The purpose of this study is to simulate and to have a better understanding of the experimental limitation of the transmission in the CTF2 drive beam in function of the beam charge. In this note, the low energy part of the beam line is studied with the particle-tracking program PARMELA from the gun to the first 30 GHz section. Assuming the beam loading and space charge at 10nC/bunch, the settings of the magnetic components in Nov 98 configuration are discussed and slight modification have to be proposed to obtain a good matching and total transmission until the first 30GHz section. The possible limitation of transmission in the HCS for bigger beam size generation on the cathode is pointed out. Some particularities of the bunch compressor are shown as well as the effect of the bunch correlation quality, the deflection angle limitation and the phase shift induced in the train. Finally, transverse results are resumed and the simulated total transmission is validated by experiments. Beam parameters will be used by A. Riche [5] to study the wake-fields and the beam transmission in the downstream 30GHz sections
