Properties of the 90° cylindrical energy analyzer with internal deceleration of the beam in real conditions of operation

Abstract

To measure plasma potential in toroidal magnetically confined fusion plasma, the Heavy Ion Beam Diagnostic (HIBD) employs of the electrostatic energy analyzer. The 90° cylindrical energy analyzer (CEA) developed for the HIBD on the ISTTOK tokamak due to compactness and multislit operation capability, is characterized by large angular aberrations, if operated in conventional mode. A new operation regime was proposed based on numerical investigations, which demonstrated the efficiency of the 90°CEA operation in reducing angular aberrations via the deceleration of the ion beam inside analyzer, resulting also on an improved energy resolution. In this paper, the properties of the 90 ° CEA featuring with guard rings (GR) to shield the analyzer from the surrounding structures are investigated in the internal deceleration mode (ID-mode) for “centric” and “paracentric” injection geometries. The numerical simulations are conducted using SIMION code. Special attention is paid to uniformity of the beam image on detector plate, which is of importance in ion beam energy measurements by split-plate detector technique. The results confirm coupling of analyzer angle aberration properties with beam deceleration inside analyzer. For both “centric” and “paracentric” entrances the existence of the first-order focus in ID-mode is obtained. However, the first order focus ID-mode of 90°CEA with GR is characterized by heterogeneity of the image for the initially uniform beam, determined by modification of electric field inside analyzer. It is revealed that the energy dispersion of the initially monoenergetic ions due to chromatic-like aberration introduced by analyzer plays a contributive role in the beam image non-uniformity. From the beam energy measurements point of view, it is shown that the energy variation due to this chromatic aberration is of the second order of the analyzer intrinsic resolution. The results show that “centric” entrance in ID-mode is preferable as the image non-uniformity is less pronounced and can be effectively restored by manipulation of entrance slit dimensions and potentials on GR. It is concluded that besides detail preparatory numerical optimization, the beam image shape and uniformity control is essential, which can be realized with the dedicated multiple cell detection.