Longitudinal Magnetic Fields
Longitudinal magnetic fields refer to magnetic fields that are oriented parallel to the direction of propagation of a wave or particle. These fields play a crucial role in various scientific and technological applications, including particle accelerators, plasma physics, astrophysics, and magnetic resonance imaging (MRI).
In the context of particle accelerators, longitudinal magnetic fields are used to control the motion of charged particles. In circular accelerators such as synchrotrons or cyclotrons, these fields are employed to keep the particles in a stable orbit. By adjusting the strength of the longitudinal magnetic field, scientists can manipulate the energy and trajectory of the particles, allowing them to achieve higher energies and study fundamental particles more effectively.
In plasma physics, longitudinal magnetic fields are utilized to confine and control high-temperature plasmas. Plasmas are ionized gases consisting of charged particles that respond to electromagnetic forces. By applying a longitudinal magnetic field, researchers can shape and stabilize plasmas for fusion experiments or other applications. This field configuration helps prevent plasma instabilities and enhances confinement properties.
Astrophysical phenomena also involve longitudinal magnetic fields. For instance, in stars like the Sun, these fields play a crucial role in various processes such as stellar structure, energy transport, and solar flares. The interaction between the plasma and the magnetic field generates complex phenomena like sunspots and coronal mass ejections. Understanding these phenomena is essential for studying stellar evolution and space weather.
In medical imaging, longitudinal magnetic fields are utilized in MRI machines. MRI is a non-invasive imaging technique that uses strong magnetic fields and radio waves to generate detailed images of internal body structures. The main magnet in an MRI machine produces a strong static longitudinal magnetic field that aligns the protons in the patient’s body. By manipulating this field using gradient coils, radiofrequency pulses can be applied to excite the protons and create signals that are detected by receivers to form images.
The study of longitudinal magnetic fields involves various theoretical and experimental techniques. In theoretical physics, Maxwell’s equations, which describe the behavior of electromagnetic fields, are used to analyze and predict the properties of these fields. Experimental techniques include the use of specialized instruments such as magnetometers to measure the strength and orientation of magnetic fields.
In conclusion, longitudinal magnetic fields are magnetic fields that are oriented parallel to the direction of propagation of a wave or particle. They have significant applications in particle accelerators, plasma physics, astrophysics, and medical imaging. Understanding and controlling these fields are crucial for advancing scientific knowledge and technological developments in these fields.
After reading this section you will be able to do the following:
Describe how longitudinal magnetic fields are established and what they can be used for.
Longitudinal magnetic can be produced in a material by placing the material inside of a current carrying coil (or a solenoid).When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a “coil shot.”
Compared to the magnetic flux lines that are outside of a magnetic material (like a bar magnet for example), the flux within the material is straight.The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. Keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much more dense inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component.
When a component is magnetized along its complete length, the flux loss is small along its length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density will be relatively uniform throughout the component. This can be applied to NDT: flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component.
The flow orientation fo the magnetic field along with the orientation of the defect in a material will determine is the defect is detectable or not.
Solenoid – An electrically energized coil of insulated wire, which produces a magnetic field within the coil.
When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on each side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the solenoid and return to the part at a opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid. As a result, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage.