Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets
  • H01F7/06—Electromagnets; Actuators including electromagnets
  • H01F7/20—Electromagnets; Actuators including electromagnets without armatures
  • H01F7/202—Electromagnets for high magnetic field strength

Definitions

• the invention due to the collaboration of the National Service of the Intensive Fields of the CNRS (Director M. AUBERT) generally relates to a solenoid magnet with high homogeneity of magnetic field, consisting of several coils spaced from each other; more particularly, it relates to a connection structure between the coils, making it possible to avoid the creation of components of stray fields.
• NMR imaging installations require a large magnet capable of generating a uniform magnetic field in a determined region of space. Typically, it is necessary to generate a magnetic field of 0.15 to 0.5 teslas with a homogeneity of 1 to 10 parts per million (ppm) in a sphere of 0 cm in diameter at least.
• the invention relates to a solenoid magnet with high magnetic field homogeneity comprising several coils of the same internal and external diameter spaced along a common longitudinal axis, characterized in that said coils are connected in series by first conductors willing parallel to said axis and extending respectively in the spaces between said coils, in that second conductors are arranged longitudinally in the vicinity of said first conductors and in that these second conductors are connected to third conductors shaped and / or arranged lengthwise of each coil to distribute a current flow substantially uniformly over a cylindrical surface coaxial with said axis, said second and third conductors being interconnected and connected as a whole in series with said coils to ensure the return of current to an axial end of said magnet .
• the coils constituting the magnet described above are preferably Bitter coils.
• Bitter coils are well known for the production of strong magnetic fields.
• the structure proposed by Bitter is a winding made up of metallic annular discs (generally copper or aluminum), split to form as many turns and connected to define a substantially hecoidal winding with flat turns.
• the stack of discs is maintained by a plurality of tie rods.
• This structure is advantageous because it allows efficient cooling of the magnet, by making holes in the discs (and in the insulators separating these discs), these holes being arranged in the same configuration from one disc to another to materialize a set of channels parallel to the axis of the coil, in which circulates a cooling fluid, for example deionized water, erozene or oil.
• a cooling fluid for example deionized water, erozene or oil.
• the invention preferably applies to a magnet constructed from such Bitter coils insofar as, in particular, at least some of the aforementioned tie rods can be used to produce the aforementioned third conductors, distributing the return current over a surface substantially cylindrical and coaxial with the coils, in the sections of the magnet occupied by the latter.
• the ends of the tie rods can be electrically connected by placing at each axial end of each coil, an open ring, that is to say comprising a radial slot through which the connection point between said first corresponding conductor and said coil is established.
• the crown is of course isolated from the coil and a pair of aforementioned second conductors is arranged on either side of said first conductor, each second conductor being connected to one end of said crown.
• FIG. 2 is a section II-II of Figure 1.
• a solenoid magnet li Bitter annular disks known per se ', consisting of a plurality of coils aligned along of the same main axis of symmetry z'z. More specifically, the two coils 14 and 15 are shown closest to an axial end of the magnet opposite to that where the means for connection to a DC power source are arranged, not shown.
• the term “Bitter coil” is understood to mean any coil meeting the definition recalled above.
• the radially split discs constituting the turns are connected, for example welded, end to end and held in a tight stack by means of a plurality of tie rods 18 regularly distributed over a cylindrical surface of axis z'z . All the coils are connected in series.
• the tie rods 18 are specific to each reel; they do not extend into the spaces defined therebetween, they are of course electrically isolated from the Bitter discs which they maintain.
• the tie rods or some of them are also used to bring the current back to the current source, that is to say towards the axial end of the magnet which is not shown.
• a current distribution plate 19 (not split) is located at the outer end of the coil 15 and welded to the last turn thereof to ensure a substantially uniform distribution of the flow of return current between the tie rods, which are in electrical contact with said distribution plate.
• the tie rods of each coil are connected to each other at the two ends of this coil so as to define a sort of squirrel cage. The interconnection of the tie rods will be described later.
• the tie rods are traversed by substantially equal fractions of the return current. They thus compensate for the small axial component of current due to the pitch of the helix of the winding defined by the Bitter discs, in the corresponding coil.
• tie rods of the Bitter coils it is not essential to use the tie rods of the Bitter coils to compensate for the axial component of the current flowing in this coil.
• the coils are connected in series by first conductors such as 20 extending respectively in the spaces between neighboring coils while second conductors 21, participating in the return of the current, are arranged longitudinally in the vicinity of said first conductors, to surround them at least partially.
• first conductors such as 20 extending respectively in the spaces between neighboring coils while second conductors 21, participating in the return of the current, are arranged longitudinally in the vicinity of said first conductors, to surround them at least partially.
• each conductor 20, 21 has a rectangular section and two conductors 21, isolated from the conductor 20, are joined along two parallel faces of the latter. The entire current flowing through the coils therefore flows in each conductor 20 while the reverse current flowing in the opposite direction is shared more or less equally in the two conductors 21.
• the connection structure between two coils which has just been described does not create stray field in the space between the two coils considered.
• third conductors interconnected between the second conductors these third conductors being nothing other than, in the example described, the groups of tie rods 18 associated with the different coils or at least part of these tie rods regularly spaced from each other.
• the second conductors 21 and the tie rods 18 are connected in series as a whole with the coils themselves in series, by means of the distribution plate 19.
• the connections between said second conductors and the groups of tie rods are provided by necks.
• ⁇ open rings 22 arranged at the axial ends of the coils and with which the tie rods 18 on the one hand and the ends of the conductors 21 on the other hand are interconnected.
• each ring isolated from the coil at the end of which it is fixed using tie rods, comprises a slot 23 in which is located the connection point between the first conductor 20 and the end of the corresponding coil while the ends 24 of the crown materialized by the slot 23, are connected to the ends of the two corresponding conductors 21.
• the tie rods of each coil pass through the corresponding crowns 22 (a crown and the plate 1 with regard to the coil 15) and are tightened by means of nuts between said crowns, which ensures correct electrical contact between these crowns and these tie rods.
• each coil has an odd number of half-turns so that, on either side of the same coil, the arrangements of conductors 20, 21 are opposite with respect to the z'z axis.
• the tie rods 18 are more or less "distant" from the connection points of the conductors 21 to the ring 22 and this distance depends on the position of the tie rod considered relative to the slot 23. This could result in differences in intensity between the currents flowing in the tie rods, due to the more or less long portion of crown 22 placed in series with each of them. This imbalance is avoided if the coil has an integer number of half-turns because, in this case, the tie rods most "distant" from the conductors 21 at an axial end of a coil are the closest to the conductors 21 homologous to the another axial end of this same coil.
• the resistances distributed between the tie rods by the presence of the crowns are substantially equal so that the return current is shared substantially regularly between the tie rods in each coil and is divided substantially into two equal fractions in the conductors 21 in each space between two neighboring coils.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)
• Electromagnets (AREA)

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• 1985
  • 1985-05-10 FR FR8507151A patent/FR2581760B1/fr not_active Expired
• 1986
  • 1986-04-22 DE DE8686902431T patent/DE3669009D1/de not_active Expired - Lifetime
  • 1986-04-22 EP EP86902431A patent/EP0221920B1/de not_active Expired - Lifetime
  • 1986-04-22 WO PCT/FR1986/000136 patent/WO1986006869A1/fr not_active Ceased
  • 1986-04-22 US US07/014,048 patent/US4743879A/en not_active Expired - Fee Related

Non-Patent Citations (1)

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