The Advanced Hydrotest Facility, under study by LANL, utilizes large-bore superconducting quadrupole magnets. In the paper we discuss the conceptual design of such quadrupoles using active shielding. The magnets are specified to achieve gradients of up to 24T/m with a 28-cm warm bore and to have 0.01% field quality. Concepts for the magnet cryosystems and quench protection are also briefly discussed to confirm the reliability of the proposed design

Jason, A. J. (Andrew J.)

Kashikhin, V. S. (Vladimir S.)

Kashikhin, V. V. (Vadim V.)

Kelley, J. P. (John Patrick)

Tompkins, J. (John)

Walstrom, P. L. (Peter L.)

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LA-UR-03- 3om Approved forpublic release; distribution is unlimited. \ cm Title Author@), Submitted to. CONCEPTUAL DESIGN OF LARGE-BORE SUPERCONDUCTING QUADRUPOLES WITH ACTIVE MAGNETIC SHIELDING FOR THE AHF Andrew J. Jason, LANSCE-1 J. Patrick Kelley, LANSCE-1 Peter L. Walstrom, LANSCE-1 Vladimir S. Kashikhin, FNAL Giorgio Ambrosio, FNAL Nikolai Andreeev, FNAL Srinivas Bhashyam, FNAL Vadim V. Kashikhin, FNAL J - 5- Thomas Peterson, FNAL John Tompkins, FNAL > W O  = 0“- o s f l  g z z d  !rn% z a g  5-0 S=% 2 E s m  *-a 3-m  S E S  Alexander V. Zlobin, FNAL VI- 2003 Particle Accelerator Conference (PAC2003) Portland, Oregon May 12-1 6,2003 Alamos ,.”’ N A T I O N A L  L A B O R A T O R Y  Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the US. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher’s right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (6/00) CONCEPTUAL DESIGN OF LARGE-BORE SUPERCONDUCTING QUADRUPQLES WITH ACTIVE MAGNETIC SHIELDING FOR THE AHF V.S. Kashikhin, G, Ambrosio, N. Andreev, S. Bhashyam, V.V. Kashikhin, T, Peterson, J. Tompkins, A.V. Zlobin, FNAL, Batavia, IL 60510, USA A. Jason, P. Kelley, P, Walstrom, LANL, Los Alamos, USA Abstract The Advanced Hydrotest Facility, under study by LANL, utilizes large-bore superconducting quadrupole magnets. In the paper we discuss the conceptual design of such quadrupoles using active shielding. The magnets are specified to achieve gradients of up to 24T/m with a 28- cm warm bore and to have 0.01% field quality. Concepts for the magnet cryosystems and quench protection are also briefly discussed to confirm the reliability of the proposed design. INTRODUCTION The LANL Advanced Hydrotest Facility (AHF) [ l ]  utilizes large-bore superconducting (SC) quadrupole magnets to image protons for radiography of fast events [2]. Since twelve imaging lines converge on the object to be radiographed [3], size limitations are an important consideration as is magnetic coupling between lines. The pre-conceptual magnetic analysis of warm, cold iron core magnets and active shield version was described in [4]. In this paper we investigate an active shielding version, which is very promising, approach for this application. The quadrupoles have two concentric windings connected in series and configured so that the outer winding fully eliminates the outer fringing magnetic field. This design also eliminates problems connected with a warm or cold ferromagnetic core. The active shielding eliminates fringing fields and Lorentz forces between adjacent quadrupoles, reduces magnet weight and cold mass. Proton beam lines consist of several large-bore quadrupoles which can be connected in series using common current leads and LHe forced flow or each doublet of quadrupoles has separate current leads and pool boiling cryosystem. The first approach widely used in accelerators, the second one in MRI, SC spectrometers, etc. The brief discussion of both options with preliminary quench analysis is presented in this paper. QUADRUPOLE DESIGN Specification and Parameters In order to meet the imaging system requirements for AHF, the focusing quadrupole magnets should satisfy the criteria presented in Table 1 [5]. In addition, due to the limited space, the magnets should generate a minimum fringe field outside to avoid interaction between magnets in the doublets and adjacent strings. . $  Table 1, Magnet parameters Design Concept The quadrupole design is based on the active shielding concept, where the return flux from the main winding is suppressed