Geometrical divergence does not match FWHM of build_gauss_wavefront

[mabelruiz]

The size of the beam at a distance z in fwhm is geometrically defined by: w_fwhm=div_fwhm*z, where

div_fwhm= lambda/pi()*width_fwhm

The value of the beam size are allways 1.6 times smaller than the given by buid_gaussian_wavefront command.

[mabelruiz]

src_to_m0 = 21.4                                # m

''' #Source size (FWHM, Calculations)
             4 nm - 120 mum
             8 nm - 140 mum
             12 nm - 160 mum
             20 nm - 190 mum
             60 nm - 270 mum 
'''

wl=7.0e-9                                          # m
ekev=1239/(wl*1e12)
coh_time= 25e-15                                #[fs=1E-15] parms http://adweb.desy.de/home/faatz/www/FLASH-parms4.html
pulseEnergy= 0.465e-5;                         #[J]  parms http://adweb.desy.de/home/faatz/www/FLASH-parms4.html
w_fwhm =100*1e-6                                # source width. wl/(np.pi*theta_fwhm)# FWHM beam waist; 
w_0 = w_fwhm/(2*np.sqrt(np.log(2)))            # sigma of amplitude
w_rms= w_fwhm/1.36



M=1.6
zR= (np.pi*w_fwhm**2)/(wl*M)
wZ=w_fwhm*np.sqrt(1+(src_to_m0/zR)**2)
wRMS=wZ*0.735
theta_fwhm=wl*M/(np.pi*w_fwhm)

#defining limits and sample
num_points = 900
nslices = 30  
sig_num = 10                                    #range is 5 times the size of the beam width radius;
range_xy =w_rms * np.sqrt(1+(src_to_m0/zR)**2)/np.sqrt(2)*sig_num;     





print('Divergence (FWHM)  :{:.2f} urad'.format(theta_fwhm*1e6))
print('Waist FWHM@{:.2f}m :{:.2f} um'.format(src_to_m0,wZ*1e6))
print('Waist RMS@{:.2f}m  :{:.2f} um'.format(src_to_m0,wRMS*1e6))
print('Range x and y      :{:.2f} mm'.format(range_xy*1e3))

data_path='KB';mkdir_p(data_path)
fname='KB2'
bSaved=True

[mabelruiz]

srwl_wf = build_gauss_wavefront(num_points, num_points, nslices, 
                                ekev, -range_xy/2, range_xy/2, -range_xy/2, range_xy/2, coh_time/np.sqrt(2), 
                                w_rms/2, w_rms/2, src_to_m0, pulseEn=pulseEnergy, 
                                pulseRange=None, _mx=None, _my=None)

FWM=0.76x0.76 mm

[tyjisinap]

I think the module build_gauss_wavefront can only build the perfact gauss beam that M2=1. And I don't know how to simulate the gauss beam that M2 >1. Have you find the answer?