# Creating a magnetic field?

1. Oct 26, 2009

### nickdk

Is it possible to create a strong enough (artificial) magnetic field to stop high energy particles? Much like the earth's magnetic field stops high energy particles.

2. Oct 26, 2009

### Bob_for_short

Yes, in TOKAMAKs and Stellarators they create strong magnetic fields to confine high energy charged particles.

Last edited: Oct 26, 2009
3. Oct 26, 2009

### Staff: Mentor

"Confining" and "stopping" are not the same thing.

If by "stopping", nickdk means "bringing to rest" or "making stationary", it's impossible for a magnetic field (by itself) to do that, because the magnetic force on a (charged) particle is always perpendicular to the particle's motion. It cannot change the particle's speed, only its direction of motion.

4. Oct 26, 2009

### Bob_for_short

You are right, the total velocity cannot be put to zero by a magnetic field but there are magnetic "bottles" where a non-uniform magnetic field prevents particles from escaping (it "reflects" particles axially in the throat area). In certain sense it "stops" particles at some moment of reflection (I mean the axial part of motion).

Last edited: Oct 26, 2009
5. Oct 26, 2009

### Bob S

Think of the betatron accelerator using the Faraday induction law to accelerate electrons to relativistic energies. The highest energy betatron ever built was over 300 MeV. Could the betatron dB/dt be reversed to decelerate 300 MeV electrons?
See
http://en.wikipedia.org/wiki/Betatron
The only requirement for the betatron magnetic field is that the guide field on the vacuum chamber be half the average field inside the area of the orbit (easy physics 201 problem). If B(t) stops increasing (it is a sine wave) and starts decreasing while maintaining this ratio with accelerated electrons in the vacuum chamber, the electrons will decelerate.
Bob S

Last edited: Oct 26, 2009
6. Oct 26, 2009

### Bob_for_short

We speak of a constant magnetic field, not of varying one. dB/dt = rotE, so it is E that actually makes this work.

Last edited: Oct 26, 2009
7. Oct 26, 2009

### Bob S

The OP asked Is it possible to create a strong enough (artificial) magnetic field to stop high energy particles?, not strong enough constant magnetic field.
Bob S

8. Oct 26, 2009

### Bob_for_short

You are right.

9. Oct 26, 2009

### nickdk

Interesting stuff. Thank you all :)