using another winding carrying an opposite current. The cross-sections of the small and large aperture quadrupoles based on this concept are shown in Fig. 1 and Fig.2. This approach allows keeping the main part of magnetic flux within the cold mass preventing interaction between the cold mass and iron screen, inherent to the warn1 yoke concept. It also allows avoiding the iron saturations effects leading to the field distortions. A simple estimation shows that limiting of the magnet current within reasonable boundaries of 15-20 kA leads to several hundred of turns in the windings for the given apertures and gradients. A traditional shell type coil with that number of turns would suffer from the stress accumulation in the midplane and large random field harmonics coming from the uncertainties of individual cable position within the shells. Thus it was imperative to split the shells into a number of mechanically decoupled blocks, providing the stress management and individual support for each block. In order to accomplish this task, winding into the support structure approach was chosen. The winding mandrel is a cylinder with rectangular slots machined in longitudinal direction, For an easier stacking and prestressing inside the slots, the cable is to be wound in the "hard bend" way with the long edge tangential to the mandrel. In addition, to simplify the manufacturing, all slots in the mandrel should be oriented radially. After the coil is wound and cured, the mandrel serves as the mechanical support structure for the coil. fYEW C Q W " * " l  3.30 gmt< - "- _ _ _  __ -"-"F!- - - j__ -_ --+ Fig. 1 Small-bore quadrupole field quality and flux lines laEd4 Cornmnam tGlP L - 2 . - 2 7  _-=-c;.- I Fig. 2 Large-bore quadrupole field quality and flux lines Apart from the traditional shell type magnet, the cable width in the proposed design concept does not drive the shell thickness and therefore the maximum gradient. It offers an opportunity of tuning the operating parameters by simply changing the number of cables in the blocks and neither the cable dimensions nor the number of layers. However, the cable width does drive the number of blocks and thus cost of the support structure, which should obviously be minimized. From this viewpoint, the maximum cable width acceptable for the cable winding in the hard-bend direction should be chosen. Simple bending experiments demonstrated that the Rutherford type cable with 28 strands, 1 mm in diameter can be hard-bent around -50 mm round mandrel without the stability loss. Given that the cable mechanical stability will be additionally enhanced by the support from the mechanical structure during the coil winding, the number of strands was fixed at 32. Sbperconducting Cable Both magnets are based on the same 32-strand cable with either NbTi or Nb3Sn strands. The strand and cable parameters are summarized in Table 2. The higher critical current density and lower Cu:nonCu ratio in NbsSn strands allow replacing significant amount of SC strands in Nb3Sn cable with pure Cu strands preserving the magnet maximum field gradient. Table 2: Cable parameters Field Quality Field quality in an air core magnet with large number of conductor blocks can be rather easily optimized to values of two orders of magnitude better than specified field harmonics In Table 3 presented results of both quadrupoles optimization shown in Figs. 1-2. So, the manufacturing accuracy will define the final field quality. Table 3: Field harmonics b,, lo4 n Small-bore Large-bore -0.0002 6 -0.0012 -0.0001 10 0.0005 -0.0001 14 -0.0035 18 0,0005 0.0002 The random field harmonics were calculated as standard deviations among a large number of cases in assumption of 4-50  pm random block displacements from the nominal position. The results show that the small bore magnet with this tolerance on the block positions will meet the field quality requirements for individual harmonics with the probability of 68%. However, due to the larger number of blocks, the large bore magnet with the same positioning tolerances will meet the field quality requirements with 99% probability or one can relax the tolerances to 4-140 pm to get the same probability. Quench