 3 is designated I and the 
intensity of the recycling current corresponding to the electron beam 
recovered by said accelerator is designated Ir. The leakage circuit 42 is 
grounded on one side, whilst the other is at potential U. The intensity of 
the leakage current corresponding thereto is designated If. 
The generators 28, 36 and 38 are driven by a rotary shaft which is not 
shown in FIG. 3, but which is visible in FIG. 2 (reference 30). The 
generators 36 and 38 belong to the same branch of the circuit of FIG. 3 
and the generator 38 is connected on one side to the generator 36 and on 
the other side is raised to the same potential as the cathode 44 of the 
electron gun 24. The accelerating electrode 46 is connected to the 
terminal common to the generators 36 and 38. Therefore the generator 36 is 
connected on one side to the generator 38 and to the electrode 46 and on 
the other side is raised to the potential of the plate 48 (potential U). 
It is possible to define the recovery rate or level n of the electrostatic 
accelerator of FIG. 3 as the ratio Ir/I. 
In the system installed at the University of California - Santa Barbara, 
said recovery rate is approximately 0.95 to 0.97, when the free electron 
laser operates. 
An increase in the intensity Ich of the charging current of the 
electrostatic accelerator makes it possible to reduce the time interval 
between two pulses of the electron gun 24. Such an increase also makes it 
possible to improve the spectral stability of the light pulses emitted by 
the free electron laser associated with the electrostatic accelerator 
shown in FIG. 3. 
In a known, Van de Graaf-type electrostatic accelerator like that shown in 
FIG. 3, the intensity i1 of the current transported by the mechanical 
charge transport device 40 from earth to the high voltage terminal can be 
expressed by the following formula: 
EQU i1=s.times.V 
in which s represents the line density of the electric charges and V the 
speed of said mechanical device (speed of the belt, Pelletron chain or 
Laddertron chain). 
In order to increase the intensity of this charging current, it will be 
necessary to increase the line density of the charges and therefore the 
width of the charge support (width of the belt or chain plates) and/or 
increase the electric charge translation speed V. All these increases 
would lead to technological difficulties (vibrations, transients, wear, 
increase in size) and to high costs. 
According to the present invention, the mechanical charge transport device 
is replaced by an electric charge beam, preferably an electron beam, which 
comes from a high frequency accelerator. In this case, the intensity i2 of 
the current of the electric charges can be expressed by the following 
formula: 
EQU i2=e.N.S.v 
in which e represents the electrical charge of the electron (in absolute 
values), N represents the electronic density of the electron beam from the 
high frequency accelerator, S represents the cross-section of said beam 
and v represents the electron velocity in said beam. 
It can be considered that the velocity of the electrons of the beam is 
approximately 10.sup.7 times higher than the speed V of the mechanical 
charge transport device. 
Bearing in mind the considerable ratio between said velocities V and v, the 
present invention makes it possible to obtain much higher charging 
currents than those possible with the mechanical charge transport devices 
of the known, Van de Graaf-type electrostatic accelerators. 
In the present invention, for injecting the charging current, it is 
possible to use the high frequency accelerator known as the Rhodotron and 
to which reference was made hereinbefore. 
The electrostatic accelerator according to the invention and which is shown 
in FIG. 4 is identical to that shown in FIG. 3, except that the charging 
generator 40 (e.g. Pelletron chain) is replaced by an assembly 
incorporating a Rhodotron 54, a supplementary decelerating tube or column 
56, a supplementary electron collector 58 and a supplementary generator 
60. 
In the electrostatic accelerator of FIG. 4, the generators 28, 36 and 38 
are still driven by a not shown, electrically insulating rotary shaft, 
which is itself rotated by a not shown motor. It is also possible to use 
any other drive mechanism not sensitive to the high voltage. 
These generators are remotely voltage-controlled by a device not sensitive 
to the high voltage, e.g. an infrared control. The generator 28 is 
preferably controlled by the electromagnetic power emitted by the laser. 
The high frequency accelerator 54 makes it possible to supply to the high 
voltage terminal 18 an electron beam having an appropriate intensity and 
energy. 
