J. Joseph

Recent Test Results of the High Field

The 1 m long Nb3Sn dipole magnet HD2, fabricated and tested at Lawrence Berkeley National Laboratory, represents a step towards the development of block-type accelerator quality magnets operating in the range of 13-15 T. The magnet design features two coil modules composed of two layers wound around a titanium-alloy pole. The layer 1 pole includes a round cutout to provide room for a bore tube with a clear aperture of 36 mm. After a first series of tests where HD2 reached a maximum bore field of 13.8 T, corresponding to an estimated peak field on the conductor of 14.5 T, the magnet was disassembled and reloaded without the bore tube and with a clear aperture increased to 43 mm. We describe in this paper the magnet training observed in two consecutive tests after the removal of the bore tube, with a comparison of the quench performance with respect to the previous tests. An analysis of the voltage signals recorded before and after training quenches is then presented and discussed, and the results of coil visual inspections reported.

{\rm Nb}_{3}{\rm Sn}

In the past two years the US LARP program carried out five tests on a quadrupole magnet aimed at the high luminosity upgrade of Large Hadron Collider (HiLumi-LHC). The 1-meter long, 120 mm bore IR quadrupole magnet (HQ) with a short sample gradient of 219 T/m at 1.9 K and a conductor peak field of 15 T is part of the US LHC Accelerator Research Program (LARP). In a series of tests, carried out at 4.4 K, the magnet reached a maximum “short-sample” performance of 86%. The tests exposed several shortcomings that are now being addressed in a Research & Development program. This paper summarizes the magnet test results, reveals findings, R&D actions and future improvements.

Dipole Magnet HD2

In support of the luminosity upgrade of the Large Hadron Collider (LHC), the US LHC Accelerator Research Program (LARP) has been developing a 1-meter long, 120 mm bore Nb3Sn IR quadrupole magnet (HQ). With a short sample gradient of 219 T/m at 1.9 K and a conductor peak field of 15 T, the magnet will operate under higher forces and stored-energy levels than that of any previous LARP magnet models. In addition, HQ has been designed to incorporate accelerator quality features such as precise coil alignment and adequate cooling. The first 6 coils (out of the 8 fabricated so far) have been assembled and used in two separate tests-HQ01a and HQ01b. This paper presents design parameters, summary of the assemblies, the mechanical behavior as well as the performance of HQ01a and HQ01b.

Impact of Coil Compaction on

We report on the fabrication, assembly, and test of the Nb3Sn dipole magnet HD2. The magnet, aimed at demonstrating the application of Nb3Sn superconductor in high field accelerator-type dipoles, features a 36 mm clear bore surrounded by block-type coils with tilted ends. The coil design is optimized to minimize geometric harmonics in the aperture and the magnetic peak field on the conductor in the coil ends. The target bore field of 15 T at 4.3 K is consistent with critical current measurements of extracted strands. The coils are horizontally pre-stressed during assembly using an external aluminum shell pre-tensioned with water-pressurized bladders. Axial pre-loading of the coil ends is accomplished through two end plates and four aluminum tension rods. The strain in coil, shell, and rods is monitored with strain gauges during assembly, cool-down and magnet excitation, and compared with 3D finite element computations. Magnets training performance, quench locations, and ramp-rate dependence are then analysed and discussed.

{\hbox {Nb}}_{3}{\hbox {Sn}}

HQ01 is a superconducting quadrupole magnet under development by the LHC Accelerator Research Program (LARP) as a part of an R&D effort to demonstrate that Nb3Sn magnet technology is a viable option for a future luminosity upgrade of the LHC. The design is characterized by a 120 mm bore, a maximum gradient of 219 T/m at 1.9 K, and a support structure based on an aluminum shell pre-tensioned by water-pressurized bladders. The shell-based structure concept has already been successfully implemented in previous LARP quadrupole magnets. In HQ01, the structure incorporates additional features designed to provide full alignment between the support structure components and the coils. Specifically, the coil azimuthal alignment is achieved through outer layer pole keys which, by intercepting part of the force applied by bladders and shell, remain clamped to bolted aluminum collars from assembly to full excitation. A sequence of assemblies and cool-downs were executed with different keys sizes to characterize the alignment system and its impact on coil pre-load, at both room temperature and at 4.5 K. This paper reports on the mechanical behavior of the HQ01, by summarizing the strain gauge data and comparing them with FEM model predictions.

