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A heavy ion beam diagnostic (HIBD) developed for the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) is based on a multiple cell array detector (MCAD), which collects a "fan" of secondary ions originated along a primary beam trajectory in collisions with the plasma electrons and separated by the magnetic field of the tokamak. An alternative to the traditional electrostatic energy analyzer, a time-of-flight (TOF) technique for the plasma potential measurements by HIBD with MCAD has been elaborated on ISTTOK.
This paper describes, in detail, the design and operation of a 4-channel time-of-flight energy analyzer (TOFEA). In general, TOFEA consists of two detectors separated at some distance along the TOF path. In the ISTTOK TOFEA the TOF path between "start" (mesh) and "stop" (plate) detectors is 620 mm. The TOFEA is arranged inside the horizontal diagnostic port of tokamak. The secondary ions are deflected into the TOFEA input windows by an array of cylindrical electrostatic plates. In such a design the influence of the plasma radiation on the detectors is minimal. A set of planar electrostatic plates allows additional alignment and control of the beam. The electronics used in the TOFEA consist on charge-sensitive and fast shaping amplifiers, constant-fraction discriminator and time-to-amplitude converter. In experiments with the TOFEA the HIBD operates with a 5(10) mA of steady Cs + (Xe + ) beam extracted from the thermo-ionic source (plasma ion source) and accelerated up to 22 keV. The 0.3-1 ms pulses of the diagnostic beam are obtained by fast electrostatic scanning across the slit in primary beam-line. Preliminary results of plasma potential measurements by TOFEA will be presented. |
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