ThomX is a Compton source project in the range of the hard X rays (40/90 keV). The machine is composed of a 50/70 MeV injector Linac and a storage ring where an electron bunch collides with a laser pulse accumulated in a Fabry-Perot resonator. The final goal is to provide an X-rays average flux of 1011-1013 ph/s. Different users are partners in the ThomX project, especially in the area of medical science and cultural heritage. A demonstrator was funded and is being built on the Orsay university campus. During the commissioning phase, a standard S-band accelerating section is able to achieve around 50 MeV corresponding to around 45 keV X-rays energy. Since the maximum targeted X-ray energy is 90 keV, the Linac section design will provide an electron beam energy of 70 MeV. This requires essentially the development of more reliable high gradient compact S band accelerating section. We present here the design of high gradient compact S-band accelerating section for the upgrade program of the THOMX LINAC.