# Synchronised cyclotron? HELP

1. Nov 24, 2015

### Samiha

Hey so help me out here please. This is an Edexcel Physics A2 question (I'm doing A Levels). I really don't know if this will make sense at all lol

So there are cyclotrons- where the mass of the particle must me constant, that is, the limitation of the cyclotron is that it does not take into consideration the relativistic effects on mass, right?

Then there's the synchrocyclotron, which is basically a modifies version of the cyclotron, and it DOES take into consideration the relativistic effects on mass, in that it alters the frequency of the applied p.d according to the changing frequency of the particle which is now close to the speed of light, right? i.e the frequency of the voltage is changed.

But my question is that my teacher talked about something that I haven't been able to find on Google- he talked about a SYNCHRONISED CYCLOTRON. And this is what he said:

At greater speed, that is when speed of the particle approaches the speed of light, according to the theory of relativity, the mass increase. So frequency of the particle decreases. But to keep on accelerating the particle, the frequency of revolution of the particle must be equal to the frequency of the a.c voltage, so the frequency of the particle can just be kept constant by increasing the magnetic field.

But if the frequency AND the magnetic field were kept constant, then wouldn't it just be a synchrotron?

If tl;dr- is there such a thing as a SYNCHRONISED CYCLOTRON? Which does not change the frequesncy of the a.c voltage with some clever circuitry, as in the synchrocyclotron, but which takes into account the relativistic effects on mass, unlike the classical cyclotron?

2. Nov 24, 2015

### Staff: Mentor

The concept of relativistic (speed-dependent) mass is not used any more.

Decreasing frequency is one option (synchrocyclotron), increasing the magnetic field is the other option (isochronous cyclotron).
Keeping both constant at the same time doesn't work if relativistic effects are relevant.

Usually you cannot ramp up magnets fast enough to make it dynamically, so you need a magnetic field that increases with radius. This leads to issues with focusing, and things get complicated. It allows continuous operation, so it is still a widely used concept. There is some overlap with the concept of a synchrotron.

3. Nov 24, 2015

### Orodruin

Staff Emeritus
Stop right there! When physicists talk about mass, we generally just talk about the rest mass (and since we only talk about rest mass, we usually also skip the "rest" part), see, e.g., What is relativistic mass and why it is not used much?. Relativistic mass is an archaic concept that unfortunately lives on in introductory texts, but leads to more misconceptions than it solves (one of the more "popular" ones being that any object will form a black hole if it is accelerated enough). It is more appropriate to talk about the relativistic effects on the equations of motion, which depend both on the speed of the particles as well as the direction you are trying to accelerate them in.