I am trying to calculate the spectrum of Bremmstrahlung, which involves calculating the Fourier transformed acceleration. I am solving a non-linear ODE to numerically calculate the acceleration in the time domain. After taking the Fourier transformation using Numpy's fft, the resultant spectrum looks highly non-smooth and "non-physical" . I cannot paste the entire code so I am posting what I think is relevant snippet. Can someone point out what I am doing wrong?

Note: My acceleration is a function of two variables (beta, and b, the impact parameter), and I want to plot it for the different b, and in order to factor out the beta term I am just summing over all the values of acceleration for the different beta, for a given impact parameter b.

Also my spectrum is Fourier transformed acceleration square times a constant factor (Larmor's formula) enter image description here enter image description here

Fourier Transformation of the acceleration
N = 2**8
sampling_frequency = 10
a_w_normalized = [[] for a in range(len(impact_parameter))]
a_w =[[] for a in range(len(impact_parameter))]
intensity_normalized = [[] for a in range(len(impact_parameter))]
acc_summed_over_velocity = []
intensity_summed_over_velocity =[]

for index,b in enumerate(impact_parameter):
    acc_sum =[0 for x in range(N)]
    intensity_sum = [0 for x in range(N)] 
    for j in range(len(velocity_z_component)):
        #window_kaiser = signal.kaiser(N, 15)
        #window_hann = signal.hann(N,sym=True)
        window =1
        fft_input = acc_normalized[index][j]*window
        ft_acc_normalized = np.abs(np.fft.fft(fft_input,norm=None))

        acc_sum =np.add(ft_acc_normalized,acc_sum )
        intensity_list = [power_spectrum_factor * (a ** 2) for a in ft_acc_normalized]
        intensity_sum = np.add(intensity_list,intensity_sum)
    intensity_summed_over_velocity.append(intensity_sum *acceleration_factor**2)

#intensity_summed_over_velocity=  ma.masked_less_equal(intensity_summed_over_velocity,1e-5)

Plotting acceleration for selected values of impact paramters in time and frequency domain

if plot is True:
    plt.figure(figsize=(12, 8))
    for i in range(len(impact_parameter)):
        plt.plot(t,acc_timedomain_summed_over_velocity[i], label='b={:.3f}'.format(impact_parameter[i]), )
    plt.ylabel(r'$ a(\tilde t) $', fontsize=14)
    plt.xlabel(r'$ \tilde t $', fontsize=14)
    if screening is False:
        plt.title('a(t) vs time without screening',fontsize=15)
        plt.title('a(t) vs time with screening',fontsize=15)

    Fourier transformed intensity for different impact paramater

    plt.figure(figsize=(12, 8))
    freq_normalized = np.fft.fftfreq(N)*(2*np.pi*sampling_frequency)
    for index,b in enumerate(plasma.impact_parameter):
        plt.plot(np.abs(freq_normalized), intensity_summed_over_velocity[index], label=r'$ \tilde b={:.2f}$'.format(b), )
    plt.xlabel(r'$ \tilde \omega $', fontsize=14)
    plt.ylabel(r'$ I_{\omega} $', fontsize=16)
    plt.legend(loc="lower left")

    if screening is False:
        plt.title('Single particle spectrum without screening')
        plt.title('Single particle spectrum with screening')


1 Answer 1


Most of the time, a non-parametric PSD estimation based on FFT alone (called a Periodogram) will provide a random looking spectrum of the numerical data.

To reduce those random variations, or to get a smoother looking estimate, you can do one of the two things:

  • Use Periodogram averaging such as Welch's method.
  • Use a model based parametric PSD estimate instead.

Note that the first approach will trade-off spectral resolution for reduced variance, whereas the second method will be high resolution but requires a consistent knowledge of the associated physical model of the process.

  • $\begingroup$ Sorry if my comments doen't make much sense, this is the first time I am reading about PSD, and Welch's method. Are you saying that I should calculate my spectrum using PSD instead of squaring Fourier domain acceleration and then multiplying by relevant coefficient? I am interested in calculating Larmor's power which in frequency domain is given by 8*pi*e^2 a(w)^2/3c^3. $\endgroup$
    – Prav001
    Oct 17, 2019 at 20:49
  • $\begingroup$ I don't know about Larmor's Power $8 \pi e^2 a(w)^{2/3} c^3$... Is $a(w)$ the Fourier transform of acceleration ? $\endgroup$
    – Fat32
    Oct 17, 2019 at 20:54
  • $\begingroup$ Yes, $a(\omega)$ is the Fourier transformation of acceleration in the time domain. So essentially I am performing FFT on my acceleration data which I show in the question. Now, my acceleration depends on two parameters, b and $\beta$. What I show in my acceleration plot is acceeleration summed over all values of $\beta$ for different b's. $\endgroup$
    – Prav001
    Oct 17, 2019 at 21:01
  • $\begingroup$ ok so what's is the expected (typical) functional form of this acceleration ? $\endgroup$
    – Fat32
    Oct 17, 2019 at 21:04
  • $\begingroup$ do you have an expression for the dependence of acceleration on $\beta$ abd b 's. $\endgroup$
    – Fat32
    Oct 17, 2019 at 21:08

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