A.D. McInturff

Time variations of fields in superconducting magnets and their effects on accelerators

The time dependence of magnetic fields in the superconducting magnets of the Fermilab Tevatron is discussed. A field variation of order 1 G at the aperture radius is observed. Studies on both full-sized Tevatron dipoles and prototype magnets have been used to elucidate these effects. Explanations based on eddy currents in the coil matrix or on flux creep in the superconducting filaments are explored with these tests. Measurement results and techniques for controlling the effect based on novel laboratory tests and the latest accelerator operation are presented. It is shown that small but important changes in the fields occur with (ln t) time dependencies. For accelerator operation, in both laboratory and accelerator measurements, some events which might be associated with nonsmooth behavior (sudden changes on the relevant time scale) and with deviations at long times from observed (ln t) behavior have been seen. >

Cryogenic Instrumentation of an SSC Magnet Test Stand

This paper describes the system used to acquire cryogenic data for the testing of SSC magnets at the Fermilab Magnet Test Facility. An array of pressure transducers, resistance thermometers, vapor pressure thermometers, and signal conditioning circuits are used Readings with time resolution appropriate for quench recording are obtained with a waveform digitizer and steady-state measurements are obtained with higher accuracy using a digital voltmeter. The waveform digitizer is clocked at a 400 Hz sampling rate and these readings are stored in local ring buffers. The system is modular and can be expanded to add more channels. The software for the acquisition, control, logging, and display of cryogenic data consist of two programs which run as separate tasks. These programs (as well as a third program which acquires quench and magnetic data) communicate and pass data using shared global resources. The acquired data are available for analysis via a nationwide DECnetα network.

Model magnet studies

A design, construction, and testing program for model magnets is underway at Fermilab to evaluate alternatives in the development of small aperture magnets. The most thoroughly developed of these programs utilizes a set of model magnets based on the Tevatron quadrupoles and dipoles as a standard. These 7.6cm aperture model magnets, approximately 64cm long, have construction or material changes which are to improve quality and/or cost effectiveness. There are also superconductor parameter changes, as well as construction and materiai parameters, which impact or give insight into the choices of material specifications and construction parameters for the 5cm aperture model magnet series which are prototype candidates for the Superconducting Super Collider (SSC) ring magnets. The test program consists of performance test quenching of the windings at various temperatures until the cable critical current is reached. The quality of the magnetic field (Fourier coefficients) B(n) and quenching current is measured versus ramp rate, dB/dt, intensity, B, magnet temperature, and conductor parameters. The magnetization of the windings is measured as a function of these same parameters. There are also other special measurements made, i.e., deflection of coils. Two 5cm diameter aperture SSC candidate dipoles without iron are being prototyped: (a) a two layer, 6kA/turn version and (b) a single layer 10kA/turn model. In addition, there is a collaborative effort between KEK and FNAL in the development of a 10T (dipole, a longer range model) magnet program.

Magnetization Characterization Study for the Energy Saver/Doubler Dipoles

One of the challenging problems in superconducting accelerator dipoles is their dynamic behavior under ramping magnetic field produced loads. Their behavior can be observed with hysteretic measurement of the energy flow in the magnet. This technique, first described in detail by M.N.Wilson, will determine changes in inductance, deflection of coils, dynamic losses, the superconducting cables critical current vs. field and coupling curves, and iron shield saturation reproducibility. These measurements, when cross correlated with integral field, NMR and harmonic data yield a comprehensive characterization of the magnet. Data are presented for several magnets tested in the Magnet Test Facility of Fermilab. Measurements of inductance changes of a few percent and deflections of a few one-one thousandths of an inch are given. These data can now be routinely obtained on a production basis, therefore yielding a more complete characterization of the accelerator dipoles dynamic behavior.

High Gradient Superconducting Quadrupoles

Prototype superconducting quadrupoles with a 5 cm aperture and gradient of 16 kG/cm have been built and tested as candidate magnets for the final focus at SLC. The magnets are made from NbTi Tevatron style cable with 10 inner and 14 outer turns per quadrant. Quench performance and multipole data are presented. Design and data for a low current, high gradient quadrupole, similar in cross section but wound with a cable consisting of five insulated conductors are also discussed.

Experimental evaluation of design features of a cryostat for an iron-less COSθ- SSC magnet

A conceptual design for an iron-less cosθ SSC magnet cryostat has identified several areas for experimental study. Included are bowing of thermal radiation shields due to cooldown and warmup; thermal performance of the suspension systems; cryostat thermal performance; structural responses to decentering forces between the coil and the steel vacuum vessel; and response of thermal shields to forces due to quench induced eddy currents. Studies were carried out with 6m long thermal bowing and magnetic effects models, a suspension heat leak measurement dewar and a 12m long thermal model. The models incorporate important features of the conceptual cryostat design. The details of the test arrangements, procedures and results are presented.

Tests of High Gradient Superconducting Quadrupole Magnets for the Tevatron

Tests have been completed on three prototype magnets and two production magnets to be used for the Tevatron DO/BO low-β insertion. These cold iron, two shell quadrupoles are made of 36 strand Rutherford type NbTi superconducting cable. Magnet field gradients well in excess of the design 1.41 T/cm have been achieved at a transfer function of 0.291 T/cm/kA. Quench performance at 4.2 K and 3.7 K and magnetic multipole measurement data are presented and discussed.

Transverse quench propagation measurement

We have experimentally simulated the wedge regions of the winding of the Superconducting Super Collider dipole and studied the propagation of quenches in them. This study is relevant for proper selection and design of the quench protection scheme. The windings of these 16.6 m long dipoles incorporate copper wedges in order to achieve the required magnetic field uniformity, and the delay that they impose on the transverse spreading of normal zones is one of the needed data that we present here. Quenches under constant currents were triggered with spot heaters, and their development recorded from voltage taps strategically located. Currents as high as 6 kA were used. Under zero magnetic field conditions the delays are too long for self-protecting schemes.

High Field Accelerator Magnet Design (10T, 2T/cm), Material and Model Development

Two accelerator magnets have been designed and developed to a manufacturable prototype. Most of the dipole calculations presented will be based on a four shell, five kiloampere/turn, five centimeter aperature magnet, approximately six meters long. Two designs are considered: a dipole bending magnet and the quadrupole focusing magnet. The mechanical and electrical coil parameters are listed.

The magnetic properties of the SLC intersection region superconducting quadrupole triplets

The measured magnetic-field parameters of the quadrupoles comprising the final triplet lens system for the Stanford Linear Accelerator Center (SLAC) Linear Collider (SLC) intersection region are presented. The minimum design gradient specifications for these quadrupoles are 1.7 T/cm at 4.6 K and 1.6 T/cm at 4.6 K in a 0.6 T external solenoidal field. Superconducting quadrupoles of two lengths have been specified for the SLC triplets. The effective magnetic length of type Q/sub 1/ is 66.498+or-0.305 cm and Q/sub 2/ is 121.106+or-0.61 cm. The superconducting performance characteristics of the quadrupoles that have been measured are: maximum critical current as a function of bath temperature, as a function of rate of change of magnetic field, and as a percentage of the short sample. Short-sample performance is defined as the current reached by the cable in a perpendicular magnetic field equal to the peak field in the winding at bath temperature. The maximum gradient achieved during testing was 1.04 T/cm (4.25 K) and 2.07 T/cm (3.2 K). This represented 95% of the strand critical current value. The magnetic length of the first Q/sub 2/ was measured to be 120.85+or-0.1 cm. The Fourier harmonic coefficients of the magnetic field were measured as a function of current and are reported. >