The decelerating column 56, which is similar to the decelerating column 22, 
is located outside the high frequency terminal 18 and decelerates the 
electron beam from the high frequency accelerator 54. The thus decelerated 
electron beam enters the high voltage terminal by an opening 62 in the 
conductive plate 48. 
The supplementary charge collector 58 is located in the high voltage 
terminal 18 and is similar to the collector 26. The collector 58 collects 
the decelerated electron beam, which has penetrated the high voltage 
terminal 18 by the opening 62. 
The supplementary generator 60 keeps constant the electrical potential of 
each of the collecting plates of the collector 58 compared with the 
potential of the cathode 44 of the electron gun 24. This generator 60 can 
be voltage regulated by means of a remote control and is preferably made 
to follow the energy variations of the beam emitted by the high frequency 
accelerator 54. 
A not shown tube tightly connects the high frequency accelerator 54 to the 
supplementary decelerating column 56 and the latter is tightly connected 
to the supplementary collector 58, so as to ensure that the high frequency 
accelerator 54, supplementary decelerating column 56 and collector 58 form 
the same tight enclosure. 
A not shown pumping system is provided for forming the vacuum in said 
enclosure (pressure of approximately 10.sup.-4 to 10.sup.-5 Pa). Obviously 
the main electron beam from the electrostatic accelerator of FIG. 4 and 
which returns there after traversing the wiggler 4, propagates into 
another tight enclosure passing from the electron gun 24 to the collector 
26, whilst successively traversing the accelerating column 20, the wiggler 
4 and the decelerating column 22. 
Not shown pumping means are provided on said other tight enclosure in which 
travels the main electron beam in order to form a vacuum therein. 
Appropriate, not shown polarization means are provided for respectively 
polarizing the different plates of each of the columns 20, 22 and 56. 
An embodiment of the Rhodotron 54 usable in the accelerator of FIG. 4 is 
diagrammatically shown in FIG. 5. It comprises a high frequency source 
SHF, an electron source K, a coaxial cavity CC and two deflectors D1 and 
D2. The coaxial cavity CC is formed by an external cylindrical conductor 
64 and an internal cylindrical conductor 66. The electron source K emits 
an electron beam Fe contained in a plane perpendicular to the axis of the 
coaxial cavity CC. Said plane encounters said axis at a point O, FIG. 5 
being a cross-sectional view along said plane. Said beam penetrates the 
cavity CC by an opening 68 and traverses the cavity CC along a first 
diameter d1 of the external conductor 64. 
The internal conductor 66 has two diametrically opposite openings 70, 72 
and which are successively traversed by the beam. The electron beam is 
accelerated by the electric field if the phase and frequency conditions 
are satisfied, i.e. said electric field must have the opposite sense to 
the velocity of the electrons. 
The accelerated beam passes out of the coaxial cavity CC through an opening 
74 diametrically opposite to the opening 68. It is then deflected by the 
electron deflector D1. The beam is reintroduced into the cavity CC by an 
opening 76. It then follows a second diameter d2 and undergoes a second 
acceleration in the coaxial cavity CC. It passes out through an opening 78 
diametrically opposite to the opening 76. 
On leaving, the beam is again deflected by the deflector D2 and 
reintroduced into the coaxial cavity CC by an opening 80. It then follows 
a third diameter d3 and undergoes a third acceleration, passing out of the 
coaxial cavity CC by an opening 82 diametrically opposite to the opening 
80. 
Thus, the Rhodotron can be designed in such a way that the electron beam 
which it accelerates reenters and leaves the coaxial cavity CC a larger 
number of times. This is the case with the Rhodotron 54, which is 
diagrammatically and partly shown in FIG. 4, where the trajectory of the 
electrons is in the form of a rosette (hence the name of this high 
frequency accelerator). 
FIG. 4 shows that a buncher G can optionally be placed on the trajectory of 
the beam supplied by the electron source K before said beam penetrates the 
accelerator 54 for the first time. It is possible to use a tube which is 
commercially available from N.E.C. (National Electrostatic Corporation - 
U.S.A.) for producing each of the columns 20, 22 and 56. 