LARP HQ Magnet

YBa2Cu3O7-¿ (YBCO) tapes carry significant amount of current at fields beyond the limit of Nb-based conductors. This makes the YBCO tapes a possible conductor candidate for insert magnets to increase the bore field of Nb3Sn high-field dipoles. As an initial step of the YBCO insert technology development, two subscale racetrack coils were wound using Kapton-insulated commercial YBCO tapes. Both coils had two layers; one had 3 turns in each layer and the other 10 turns. The coils were supported by G10 side rails and waxed strips and not impregnated. The critical current of the coils was measured at 77 K and self-field. A 2D model considering the magnetic-field dependence of the critical current was used to estimate the expected critical current. The measured results show that both coils reached 80%-95% of the expected values, indicating the feasibility of the design concept and fabrication process.

Test Results of 15 T

The US LHC Accelerator Research Program (LARP) is developing Nb3Sn prototype quadrupoles for the LHC interaction region upgrades. Several magnets have been tested within this program and understanding of their behavior and performance is a primary goal. The instrumentation is consequently a key consideration, as is protection of the magnet during quenches. In all LARP magnets, the flexible circuits traces combine the instrumentation and the protection heaters. Their fabrication relies on printed circuit technology based on a laminate made of a 45-micron thick kapton sheet and a 25-micron thick foil of stainless steel. This paper reviews the protection heaters designs used in the TQ (Technology Quadrupole) and LR (Long Racetrack) series as well as the one used in LBNL HD2a high field dipole and presents the design of the traces for the Long Quadrupole (LQ), addressing challenges associated with the stored energy and the length of the magnet.

{\rm Nb}_{3}{\rm Sn}

This article describes the development of a direct-current (dc) superconducting transformer system for the high current test of superconducting cables. The transformer consists of a core-free 10,464 turn primary solenoid which is enclosed by a 6.5 turn secondary. The transformer is designed to deliver a 50 kA dc secondary current at a dc primary current of about 50 A. The secondary current is measured inductively using two toroidal-wound Rogowski coils. The Rogowski coil signal is digitally integrated, resulting in a voltage signal that is proportional to the secondary current. This voltage signal is used to control the secondary current using a feedback loop which automatically compensates for resistive losses in the splices to the superconducting cable samples that are connected to the secondary. The system has been commissioned up to 28 kA secondary current. The reproducibility in the secondary current measurement is better than 0.05% for the relevant current range up to 25 kA. The drift in the secondary current, which results from drift in the digital integrator, is estimated to be below 0.5 A/min. The systems performance is further demonstrated through a voltage-current measurement on a superconducting cable sample at 11 T background magnetic field. The superconducting transformer system enables fast, high resolution, economic, and safe tests of the critical current of superconducting cable samples.

Quadrupole Magnet HQ01 with a 120 mm Bore for the LHC Luminosity Upgrade

The US LHC Accelerator Research Program (LARP) is developing Nb3Sn prototype quadrupoles for the LHC interaction region upgrades. Several magnets have been tested within this program and understanding of their behavior and performance is a primary goal. The instrumentation is consequently a key consideration, as is protection of the magnet during quenches. In all LARP magnets, the flexible circuits traces combine the instrumentation and the protection heaters. Their fabrication relies on printed circuit technology based on a laminate made of a 45-micron thick kapton sheet and a 25-micron thick foil of stainless steel. This paper reviews the protection heaters designs used in the TQ (Technology Quadrupole) and LR (Long Racetrack) series as well as the one used in LBNL HD2a high field dipole and presents the design of the traces for the Long Quadrupole (LQ), addressing challenges associated with the stored energy and the length of the magnet.

Assembly and Test of HD2, a 36 mm Bore High Field

Development of high-field magnets for future accelerators brings new challenges and in particular the problem of reliable quench detection and localization. Traditionally, quench locations are determined by timing the propagation of the normal zone across the cable segment bounded by the neighboring voltage taps. However, applicability of this method is limited in high-field magnets due to a short time window allowed for quench propagation prior to firing the protection heaters. It becomes even more problematic for longer magnets, because a proportional increase in the number of voltage taps is required to see tap-to-tap propagation. Therefore, development of alternative quench localization techniques and improvement of the existing ones are needed. Here, we analyze three-dimensional magnetic field profiles due to a developing quench using the current redistribution model for Rutherford cable. We simulate transient field variations caused by the moving boundary of the normal zone and, as an example, attempt the model verification with the inductive quench antenna signals measured on the Nb3Sn quadrupole magnet, HQ01. Further steps on optimizing inductive quench antenna design will be discussed.