Control number of FFT modes in longitudinal space charge solver

[austin-hoover]

This PR gives the option to restrict the number of modes used in longitudinal space charge kicks. From Chao, the formula for the energy gain from longitudinal impedance $Z_{0}^{\parallel}(\omega)$ is

$$ \Delta \mathcal{E} = -\frac{1}{2\pi} \int_{-\infty}^{\infty} d\omega | \rho(\omega) |^2 \text{Re} \left\{ Z_{0}^{\parallel} (\omega) \right\} . $$

From the FFT, the solver uses $N / 2$ frequencies/modes, where $N$ is the number of longitudinal bins.

PyORBIT3/src/spacecharge/LSpaceChargeCalc.cc

Lines 211 to 227 in e4ac4ce

	double LSpaceChargeCalc::_kick(double angle)
	{
	// n=0 term has no impact (constant in phi)
	// f(phi) = _FFTMagnitude(1) + sum (n = 2 -> N / 2) of
	// [2 * _FFTMagnitude(i) * cos(phi * (n - 1) +
	// _FFTPhase(n) + _chi(n))]
	double kick = 0.;
	double cosArg;
	for(int n = 1; n < nBins / 2; n++)
	{
	cosArg = n * (angle + OrbitConst::PI) + _fftphase[n] + _chi[n];
	kick += 2 * _fftmagnitude[n] * _z[n] * cos(cosArg);
	}
	return kick;
	}

This PR lets the user restrict the number of modes used in the energy kick, acting as a low-pass filter.

[austin-hoover]

Example with decreasing number of modes:

rotation_fft_modes.mov