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Homework Help: Pulsed ion beam

  1. Jul 13, 2010 #1
    1. The problem statement, all variables and given/known data

    Problematic Part of the problem:

    Consider a negative ion beam of He- ions. The given beam characteristics are the beam energy (E) and the beam current (I).

    An electrostatic chopper is placed on the beam line, which pulses the DC beam with a pulse width of say [tex]t_1[/tex] nanoseconds.

    What are the new beam characterisitcs?

    2. Relevant equations
    Normal Distribution:
    f(x)= \frac{1}{\sqrt{2 \pi \sigma^2}} e^{-\frac{t^2}{2\sigma^2}}

    3. The attempt at a solution

    I would have said the new current profile would be, for an incident DC beam of beam current I0

    [tex]I(t)=I_0\frac{1}{\sqrt{2 \pi \sigma^2}} e^{-\frac{t^2}{2\sigma^2}}[/tex]

    if the beam emerges from the chopper at t=0. Also, since the FWHM of the beam is given to be [tex]t_1[/tex] the standard deviation would be


    (from http://en.wikipedia.org/wiki/Full_width_at_half_maximum)

    This seems pretty straightforward, however, this appears to be wrong because according to this, the peak current has changed, and become .046 times its initial value

    [tex]I(0)=I_0 f(0)[/tex] where f(x) is the normal distribution...

    can anyone tell me what I am doing wrong.
    Last edited: Jul 13, 2010
  2. jcsd
  3. Jul 13, 2010 #2

    you can just tell me how a dc beam is chopped to a gaussian pulse...
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