View source for Electron cyclotron resonance

{{Use dmy dates|date=July 2023}}
'''Electron cyclotron resonance''' ('''ECR''') is a phenomenon observed in [[plasma physics]], [[condensed matter physics]], and [[accelerator physics]]. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in magnetic fields. A free [[electron]] in a static and uniform [[magnetic field]] will move in a circle due to the [[Lorentz force]].  The circular motion may be superimposed with a uniform axial motion, resulting in a [[helix]], or with a uniform motion perpendicular to the field (e.g., in the presence of an electrical or gravitational field) resulting in a [[cycloid]].  The [[angular frequency]] (ω&nbsp;= 2π[[Frequency|''f'']] ) of this ''cyclotron'' motion for a given magnetic field strength ''B'' is given (in [[SI]] units)<ref>In SI units, the elementary charge ''e'' has the value 1.602×10<sup>−19</sup> [[coulomb]]s, the mass of the electron ''m''{{sub|e}} has the value 9.109×10<sup>−31</sup> kilograms, the magnetic field ''B'' is measured in [[tesla (unit)|tesla]]s, and the angular frequency ω is measured in [[radian]]s per second.</ref> by

:<math>\omega_\text{ce} = \frac{eB}{m_\text{e}}</math>.

where <math>e</math> is the [[elementary charge]] and <math>m</math> is the mass of the electron. For the commonly used [[microwave]] frequency [[ISM band|2.45&nbsp;GHz]] and the bare electron charge and mass, the resonance condition is met when ''B'' = 875{{nbsp}}[[gauss (unit)|G]] = 0.0875{{nbsp}}[[tesla (unit)|T]].

For particles of charge ''q'', electron rest mass ''m''<sub>0,e</sub> moving at relativistic speeds ''v'', the formula needs to be adjusted according to the [[special theory of relativity]] to:

:<math>\omega_\text{ce} = \frac{eB}{\gamma m_{0,\text{e}}}</math>

where

:<math>\gamma = \frac{1}{\sqrt{1 - \left(\frac{v}{c}\right)^2}}</math>.

==In plasma physics==
An ionized [[Plasma (physics)|plasma]] may be efficiently produced or heated by superimposing a static [[magnetic field]] and a high-frequency [[electromagnetic field]] at the electron cyclotron [[resonance]] frequency. In the toroidal magnetic fields used in [[magnetic fusion energy]] research, the magnetic field decreases with the major radius, so the location of the power deposition can be controlled within about a centimeter. Furthermore, the heating power can be rapidly modulated and is deposited directly into the electrons. These properties make electron cyclotron heating a very valuable research tool for energy transport studies. In addition to heating, electron cyclotron waves can be used to drive current. The inverse process of [[Cyclotron radiation|electron cyclotron emission]] can be used as a [[Plasma diagnostics|diagnostic]] of the radial electron temperature profile.
{|
| [[File:Cyclotron-Resonance-Motion Linearly-Pol-Fields Freq-1.0 Efield-1.0 fps-20 Image-Res-100 Image-Size-610x610.gif |thumb|400px |Example of cyclotron resonance between a charged particle and linearly polarized electric field (shown in green). The position vs. time (top panel) is shown as a red trace and the velocity vs. time (bottom panel) is shown as a blue trace.  The background magnetic field is directed out towards the observer.  Note that the circularly polarized example below assumes there is no Lorentz force due to the wave magnetic field acting on the charged particle. This is equivalent to saying that the charged particle's velocity orthogonal to the wave magnetic field is zero.]]
| [[File:Cyclotron-Resonance-Motion Circularly-Pol-Fields Freq-1.0 Efield-1.0 fps-20 Image-Res-100 Image-Size-610x610.gif |thumb|400px  |Example of cyclotron resonance between a charged particle and circularly polarized electric field (shown in green). The position vs. time (top panel) is shown as a red trace and the velocity vs. time (bottom panel) is shown as a blue trace.  The background magnetic field is directed out towards the observer.  Note that the circularly polarized example below assumes there is no Lorentz force due to the wave magnetic field acting on the charged particle. This is equivalent to saying that the charged particle's velocity orthogonal to the wave magnetic field is zero.]]
|}

