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Glossary

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AC field

A magnetic field that varies in time, as opposed to a DC field.

Accelerator

Particle accelerators were developed by physicists to study the elementary nature of matter. Nowadays, they are usually immense, highly international installations, with a permanent operational staff "servicing" a user community of scientists. Smaller accelerators are also used commercially, for example in mass spectrometry, proton therapy and semiconductor ion implantation. Even an ordinary CRT can be considered a particle accelerator!

Accuracy

Accuracy is how close a measure conforms to reality. Even if our NMR Teslameter displays 9 digits, we claim 5 ppm (7 digit) accuracy. In common speech, accuracy is often confused with precision – in fact, we may do it ourselves 😉

Aperture

In magnetism, this refers to the spacing between the poles.

B0

In MRI, B0 refers to the strong primary field used to polarize the protons in the body.

Bending magnet

In accelerators, a bending magnet is a dipole magnet used to redirect the particle beam.

B/H

Ferromagnetic materials are characterized by measuring their hysteresis loop, or B/H loop. H is the applied field, and B is the field in the material.

Bore

The centre of a solenoidal magnet, where the magnetic field is concentrated.

Bucking coil

Bucking coils are usually used in rotating-coil systems for characterizing accelerator magnets. They subtract out part of the response of the main coil, permitting a direct and precise measure of a field gradient.

Calibration

During calibration, the instrument reading is compared to one or more references to verify its accuracy. The references themselves need to be calibrated against other references, leading to a chain of reference instruments that should eventually lead back to a national standard.

Calibration laboratory

A laboratory – either commercial or governmental – that calibrates the instruments used in industry. Magnetometry is so specialized that most calibration labs send magnetometers back to the manufacturer for calibration.

Component

The magnetic field is a three-dimensional vector quantity, with three components. Magnetometers may measure 1, 2 or 3 components, or the vector magnitude.

Continuous-wave

In NMR magnetometry, continuous-wave is one approach to finding the NMR resonance frequency, using a continuous excitation procedure. The other common approach is pulsed mode.

Control

An NMR-controlled field is one whose strength is monitored by NMR. As opposed to NMR-regulated.

Cyclotron

A compact particle accelerator that allows the particle beam to spiral out as it gains energy.

DC field

A magnetic field that does not vary in time, as opposed to an AC field.

Decay

For superconducting magnets, decay refers to the infinitesimally small and slow loss of current and corresponding decrease in magnetic field. The decay rate is more rapid right after a magnet – especially a new magnet – is first ramped up.

Dipole magnet

A magnet with two poles, a north and a south, separated by a gap. As opposed to a multipole or solenoid magnet.

Drift

The gradual loss of an instrument's accuracy. NMR teslameters drift because their time base drifts; this can be easily checked and limited to very low values. Fluxmeters drift during a single measurement because they integrate a constant offset voltage; this is often called baseline drift. Hall magnetometers can drift for many reasons; short-term drift is often caused by temperature fluctuations, and long-term drift by aging of the semiconductor.

Electromagnet

A magnet driven by an electrical current, as opposed to a permanent magnet.

Encoder

In this context, refers to a device that measures linear or angular position. Usually used with moving- or rotating-coil systems, to drive the acquisition of the integrator output.

EPR

Electron Paramagnetic Resonance – same as ESR.

ESR

Electron Spin Resonance. Effect similar to Nuclear Magnetic Resonance, except we're manipulating the spin of the electron rather than the nucleus.

Fixture

See jig.

Flip-coil

A time-honoured method of measuring a magnetic field in a given location, using a fluxmeter. The idea is to flip a coil so it makes a half turn; the field strength can be calculated from the height of the voltage curve.

Flowing liquid / Flowing water

An elegant NMR magnetometry technique that is capable of measuring a very broad range of magnetic fields, including low fields not otherwise measurable with reasonably-sized samples. The basic idea is to prepolarize a liquid sample (like water) in a magnet, pump the sample to the location to be measured, wait for the spins to align, and pump it back to analyze the results.

Flux

The magnetic flux density, B, integrated over an area. The voltage induced in a coil is proportional to the flux change.

Fluxmeter

An instrument that measures flux changes by integrating the voltage induced on a coil.

Focusing magnet

In accelerators, a magnet used to focus the particle beam. Often a quadrupole or higher multipole.

Fringe field

The field around a magnet, not in the gap or bore.

Gap

The area between the two poles of a dipole magnet, where the magnetic field is concentrated. Also used to refer to the aperture.

