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A flux loop is a loop of wire placed inside a plasma at a right angle. Changes in the field create a current in the loop, which may be interpreted to measure the properties of the plasma. Flux loops are key
A flux loop is a loop of wire. The magnetic field passes through the wire loop. As the field varies inside the loop, it generates a voltage by 
diagnostic
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engine ...
s in fusion power
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion, nuclear fusion reactions. In a fusion process, two lighter atomic nucleus, atomic nuclei combine to form a heavier nucleus, whi ...
research.
Theory
A flux loop is a loop of wire. The magnetic field passes through the wire loop. As the field varies inside the loop, it generates a voltage by Faraday's law of induction
Faraday's law of induction (briefly, Faraday's law) is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (emf)—a phenomenon known as electromagnetic inducti ...
, driving a current. This was measured and from the signal the magnetic flux was measured. The voltage induced is determined by:"Principals of Plasma Diagnostics" Second edition, I H Hutchinson, page 11, 2002
:
Typically, you need to integrate the signal over a period of time to get the magnetic field at that instant (not just the change in magnetic fields). This is normally done by adding an integrator circuit
A low-pass filter is a Filter (signal processing), filter that passes signal (electrical engineering), signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exa ...
which will passively integrate the electrical signal.
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Common Uses
The flux loop is common ontokamak
A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being d ...
s, magnetic mirror
A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement device used in fusion power to trap high temperature plasma using magnetic fields. T ...
s, Field-reversed configuration
A field-reversed configuration (FRC) is a type of plasma device studied as a means of producing nuclear fusion. It confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stabl ...
s, the Levitated dipole
A levitated dipole is a type of nuclear fusion reactor design using a superconducting torus which is magnetically levitated inside the reactor chamber. The name refers to the magnetic dipole that forms within the reaction chamber, similar to Eart ...
experiment and the polywell
The polywell is a proposed design for a fusion reactor using an electric field to heat ions to fusion conditions.
The design is related to the fusor, the high beta fusion reactor, the magnetic mirror, and the biconic cusp. A set of electromagn ...
. In tokamaks a set of loops is used, and these are spaced slightly off-center and non-concentrically. Placing the loops intentionally asymmetrically allows the users to find the magnetic field density at different points inside the tokamak. The distance between the loops was determined by shafranov, and is sometime called the "shafranov distance".shafranov, v,d 1963, plasma physics 5:251
References
Plasma physics {{plasma-stub