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The spectrum of initial state radiation 
(ISR, photon radiation from the incoming 
electrons and positrons) contains the ISR 
scale inherent to the process under 
consideration which should be defined by 
the user.

Beamstrahlung is the process of energy 
loss by the incoming electron/positron
in the field of the positron/electron 
bunch moving in the opposite direction.  
Beamstrahlung spectrum is an attribute of
the LC design and depends on the bunch 
geometry, bunch charge and the collision 
energy.

See details in the Manual.