This tube is diagrammatically shown in FIG. 6 and which comprises internal 
annular electrodes 84 electrically insulated from one another by tubular, 
electrically insulating segments 86, the latter being tightly connected to 
one another. The thus obtained tube has flanges 90 and 92 at its two ends. 
Another known tube, which can also be used for producing each of the 
columns 20, 22 and 56, is diagrammatically and partly shown in FIG. 7. 
This tube known as the inclined field tube comprises series of spaced, 
equipotential, conductive rings 94, whose inclinations alternate and which 
are interconnected by electrically insulating rings 96, the whole forming 
a tight assembly. 
An embodiment of the collector 58 (which is of the same type as the 
collector 26) is diagrammatically shown in FIG. 8. This collector 58 has a 
succession of collecting plates or electrodes 98 separated from one 
another by electrically insulating rings 110. An electrically insulating 
tubular element 102 separates the collector 58 from the corresponding 
decelerating tube 56. 
FIG. 8 also shows a grid 104 for repelling the secondary electrons produced 
in the collector 58 of FIG. 8. It is also possible to see a focussing coil 
106 fitted at the outlet of the decelerating tube 56 and which focusses 
the electron beam on leaving said tube 56 before the beam penetrates the 
collector 58. 
The different plates 98 are raised to appropriate potentials so as to 
create an electric retarding field Ef, whose direction passes from the 
first plate (located on the side of the decelerating tube 56) to the last 
plate (which is the furthest from the decelerating tube 56). 
It is pointed out that such a collector structure is known and the 
polarization of each of the electrodes 98 is chosen so as to bring about 
an optimum recovery of the energy of all the electrons of the beam from 
said decelerating tube 58. 
The plates 98 are cooled by not shown means, e.g. a circulation of SF.sub.6 
in a thermally conductive tube welded around each of the plates 98. 
Reference has already been made to the similarity of the columns 20, 22 and 
56, which are respectively provided for accelerating the main beam, 
decelerating the main beam (following the use of the latter and prior to 
the collection of the used beam) and the deceleration of the charging 
beam. 
In a special embodiment of the invention and which is less costly than that 
shown in FIG. 4, these three columns 20, 22 and 56 are combined into a 
single column for accelerating the main beam, decelerating the main beam 
and decelerating the charging beam. This single column is diagrammatically 
and partly shown in FIG. 9, wherein it carries the reference 108. 
Column 108 comprises electrically conductive, parallel plates 110 tightly 
connected to one another by tubular, electrically insulating elements 112. 
In the embodiment of FIG. 9, the column 108 is a tight assembly of sections 
109, each section 109 incorporating an element 112 and, on either side of 
the latter, two half-plates 110a and 110b, each plate 110 being the tight 
assembly of two adjacent half-plates, as can be seen in FIG. 9. The plates 
110 and the tubular elements 112 have the same axis constituting the axis 
of the column 108. Each plate 110 has four openings 114, 116, 118 and 120, 
all of the openings being in the volume defined by the tubular elements 
112. The openings 114 are placed in the center of the plates 110 and all 
have the same axis as the column 108. These openings 114 are pumping 
orifices making it possible to form the vacuum in the column 108. 
On each plate 110, the openings 116, 118 and 120 are at 120.degree. for one 
another around the opening 114 of said plate 110. Moreover, the openings 
116 of he plates 110 are coaxial and are provided for the passage of the 
accelerated main beam. The openings 118 of the plates 110 are coaxial and 
are provided for the passage of the decelerated main beam. The openings 
120 of the plates 110 are coaxial and provided for the passage of the 
decelerated charging beam. 
Not shown means are provided for raising the plates or electrodes 110 to 
potentials appropriate for said accelerations and for said decelerations. 
These potentials are distributed in know manner, e.g. by means of not 
shown, strong electrical resistors. 
Obviously, the openings 120 respectively located at the two ends of the 
column 108 are respectively connected to the collector 58 and to the not 
shown duct supplying the electrons from the high frequency accelerator 54. 
The openings 118 respectively located at the two ends of the column 108 are 
tightly connected respectively to the collectors 26 and to the duct 
supplying the electrons which have traversed the wiggler 4. 