Margin The maximum field gradient was calculated for the quadrupoles made of NbTi and Nb3Sn conductors. Conductor properties are shown in Table 2. Both large and small quadrupoles based on the NbTi conductor achieve the maximum operating gradients with 1520% critical current margin. Using Nb3Sn cable in these magnets allows enhancement of the operating gradients by a factor of 1.5 with the same IC margin. The radiation losses in magnet coils will produce an extra heat load of 0.3-1.0 mJ/g, Fig. 3 shows the quench limit for NbTi and Nb3Sn coils vs the magnet critical current margin. To provide magnet operation at 1.0 mJ/g heat deposition the critical current margin for above designs in case of NbTi coil has to be more than 25% and in case of Nb3Sn coil only 10%. -+- Wq-Nb3Sn + WtNbTi  7 E 5.0 0.65 0.70 0.7.5 0.80 0.85 0.90 0.95 1.00’ lnllq Fig. 3 Quench energy vs. coil critical current margin for NbTi and Nb3Sn quadrupoles. Quench Protection Magnets will be protected with an active quench protection system based on the internal quench heaters as it is used in modern SC accelerators. Analysis shows that at quench detection time of -50 ms and 50% coil volume quenched by the heaters, the coil maximum temperature does not exceed 300 K and maximum voltage between the coil and ground during a quench is less than 100 V. Cryosystem Tom Peterson’s proposa1:Pat Kelley should briefly, describe the cryogenic system concept here, with reference to published papers for more details. I suggest emphasizing the difSicult geometry-the lack of a nice, linear magnet arrangement--as one of the unique difsiculties for this cryosystem design. Somewhere in this section, wherever it Jits, he would like to udd something like the following sentences: One potential problem with pool-boiling combined with large heat loads is the large volumetric flow rate required for removing vapor generated in tight spaces, particularly with the flow driven by density gradients (natural circulation). Especially with spaces of less than 1 mm, which are not vertical, heat fluxes that can be removed by boiling and natural circulation start to drop off. Our extensive experience with magnets cooled by pool-boiling helium in vertical test dewars may not be relevant, since heat loads there are typically minimal and local heat fluxes are very low. In spite of the geometric difficulties presented by the radial magnet arrangement, forced flow cooling, which has been used successfully in long accelerator magnet strings such as Tevatron, HERA, and RHIC, might have to be developed for the AHF system as an integral part of the magnet design. SUMMARY The conceptual design of the AHF SC quadrupoles confirmed the feasibility of design based on the active shielding approach. The results, from this analysis, are: 0 magnetic design is in an agreement with the specified field quality ; 0 quadrupoles windings can be made from NbTi or Nb3Sn superconductor; 0 proposed mechanical structure with stress management; 0 quench protection system is visible; 0 cryosystem is briefly discussed. REFERENCES [l] P.W. Lisowski and J.A. Paisner, “An Advanced Hydrotest Facility for the Stockpile Stewardship Program”, Proc, of the Fourth International Meeting on Nuclear Applications of Accelerator Technology”, November 12-15, 19??, Washington D.C., p. 13. [2] G.E. Hogan et al., “Proton Radigraphy”, Proc. of the 1999 Particle Accelerator Conference, 1999, p.579. [3] C.T. Mottershead, J.D. Zumbro, “Magnetic Optics for Proton Radiography”, Proc. of the 1997 Particle Accelerator Conference, Vancouver, 1997, p. 1397. [4] N. Andreev et al., “Magnetic Design of Large-Bore Superconducting Quadrupoles for the AHF”, ASC 2002, [5 ]  P.W. Walstrom, “Magnetic Field Quality Specifications for the AHF Magnetic-Lens Superconducting Quadrupoles”, Lansee Technical Note, LANSE-1:Ol-063, August 23,2001. [6] P.Kelley, ,.... 

Conceptual design of large-bore superconducting quadrupoles with active magnetic shielding for the AHF

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Conceptual design of large-bore superconducting quadrupoles with active magnetic shielding for the AHF

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