Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
1999-07-27
2001-02-27
Shingleton, Michael B (Department: 2817)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C118S7230MR, C118S7230MA
Reexamination Certificate
active
06194836
ABSTRACT:
DESCRIPTION
Technical Field
The invention relates to the field of magnetic devices to create a magnetic field, particularly for an application for an ECR (electronic cyclotronic resonance) source. This type of source is used to produce ions.
Document FR-2 475 798 describes a process and a device for production of highly charged ions. The device comprises a hyperfrequency cavity excited by at least one high frequency electromagnetic field. This field is associated with a magnetic field, the amplitude of which is chosen such that the electronic cyclotronic frequency associated with the said magnetic field is equal to the frequency of the electromagnetic field set up in the cavity. The condition is:
B=f.2&pgr;m/e (1)
where m is the mass of the electron, e is the electron charge and f is the frequency of the electromagnetic field.
In this device, the magnetic field is created by superposing:
a multi-pole radial magnetic field with a minimum amplitude in the central part of the cavity,
and an axial magnetic field with a symmetry of revolution, with a gradient along the said axis, the total resultant magnetic field being adjusted such that there is at least one completely closed magnetic layer in the cavity, without any contact with the walls of the cavity. The electronic cyclotronic resonance condition is satisfied on this layer, so that the gas passing through it will be ionized.
The device described in this document will now be summarized with reference to FIG.
1
. Two sources (not shown) are used to send ionizable gas through pipes
2
,
4
that lead into a confinement chamber
6
in which a vacuum pump creates a high vacuum. An electromagnetic field is introduced through wave guides
8
,
10
.
Pairs of axial coils
14
,
16
are used to produce the axial magnetic field. The multi-pole radial magnetic field with zero amplitude at the center of the cavity is created using bars
18
placed parallel to each other. The resultant magnetic field superposed on the HF electromagnetic field enables electronic cyclotronic resonance.
Document EP-138 642 also describes a magnetic confinement structure (
FIG. 2
) in which the solenoids
20
supply an axial field Ba that is superposed on a radial field Br generated by permanent magnets
22
mounted on the inside wall of a cylindrical casing
24
. The assembly of the two solenoids is shielded by a ferromagnetic casing
26
. This type of device magnetizes a useful volume
28
.
The device illustrated in
FIG. 3
diagrammatically shows a target-source assembly called “Nanomafira”. This assembly is described in the paper by P. Sortais et al. entitled “Developments of compact permanent magnets ECRIS”, 12
th
International Workshop on ECR Ion Sources, Apr. 25-27, 1995, Riken, Japan. A set of magnets
32
,
34
,
36
is placed around a plasma confinement zone
38
in order to define an axial field. A multi-pole structure
37
defines a radial field that is superposed on the axial field.
The fact that a certain volume called the confinement volume is magnetized is common to all these devices. Experience shows that the use of high volume plasma chambers can result in increased performances. Making a high volume plasma chamber involves magnetizing a large volume, and therefore involves the use of significant magnetic resources and the use of a large electric power, or the use of a large number of magnets.
Furthermore, the UHF frequency that has to be used to produce monocharged ions, or ions with low charged states, is low. Consequently, the magnetic field to be used is also low (relation (1)). However, the UHF frequency must be high to produce multicharged ions. This means that a magnetic field with a high modulus has to be used in the confinement volume.
For example, all that can be produced using a Nanomafira source (described above) with a 26 mm diameter and 90 mm long confined volume, is 55 &mgr;A of Ar
8+
ions and 3 &mgr;A of Ar
11+
ions.
Another disadvantage of conventional ECR sources is that they have a clearly defined magnetic structure, and consequently a very narrow frequency usage range around a given central value. Thus, these sources can operate around 2.5 GHz, or 6.5 GHz or 10 GHz, or 14 GHz, or 18 GHz. But they are not compatible with use in wide band.
Another disadvantage of known ECR sources is that they cannot be installed at the center of a cyclotron.
FIG. 4
diagrammatically shows a sectional view of two plates
48
,
50
in a cyclotron, between which a magnetic field B is setup. Reference
52
denotes an ECR ion source and reference
56
diagrammatically shows an injection line of ions produced by source
52
, and means of adapting the beam in order to inject it at the cyclotron input. The particle beam
58
inside the cyclotron is deflected by electrostatic means
54
. The beam can then be accelerated in the hyperfrequency cavities. A conventional ECR source
52
cannot be built into the inside the cyclotron, due to its environment and its radial multi-pole magnetic components.
Another disadvantage of conventional ECR sources is the need to extract the ion beam along the axis of symmetry of the source, and only along this axis.
A device with lateral extraction has to be made if it is required to couple an ECR source in a cyclotron.
Furthermore, a source with lateral extraction may have an advantage for many applications, not only for applications to a cyclotron. In particular, an ion source with lateral extraction, either partial or even over 360°, would be very useful for the ion implantation technique. At the present time sources used for this technique have an extraction hole with a diameter of about 10 mm, which can only give a single beam. Furthermore, the principle of extraction through a hole on the axis and the radial components of the field created by the multi-pole system, cause non-uniformities in the beam extracted from the source.
DISCLOSURE OF THE INVENTION
The purpose of the invention is a device for generating a magnetic field B comprising a multi-pole structure for which the elements have polarities such that the vector sum of the fields created by each of these elements at each point in space delimited by the said elements is sufficient to define at least one continuous and closed line of minima inside a surface with constant modulus closed in the said space.
In the invention, the closed surface with modulus B
f
surrounds an internal volume in which in particular the magnetic field may have a very low minimum B, unlike what takes place in known ECR sources.
With this type of source, the maximum operating frequency is defined by the closed surface with constant modulus for which the field is a maximum and equal to |B
f
|, obtained inside the multi-pole structure. Therefore, the same source can operate at low frequency f
0
without modification if a closed line of minima |B
0
| is compatible with this frequency, in other words satisfies the relation B
0
=f
0
2&pgr;m/e. Therefore, the device according to the invention is compatible with operation in wide band.
Furthermore, this type of device can easily be modulated and its volume may be adjusted; the increase in volume does not require the use of much larger magnetic means. In the device according to the invention, there is no need to create a large magnetic field far from elements in the multi-pole structure; the magnetic field can decrease quickly when moving away from these elements.
Therefore the invention relates particularly to a device capable of generating a magnetic field in a cavity in which an HF electromagnetic wave is injected using appropriate means, the modulus of the magnetic field being such that the electronic cyclotronic frequency associated with this magnetic field is equal to the frequency of the electromagnetic field, on a closed magnetic layer, in order to cause ionization of the gas passing through it and to create a plasma, particularly to produce ECR type ion sources.
In order to increase the volume of the plasma chamber for an ion source without excessive magnetic means
Leroy Renan
Pacquet Jean-Yves
Burns Doane Swecker & Mathis L.L.P.
Commissariat A l'Energie Atomique
Shingleton Michael B
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