Magnetic Field Distribution Generated by Screening Current Flowing in Coated Conductor Arranged Edge-By-Edge and/or Face-To-Back

To study the influence of coated-conductor magnetization on the field quality of accelerator magnets, we made a small dipole magnet consisting of four racetrack coils wound with GdBCO coated conductors and measured its magnetic field in liquid nitrogen by using rotating pick-up coils. We focused on the dipole and sextupole components (coefficients) of the magnetic field, which vary with time owing to the decay of the magnetization of the coated conductors. About 50 min (3055 s) after the current was ramped up to 50 A, the dipole coefficient normalized by the design value of the dipole component, i.e., the value calculated with the designed coil shape and the uniform current distribution in the coated conductors, increased by 7.4 × 10 −4 , and the sextupole coefficient normalized by the design value of the dipole component increased by 1.8 × 10 −4 . The magnitudes of the dipole and sextupole components depended on the excitation history of the magnet. Electromagnetic field analyses were carried out to calculate the current distributions in coated conductors, considering their superconducting properties; the dipole and sextupole coefficients were then determined from the calculated current distributions. Although the analyses were based on the two-dimensional approximation of the cross-section of the magnet, the temporal behaviours as well as the hysteretic characteristics of the calculated dipole and sextupole coefficients agree qualitatively with those of the dipole and sextupole coefficients measured in the magnet.

Section: Introductionmentioning

confidence: 99%

Temporal behaviour of multipole components of the magnetic field in a small dipole magnet wound with coated conductors

Amemiya

Otake

Sano

et al. 2015

“…SCFs, induced when an REBCO magnet is exposed to a timevarying magnetic field, is a major source of temporal field instability in the magnet [11][12][13][14][15][16][17][18][19][20][31][32][33][34]. Time-varying field distributions, resulting from SCFs, make it difficult to calculate accurate field gradients and implement proper arrays of ferromagnetic shims for REBCO magnets.…”

Section: Protocol To Mitigate Scfsmentioning

confidence: 99%

“…No-insulation REBCO magnets have in particular attracted interest because they remain undamaged subsequent to quenching as opposed to the insulated types, which are usually damaged [5][6][7][8][9][10]. REBCO conductors are only available in a tape form, and no-insulation REBCO magnets hence have very large screening currents all around the conductors [11][12][13][14][15][16][17][18][19][20][21]. Unfortunately, the screening currents become relaxed over the long term to cause a considerable drift of the magnetic field [22], and the currents are very difficult to predict and take into account while such magnets designed, a situation greatly detrimental to their field homogeneity as well [23].…”

Section: Introductionmentioning

confidence: 99%

Reproducibility of the field homogeneity of a metal-clad no-insulation all-REBCO magnet with a multi-layer ferromagnetic shim

Jang

Hwang

Han

et al. 2020

We report that the field homogeneity of a large-scale metal-clad no-insulation all-ReBa 2 Cu 3 O 7−x magnet is reproducible after occasional operating conditions, in this case the current ramp down/ up and magnet warm-up/cool-down conditions. First, we shimmed the magnet by cylindrically arraying ferromagnetic shims on the wall of the room-temperature bore of the magnet to improve the spatial field homogeneity. As a result, the field homogeneity of the magnet was improved from 557.7 to 14.1 ppm in a 10 mm diameter spherical volume. The resultant acquisition of field homogeneity tremendously enhanced from an inhomogeneous state greatly degraded by screening currents qualified us to study the temporal stability of the field homogeneity. We used a magnet-energizing protocol with the field drift minimized to shrink a nuclear magnetic resonance (NMR) lineshape into a much narrower super-imposable lineshape. We demonstrate the technique by NMR experiments, showing excellent reproducibility in every case, presenting strong evidence of recovery to certain time-unvarying field gradients and to a state of homogeneity. These findings suggest a new route for high-resolution high-temperature superconducting NMR and magnetic-resonance-imaging magnet technology.

“…However, the wide monolithic shapes of coated conductors would lead to significant magnetisation (screening or shielding current), which might cause serious deterioration of the field qualities of the magnets. Various authors have studied the influence of the magnetisation of high T c superconductor tapes on the field qualities of magnets [7][8][9][10][11][12][13][14][15]. Most of these previous works focus on axisymmetric coils whose applications are mainly related to nuclear magnetic resonance (NMR) [7][8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Magnetisation and field quality of a cosine-theta dipole magnet wound with coated conductors for rotating gantry for hadron cancer therapy

Amemiya

Sogabe

Sakashita

et al. 2015

Electromagnetic field analyses were carried out to study the influence of coated-conductor magnetisation, i.e. the screening (shielding) current, on the field quality of a dipole magnet in a rotating gantry for hadron cancer therapy. The analyses were made on the cross section of a cosine-theta dipole magnet in a rotating gantry for carbon ions, which generated 2.90 T of magnetic field. The temporal profile (temporal variation) of the magnet current was determined based on the actual excitation schemes of the magnets in the rotating gantry. The experimentally determined superconducting property of a coated conductor was considered, and we calculated the temporal evolutions of the current-density distributions in all the turns of coated conductors in the magnet. From the obtained current-density distributions, we calculated the multipole components of the magnetic field and evaluated the field quality of the magnet. The deviation in the dipole component from its designed value was up to approximately 25 mT, which was approximately 1% of the designed maximum dipole component. Its variation between repeated excitations was approximately 0.03%, and it drifted approximately 0.06% in 10 s. Some compensation schemes might be required to counteract such influence of magnetisation on the dipole component. Meanwhile, the higher multipole components were small, stable, and sufficiently reproducible for a magnet in rotating gantries, i.e. |b3| ∼ 1.1 × 10−3 and |Δb3| ∼ 0.2 × 10−3 in 10 s.