Gauss

The cgs unit of magnetic flux density. Officially declared to be outdated, it is still popular for low fields. Equal to 10-4 Tesla.

Gaussmeter

See magnetometer.

Gradient

In this context, gradient refers to the spatial variation of the magnetic field.

Gyromagnetic ratio

In NMR or ESR, the ratio of the resonant frequency to magnetic flux density. Varies from one nucleus to the other; for hydrogen (protons) it is about 42.5 MHz/T. For the electron, it is 28 GHz/T.

Hall effect

A popular magnetometer technology, using a semiconductor plate. One injects current on one axis of the plate and measures a voltage on the other axis. The maximum response is with the field perpendicular to the plate.

Homogeneous

A magnetic field with no or only a very small gradient. NMR teslameters can only be used in fields that are relatively homogeneous. MRI magnets are shimmed to make them as homogeneous as possible.

Hysteresis loop

The characteristic of ferromagnetic materials that they remain magnetized when the external field is removed. See also B/H loop.

Insertion device

See Wiggler.

Integrator

The part of a fluxmeter that integrates the voltage

Jig

In our context, refers to a mechanical structure that allows a probe to be placed in known locations in the field.

Larmor frequency

The frequency at which an NMR sample resonates. Depends linearly on the applied external field.

Magnet model

A computational simulation of the field generated by a magnet.

Magnetic circuit

A way of thinking about magnetic systems, similar to an electric circuit. Allows predicting, for example, the flux density in the various parts of a permanent magnet system.

Magnetometer

An instrument to measure magnetic flux density (B) or magnetic field intensity (H).

Magnitude

The magnetic field is a vector quantity. We may be interested in its individual components or its total magnitude.

Mapping

The process of measuring magnetic field intensity at many different points, in order to understand the structure of the field within a volume. For example, in MRI one typically maps the field by measuring many points on a sphere containing the imaging region.

Modulation

In a continuous-wave NMR teslameter, the Larmor frequency has to be crossed and re-crossed to perform a measurement. This is achieved by modulating either the frequency of the RF field or the magnetic field.

Monitor

See Control.

Moving coil

A fluxmeter approach to measure the field at a point, by moving a coil from the point of interest to an area of zero field.

Moving wire

A fluxmeter approach to mapping very narrow-gap magnets, by measuring the voltage changes in a wire swept through the gap.

MRI

Magnetic Resonance Imaging. Using NMR to image the interior of a solid body, usually for a medical diagnosis.

Multipole magnet

In the accelerator world, multipole is a term used to refer to N-pole magnets, usually quadrupole, sextupole, octupole, or higher order.

Multiplexer

An adaptor that allows multiple probes to be connected to a single magnetometer.

NMR

Nuclear Magnetic Resonance. A resonance phenomenon seen when you irradiate a sample in a magnetic field with an RF field. NMR teslameters are the most accurate and precise of magnetometers.

Normalisation

For NMR probe arrays, normalisation refers to a procedure to compensate for the very slight probe-to-probe variations caused by the magnetic susceptibility of some of the array's components in high fields. The procedure requires a large-bore, homogeneous magnet at the correct field strength, and simply consists of placing each probe in turn at exactly the same location and measuring its response. During operation, the Magnetic Field Camera automatically reads the normalisation data from the probe array and compensates all measurements.

Particle physics

Or high-energy physics. The study of the elementary structure of matter, usually using accelerators.

Permanent magnet

A magnet using the magnetism "locked into" the structure of a suitable material rather than a current in a coil. Commonly used permanent magnet materials are iron-cobalt, ferrite, samarium-cobalt, or neodymium.

ppb

Parts per billion, or 10-9. 1 ppb is 10-7 % - not very much at all!

ppm

Parts per million, or 10-6. Bigger than a ppb, but still pretty darn small.

Pole

Part of a magnet used to focus the flux in a gap. The pole face is the interior surface of the gap.

Precision

Precision is how closely multiple measurements will be clustered. Also called reproducibility or repeatability. In everyday speech, often confused with accuracy and resolution.

Probe

The actual sensor that is placed in the magnetic field. The NMR probe contains the NMR sample; the Hall probe contains the Hall elements; and for a fluxmeter the probe is a coil.

Probe array

For Metrolab's Magnetic Field Camera, the probe array is a semi-circular plate holding up to 32 NMR probes.

Pulsed wire

A measurement technique used for accelerator magnets, in which an electrical pulse is applied to a wire stretched through the aperture. An analysis of the resulting wire motion yields a longitudinal map of the field.

