Improvements of the ion beam quality of the Magurele FN tandem accelerator

The Pelletron charging chain has been installed in 2007 at the Tandem accelerator of the National Institute of Physics and Nuclear Engineering, Magurele. The chain does not limit the ultimate terminal potential, and it is in use in electrostatic accelerators up to 9 MV, and has excellent voltage stability; no spark damage, intrinsically protected; high efficiency; isolation from line voltage ripple. A new generating volt meter was developed to get more accuracy for the terminal voltage measurement and tandem stabilization, running under GVM control only. The GVM consists of two signal plates and a grounded rotating plate. Alternatively the rotating plate covers and uncovers the signal plates so that, due to the alternating field, a current is induced to the signal plates. This current I follows the equation: I - UT dC/dt, where UT is the terminal voltage and C is the terminal-signal plate capacitance. The rotating plate covers more than the area of one signal plate, resulting in a capacitance, C1 and C2 respectively. Therefore the readout signal i1 and i2 is shaped in that way that its derivative is zero at zero crossing. So the readout electronic can be slow and there is enough time to control the electronic switches. The motor axe is grounded via a massive collector. Position holes in the rotating plate are read out by a photo-logic sensor to control the complete read out electronics. The positive and negative half cycles of the input signals i1 end i2 are integrated in different capacitors and in the next state they are sampled. An output amplifier makes the difference of the sampled voltage. The output voltage, representing the terminal voltage, is independent of the revolution speed. Only the temperature coefficient of the integration capacitors influences the accuracy of measurement. A stability of the tandem voltage better than 1 kV is achieved. To eliminate the effect of the temperature drift of the capacitors it is planned to use the integrators as comparators. Current pulses, representing the nominal value of the terminal voltage, with inverse polarity to the input signals, will be fed to the integrators. At the output we will get the voltage error that controls the voltage stabilizer circuit. (author)