An alternative to the synchrocyclotron is the isochronous cyclotron, which has a magnetic field that increases with radius, rather than with time. Isochronous cyclotrons are capable of producing much greater beam current than synchrocyclotrons, but require azimuthal variations in the field strength to provide a strong focusing effect and keep the particles captured in their spiral trajectory. For this reason, an isochronous cyclotron is also called an "AVF (azimuthal varying field) cyclotron".[13] This solution for focusing the particle beam was proposed by L. H. Thomas in 1938.[13] Recalling the relativistic gyroradius r = frac{gamma m_0 v}{q B} and the relativistic cyclotron frequency f = frac{f_0}{gamma}, one can choose B to be proportional to the Lorentz factor, B = gamma B_0. This results in the relation r = frac{m_0 v}{q B_0} which again only depends on the velocity v, like in the non-relativistic case. Also, the cyclotron frequency is constant in this case.
The transverse de-focusing effect of this radial field gradient is compensated by ridges on the magnet faces which vary the field azimuthally as well. This allows particles to be accelerated continuously, on every period of the radio frequency (RF), rather than in bursts as in most other accelerator types. This principle that alternating field gradients have a net focusing effect is called strong focusing. It was obscurely known theoretically long before it was put into practice.[14] Examples of isochronous cyclotrons abound; in fact almost all modern cyclotrons use azimuthally-varying fields. The TRIUMF cyclotron mentioned below is the largest with an outer orbit radius of 7.9 metres, extracting protons at up to 510 MeV, which is 3/4 of the speed of light. The PSI cyclotron reaches higher energy but is smaller because of using a higher magnetic field.
An alternative to the synchrocyclotron is the isochronous cyclotron, which has a magnetic field that increases with radius, rather than with time. Isochronous cyclotrons are capable of producing much greater beam current than synchrocyclotrons, but require azimuthal variations in the field strength to provide a strong focusing effect and keep the particles captured in their spiral trajectory. For this reason, an isochronous cyclotron is also called an "AVF (azimuthal varying field) cyclotron".[13] This solution for focusing the particle beam was proposed by L. H. Thomas in 1938.[13] Recalling the relativistic gyroradius r = frac{gamma m_0 v}{q B} and the relativistic cyclotron frequency f = frac{f_0}{gamma}, one can choose B to be proportional to the Lorentz factor, B = gamma B_0. This results in the relation r = frac{m_0 v}{q B_0} which again only depends on the velocity v, like in the non-relativistic case. Also, the cyclotron frequency is constant in this case.The transverse de-focusing effect of this radial field gradient is compensated by ridges on the magnet faces which vary the field azimuthally as well. This allows particles to be accelerated continuously, on every period of the radio frequency (RF), rather than in bursts as in most other accelerator types. This principle that alternating field gradients have a net focusing effect is called strong focusing. It was obscurely known theoretically long before it was put into practice.[14] Examples of isochronous cyclotrons abound; in fact almost all modern cyclotrons use azimuthally-varying fields. The TRIUMF cyclotron mentioned below is the largest with an outer orbit radius of 7.9 metres, extracting protons at up to 510 MeV, which is 3/4 of the speed of light. The PSI cyclotron reaches higher energy but is smaller because of using a higher magnetic field.
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