Pulsed-mode NMR

An approach to NMR magnetometry that relies on a broad-band pulse to excite the sample, and then measures the frequency at which it "rings." The other common approach is continuous-wave.

Quadrupole

In accelerators, a magnet with four poles, yielding a highly inhomogeneous magnetic field. See focusing magnet  and multipole magnet.

Ramping

The process of injecting current to bring the field up to the operational field. Used particularly for superconducting magnets.

Range

The range of a probe is defined by the minimum and maximum field strength it can measure. On an instrument, the range refers to different sensitivity levels, allowing higher or lower fields to be measured.

Reference magnet

A magnet used for calibrating a magnetometer. Reference magnets are often controlled or even regulated by an NMR teslameter.

Regulation

A NMR-regulated magnet is an electromagnet whose field is measured by an NMR teslameter, which then closes the loop by controlling the power supply driving the magnet. Such magnet systems can provide long-term stable fields in the ppm range, rivalled only by superconducting magnets.

Resistive magnet

A normal electromagnet, as opposed to a superconducting magnet.

Resolution

Resolution measures the ability of a magnetometer to distinguish ("resolve") two nearly identical field values. Related to precision, but not to be confused with accuracy.

Rotating coil

A fluxmeter mapping technique commonly used for accelerator magnets. Generates a radial map of the integrated field seen by the particle beam.

Sample

The NMR sample is the material placed in the magnetic field, whose proton spin resonates when an RF field of the right frequency is applied.

Shimming

In magnet production, the process of making a magnetic field more homogeneous, by placing pieces of iron (shims) in the appropriate place, or by adjusting the current in special shim coils.

Site preparation

Preparing a site in a hospital or clinic for the installation of an MRI system. In particular, the area has to be magnetically and electrically (RF) shielded and structurally reinforced to support the weight.

Shielding

In magnetics, this refers to shielding the outside world from a magnetic field generated by a magnet, or vice versa. The brute force way of doing this is to build a big, soft-iron box. The more subtle approach, called active shielding, is to use a magnetic measurement system and coils in the walls surrounding the magnet to cancel the unwanted fields.

Solenoid

A magnet in the form of a cylindrical coil, concentrating the field inside the cylinder. See Bore. 

Spin

Electrons and protons have spin; in other words, they act as if they were spinning on their axis. This means that they act like little bar magnets, and tend to align in an external magnetic field. NMR, ESR, ferromagnetism and permanent magnetism are all phenomena involving spin.

Stabilisation

See regulation.

Standard

A standard is the internationally agreed-upon physical representation of a unit. For example, a caesium clock is the standard for a second. It should always be possible to trace a calibration back to a series of standards. The tesla doesn't have a convenient standard, so NMR teslameters are used instead, a so-called secondary standard.

Standards laboratory

Usually a government laboratory whose responsibility it is to maintain and develop standards, in cooperation with international standards bodies.

Superconducting magnet

An electromagnet built with superconducting wire. To remain superconducting, the coil has to be maintained at cryogenic temperatures, so it is enclosed in a cryostat. Superconducting magnets can achieve very high fields, don't consume any power once ramped up, and provide a nearly perfectly stable field.

Switching magnet

In accelerators, a magnet that acts as a switch, steering the particle beam in one direction or another.

Temperature compensation

Hall magnetometers are sensitive to temperature, so they may include a temperature sensor that allows the instrument to compensate the measurement.

Tesla

The SI unit for magnetic flux density (B).

Teslameter

See magnetometer.

Three-axis

The magnetic field is a three-dimensional vector quantity. A single Hall element only measures one component. A three-axis Hall instrument has three Hall elements arranged to measure all three components.

Tracking

Once the resonance has been found (see Search), a continuous-wave NMR teslameter can lock onto a signal and continue measuring it, even though the field is slowly changing. This is called tracking.

Tuning

NMR teslameter probes usually have a capacitor placed in parallel with the RF coil, forming an LC resonator that increases the gain and therefore the sensitivity of the probe. Probes can employ fixed tuning, with a fixed capacitor / trimcap, or variable tuning, using an electronically controlled varicap.

Uniform

See homogeneous.

Vibrating wire

A mapping technique similar to pulsed wire, used to create a longitudinal map of accelerator magnets. An AC current, at varying frequencies, is fed into a wire stretched through the aperture, and an analysis of the resulting wire motion yields a longitudinal map of the field.

Wiggler

A magnet system of alternating north and south poles, used to cause an electron beam to pursue a oscillating path. This causes the electrons to radiate, creating very intense and energetic light beams.

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