The Nitrogen screen system of HT-7 tokamak consists of two toroidal shields that are located in the interspace between the internal and external vacuum chamber. They enclose the superconducting magnet, shield the 4.5 K TF coils against the thermal radiation, resist the effluence of fast change of poloidal magnetic field during discharge and plasma disruption.

Both internal and external screen torus are composed of eight octants. They are made from 15 mm thick pure copper. Eight insulating gaps has been inserted in meridian planes to allow penetration of poloidal magnetic field. In order to decrease the shear forces and to avoid current loops, an insulating break has been designed on each octant of the internal and external shields. The two toroidal screens are connected each other, formed a strong rigid torus provides a mechanical support for TF system.

The HT-7 tokamak is reconstructed from Russia tokamak T-7. The preliminary concept design was provided by Kurchatov Research Center Nuclear Fusion Institute. The NFI specialists participated the engineering design as consultant.

One of the main purpose of the reconstruction of HT-7 is to improve the accessibility of the device, increase the quantity and size of the ports of vacuum vessel for diagnostics, additional heating and current drive. From this point, The 48 TF coils of T-7 has been merged to 24 TF coils of HT-7 to get more spaces for the 34 new vacuum vessel ports. Correspond with this, 10 horizontal and 24 vertical ducts have been designed. The cooling circular pipes have to be redistributed and their inlets and outlets have been changed. In order to decrease the consume of liquid nitrogen during vacuum vessel backing, a water cooling channel has been added on each internal screen octant.

The loads that are considered in the finite element analysis are the dead weight of the screen, the gravity of the TF magnet system and the electromagnetic forces induced by plasma disruption. The result shows that the maximum stress is much less than the yield point, the deformations are negligible. The radiation area is increased due to new ducts. The increasing of radiation power corresponding with this is less than 2 kW at 80 K according to our estimation.

The main parameter, the process of reconstruction and the performance of the nitrogen screen in the operations will be described and discussed in this paper