The openings 116 formed on the plates 110 located at the two ends of the 
column 108 are tightly connected respectively tot he electron gun 24 and 
to the not shown duct bringing the accelerated electrons to the wiggler 4. 
Obviously, it is then possible to use a pumping system common to the three 
aforementioned beams, said pumping being possible by means of the orifices 
114. 
A gaseous SF.sub.6 atmosphere is provided outside the column 108 (FIG. 9), 
or outside the columns 20, 22 and 56 (FIG. 4) and also in the high voltage 
terminal 18. 
In the preceding description, the electrostatic accelerator according to 
the invention has been described in its preferred application to free 
electron lasers. However, the invention is not limited to this 
application. 
In particular, the electrostatic accelerator according to the invention can 
be used for: 
accelerating strong positive ion currents from a positive ion source at 
earth potential to a target placed in the high voltage terminal (nuclear 
physics experiments), 
accelerating strong negative ion currents from a negative ion source placed 
in the high voltage terminal to a target raised to earth potential 
(application to the heating of plasmas in controlled fusion experiments), 
accelerating strong ion currents using the configuration of tendem 
accelerators, the high voltage terminal then containing the device for 
converting the negative ions into positive ions, which is the 
characteristic of tandem accelerators. 
If Er is the energy of the electron beam emitted by the high frequency 
accelerator (e.g. a Rhodotron) and if the ions to be accelerated are 
charged n times, the final energy Ei to which said ions can be accelerated 
is such that: 
EQU Ei=n.times.Er (1) 
Typically Er is between 1 and 20 MeV and n can vary from 1 to values higher 
than 10 as a function of the particular case. 
The ion beam to be accelerated can be represented by a pulse sequence of 
duration t and period T. If Ir represents the man current of the electron 
beam supplied by the Rhodotron, the peak current Ii of the ion pulses 
under continuous operating conditions is: Ii=Ir/33 T/t. Typically Ir is 
approximately 10 to 20 mA. 
An accelerator according to the invention using a high frequency electron 
accelerator has the following advantages for accelerating ions. It is able 
to accelerate intense ion beams under long pulse operating conditions or 
even continuous conditions. It makes it possible to obtain high ion 
energies, as is shown by the relation (1). It makes it possible to 
compensate voltage variations during ion current pulses. It permits the 
emission of high power ion beams. 
FIGS. 10 to 13 diagrammatically show examples of accelerators according to 
the invention for accelerating ions. The configuration of these 
accelerators is dependent on the polarity of the ions to be accelerated 
(positive or negative ions). 
In FIG. 10, use is made of a positive ion source S1 which, like the 
Rhodotron 54, is earthed or grounded. The accelerating column of the 
electron beam and the accelerating column of the ion beam respectively 
carry the references 56 and 20. The electron beam collector 58 and the ion 
beam receiving target C1 are located in the high voltage terminal 18, 
which is brought to a negative potential. 
FIG. 11 uses a negative ion source S2. The Rhodotron 54 and the ion beam 
receiving target C2 are grounded. The electron beam collector 58 and the 
ion source S2 are located in the high voltage terminal 18, which is 
brought to a negative potential. 
In the preceding examples, a grounded high frequency accelerator has been 
used. In the examples illustrated in FIGS 12 and 13 use is made of a high 
frequency accelerator, e.g. a Rhodotron, which is raised to a high voltage 
and which is thus in the high voltage terminal, whereas the electron beam 
collector is located outside said terminal. 
Use is made of a positive ion source S1 in FIG. 12 and said source together 
with the Rhodotron 54 are located in the high voltage terminal 18, which 
is raised to a positive potential. The electron beam collector 58 and the 
ion beam receiving target C1 are grounded. 
Use is made of a negative ion source S2 in FIG. 13. The Rhodotron 54 and 
the ion beam receiving target 62 are located in a high voltage terminal 18 
raised to a positive potential. The electron beam collector 58 and the ion 
source S2 are grounded. 
In all the hitherto described embodiments use is made of a high frequency 
electron accelerator, e.g. a Rhodotron. However, in other embodiments of 
the invention, it would be possible to use a high frequency positive or 
negative ion accelerator for supplying the charging current.