next up previous
Next: Adiabatic Invariants Up: Charged Particle Motion Previous: Invariance of Magnetic Moment

Poincaré Invariants

An adiabatic invariant is an approximation to a more fundamental type of invariant known as a Poincaré invariant. A Poincaré invariant takes the form
\begin{displaymath}
{\cal I} = \oint_{C(t)} {\bf p}\cdot d{\bf q},
\end{displaymath} (102)

where all points on the closed curve $C(t)$ in phase-space move according to the equations of motion.

In order to demonstrate that ${\cal I}$ is a constant of the motion, we introduce a periodic variable $s$ parameterizing the points on the curve $C$. The coordinates of a general point on $C$ are thus written $q_i = q_i(s,t)$ and $p_i=p_i(s,t)$. The rate of change of ${\cal I}$ is then

\begin{displaymath}
\frac{d{\cal I}}{dt} =\oint\left(p_i\,\frac{\partial^2 q_i}{...
...l p_i}{\partial t} \frac{\partial q_i}{\partial s}\right)\,ds.
\end{displaymath} (103)

We integrate the first term by parts, and then used Hamilton's equations of motion to simplify the result. We obtain
\begin{displaymath}
\frac{d{\cal I}}{dt} =\oint\left( -
\frac{\partial q_i}{\pa...
... H}{\partial q_i}
\frac{\partial q_i}{\partial s}\right)\,ds,
\end{displaymath} (104)

where $H({\bf p}, {\bf q}, t)$ is the Hamiltonian for the motion. The integrand is now seen to be the total derivative of $H$ along $C$. Since the Hamiltonian is a single-valued function, it follows that
\begin{displaymath}
\frac{d{\cal I}}{dt} =-\oint\frac{d H}{ds}\,ds =0.
\end{displaymath} (105)

Thus, ${\cal I}$ is indeed a constant of the motion.


next up previous
Next: Adiabatic Invariants Up: Charged Particle Motion Previous: Invariance of Magnetic Moment
Richard Fitzpatrick 2011-03-31