Electric lamp and discharge devices – With positive or negative ion acceleration
Reexamination Certificate
2000-06-23
2002-02-19
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
C315S502000, C315S505000
Reexamination Certificate
active
06348757
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the production of structures associating a superconducting material with a mechanical reinforcement material, possessing good thermal characteristics. An example of such a structure is that of sheets, or narrow tubes, of niobium associated with a rigidification layer, for example in copper or in tungsten.
Such structures offer applications in the field of particle accelerators.
PRIOR ART
FIG. 1A
represents an accelerating structure of an electron accelerator. Such a structure takes the form of successive cavity cells
2
-
1
, . . . ,
2
-
9
. The particles are accelerated here by a radio-frequency wave generated by a klystron. Length L is 1039 mm for a frequency of 1.3 GHz.
FIG. 1B
represents a structure for accelerating protons. Cells
2
-
10
, . . . ,
2
-
13
in niobium are immersed in a bath
3
of liquid helium. Such a structure has a diameter D of 1.1 m for a frequency of 700 MHz.
The shape and the dimensions of these cavities are optimised according to a large number of parameters linked to the RF performance, dark currents, turbulence field etc. Resolution of the Maxwell equations associated with the conditions at the limits on the walls makes it possible to define space and time values for the electric and magnetic fields in such a structure.
These electric and magnetic fields contribute to the accelerating effects on the particles of the beam, but also to secondary effects, in particular the heating of materials and structures, dark currents etc.
In particular, a current j induced in the cavity walls leads to a loss of high frequency power.
At present, there is a distinction between two types of accelerators: the so-called “hot” accelerators made with copper cavities, and the “superconductor” accelerators using cavities of a superconducting material such as niobium, which is cooled below its critical temperature T
C
to make it superconducting. The critical temperature T
C
for niobium is 9.3 K, which implies cooling the structure in a bath of liquid helium (at atmospheric pressure helium is liquid at 4 K).
In the first case, for hot accelerators, a large part of the electrical power provided by the network serves to heat the cooling water in the copper structures. In the case of a superconductor accelerator, the greater part of the electrical power serves to accelerate the particle beam, which helps explain all the interest of superconductivity in terms of electrical consumption. However, a small part (but not zero) of the HF energy is dissipated in a fine layer of superconducting material called the LONDON layer. The typical thickness of the LONDON layer is about 100 nm and does not depend on the frequency, as is the case for currents induced in a normal conductor. This dissipated energy makes it possible to explain the variations in the characteristic curve of a superconductor cavity Q(E
acc
), or quality factor, depending on the accelerating field. Since the dissipated energy rises with the accelerating field, the characteristic curve follows a descending slope depending on the field.
The BCS theory on superconductivity worked out by Bardeen, Cooper and Schrieffer in 1957 makes it possible to predict the corresponding resistance, (the so-called BCS resistance) and the losses from the Joule effect, which depend on the frequency and temperature.
To this resistance R
SCS,
one should add a residual resistance linked to the defects of structures, interstitial atom impurities, included gases etc.
Thus, in a structure of superconducting cavities, one can consider that the thickness of the superconducting material (niobium, for example), constituting the superconducting cavity, plays several roles:
1—The role of superconducting layer as an internal skin, on the vacuum side of the cavity.
2—The role of heat sink for the rest of the thickness, allowing the calories generated by the BCS and residual resistances in the LONDON layer to flow towards the helium bath.
3—The role of mechanical structure making it possible to conserve the internal shape fixing the conditions at the limits to the electromagnetic field which is created in the structure of the tube and in the accelerating cavities.
The optimisation mentioned above makes it possible to reach compromises allowing in general the optimisation of the RF (or HF) shapes and specifications but leaves open the questions of mechanical stability and the thermal properties of such structures.
When an accelerating machine is used, the mechanical and geometric conditions must remain stable in order to maintain the structure of the cavities tuned to the klystron frequency. Various causes may disturb this operating stability: Lorentz forces creating pressure in the cells and tending to deform them, mechanical vibrations induced from outside, and in particular induced by variations in the pressure of the liquid helium bath etc.
In transient states, when the HF wave is introduced into the cavities, the structure is submitted to Lorentz forces which tend to deform and detune it. To avoid this, rigid structures need to be produced.
An example of such a rigidified structure is illustrated in
FIG. 2A. A
corrugated tube
4
is produced with a high thickness of superconducting material, generally niobium, which is expensive. In fact, the tube is an assembly of elementary parts
5
,
7
,
9
,
11
assembled by welds
6
,
8
,
10
.
The utilisation of thick niobium means using a large quantity of very expensive material (between 1,500 FF and 5,000 FF per kilo), a procedure which can scarcely be envisaged for machines operating with a high number of cavities.
Another known structure, represented schematically in
FIG. 2B
, consists of making cavity parts in material
12
of average thickness, of welding them together with welding seams
16
and strengthening them with a ring
14
at the level of the iris (regions or zones of lower diameter). Consequently, in order to overcome the effect of the Lorentz forces, the utilisation of niobium of lower thickness necessitates welding stiffeners
14
, complicating the production process and making it difficult to control the dimensions because of shrinking after welding. In addition, such a technique makes it difficult to obtain reproducible dimensions.
Another technique (
FIG. 2C
) consists of depositing a thin layer
22
of niobium on a substrate
20
in thick copper. This process makes it possible to solve the problem of rigidifying the structure. However, the structure obtained has limits in terms of the accelerator field which can be reached. In fact, for reasons linked to the structure of the superconducting layer of niobium deposited by “sputtering” on the copper substrate, the maximum electric fields likely to be reached remain of the order of 10 MV/m. For other machines, and in particular colliders e
+
-e
−
, this field is clearly insufficient.
Apart from the mechanical problems, there are also thermal problems. Thermal conditions must be maintained so that the internal skin of the niobium remains below the critical temperature T
C
and below the critical field H
C
. In order that the thermal conditions are maintained and the niobium remains superconducting in the LONDON thickness, it is necessary that if there is a hot spot on the internal surface, on the vacuum side of the accelerator, the calories can be evacuated rapidly towards the helium bath.
There are several reasons which can lead to the creation of a hot spot:
1)—HF losses through the Joule effect, due to the uniformly distributed global and homogeneous surface resistance described by the BCS theory and by the residual resistance. A more complete theory introducing non-quadratic losses, presented by W. Weingarten (“Progress in thin film techniques”, CERN-European Laboratory for Particle Physics, Geneva, Switzerland, 7th Workshop on RF Superconductivity, Paris, 1995), shows that it is possible to express super-losses by the following formula, giving the superconducting resistance (measured in n&OHgr;/mT):
R
s
(
B
ρ
,
ω
,
T
,
B
ext
)
=
R
0
Centre National de la Recherche Scientifique
Clove Thelma Sheree
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Nimeshkumar D.
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