==ECR ion sources==
Since the early 1980s, following the [[Tom W. Bonner Prize|award-winning]] pioneering work done by Dr. [[Richard Geller (physicist)|Richard Geller]],<ref>R. Geller, Peroc. 1st Int. Con. Ion Source, Saclay, p. 537, 1969</ref> Dr. [[Claude Lyneis]], and Dr. H. Postma;<ref>{{cite journal|author=H. Postma|journal=Physics Letters A|volume=31|issue=4|page=196|year=1970|title=Multiply charged heavy ions produced by energetic plasmas|doi=10.1016/0375-9601(70)90921-7|bibcode=1970PhLA...31..196P}}</ref> respectively from [[Commissariat à l'Énergie Atomique|French Atomic Energy Commission]], [[Lawrence Berkeley National Laboratory]] and the [[Oak Ridge National Laboratory]], the use of electron cyclotron resonance for efficient plasma generation, especially to obtain large numbers of multiply charged ions, has acquired a unique importance in various technological fields. Many diverse activities depend on electron cyclotron resonance technology, including
* advanced cancer treatment, where ECR [[ion sources]] are crucial for [[proton therapy]],
* advanced [[Semiconductor fabrication|semiconductor manufacturing]], especially for high density [[DRAM]] memories, through [[plasma etching]] or other [[plasma processing]] technologies,
* [[Electrically powered spacecraft propulsion|electric propulsion]] devices for [[spacecraft propulsion]], where a broad range of devices ([[HiPEP]], some [[Electrostatic ion thruster|ion thruster]]s, or [[electrodeless plasma thruster]]s),
* for [[particle accelerator]]s, on-line mass separation and radioactive ion charge breeding,<ref>''Handbook of Ion Source'', B. Wolf, {{ISBN|0-8493-2502-1}}, pp. 136–146</ref>
* and, as a more mundane example, painting of plastic bumpers for cars.

The ECR ion source makes use of the electron cyclotron resonance to ionize a plasma. Microwaves are injected into a volume at the frequency corresponding to the electron cyclotron resonance, defined by the magnetic field applied to a region inside the volume. The volume contains a low pressure gas. The alternating electric field of the microwaves is set to be synchronous with the gyration period of the free electrons of the gas, and increases their perpendicular kinetic energy. Subsequently, when the energized free electrons collide with the gas in the volume they can cause ionization if their kinetic energy is larger than the ionization energy of the atoms or molecules. The ions produced correspond to the gas type used, which may be pure, a compound, or vapor of a solid or liquid material.

ECR ion sources are able to produce singly charged ions with high intensities (e.g. [[Hydrogen|H]]<sup>+</sup> and [[Deuterium|D]]<sup>+</sup> ions of more than 100 [[milliampere|mA]] (electrical) in DC mode<ref>R. Gobin et al., [http://accelconf.web.cern.ch/AccelConf/e02/PAPERS/THPRI003.pdf Saclay High Intensity Light Ion Source Status] The Euro. Particle Accelerator Conf. 2002, Paris, France, June 2002, p. 1712</ref> using a 2.45&nbsp;GHz ECR ion source).

For multiply charged ions, the ECR ion source has the advantages that it is able to confine the ions for long enough for multiple collisions and multiple ionization to take place, and the low gas pressure in the source avoids recombination. The VENUS ECR ion source at [[Lawrence Berkeley National Laboratory]] has produced in intensity of 0.25 mA (electrical) of [[Bismuth|Bi]]<sup>29+</sup>.<ref>[http://cerncourier.com/cws/article/cern/29329 VENUS reveals the future of heavy-ion sources] CERN Courier, 6 May 2005</ref>

Some important industrial fields would not exist without the use of this fundamental technology, which makes electron cyclotron resonance ion and plasma sources one of the enabling technologies of today's world.

==In condensed matter physics==
Within a solid the mass in the cyclotron frequency equation above is replaced with the [[effective mass (solid-state physics)|effective mass]] tensor <math>m^*</math>.  Cyclotron resonance is therefore a useful technique to measure [[effective mass (solid-state physics)|effective mass]] and [[Fermi surface]] cross-section in solids.  In a sufficiently high magnetic field at low temperature in a relatively pure material

:<math>\begin{align}
         \omega_\text{ce} &> \frac{1}{\tau} \\
  \hbar{\omega}_\text{ce} &> k_B T \\
\end{align}</math>

where <math>\tau</math> is the carrier scattering lifetime, <math>k_B</math> is [[Boltzmann's constant]] and <math>T</math> is temperature.  When these conditions are satisfied, an electron will complete its cyclotron orbit without engaging in a collision, at which point it is said to be in a well-defined Landau level.

==See also==
* [[Cyclotron resonance]]
* [[Cyclotron]]
* [[ARC-ECRIS]]
* [[Ion cyclotron resonance]]
* [[Synchrotron]]
* [[Gyrotron]]
* [[De Haas–van Alphen effect]]

==References==
{{Reflist}}

==Further reading==
* [https://web.archive.org/web/20090920064757/http://mgm.mit.edu/historic/i1101.pdf "Personal Reminiscences of Cyclotron Resonance,"] G. Dresselhaus, Proceedings of ICPS-27 (2004).   This paper describes the early history of cyclotron resonance in its heyday as a [[band structure]] determination technique.

{{DEFAULTSORT:Electron Cyclotron Resonance}}
[[Category:Waves in plasmas]]
[[Category:Condensed matter physics]]
[[Category:Electric and magnetic fields in matter]]
[[Category:Ion source]]
[[Category:Particle accelerators]]