The Pelletron Accelerator and Associated Facilities

 The Pelletron Accelerator [1] was installed in December 1988 and experiments using the beam were started in the middle of 1989. The source for the charged particles is located at the top of the accelerator tower. A Cesium sputter ion source generates negative ions, which are initially accelerated to low energies (150-250 keV) in a short horizontal section. These low energy negative ions are then bent through 90o using an injector magnet into the vertical accelerating column. In the first stage, the acceleration results from the electrostatic attraction of the negative ions by the positively charged high voltage terminal situated at the center of the column. The high electric potential at the terminal is achieved by a continuous transfer of charge to the terminal by means of the chain of steel pellets and hence the name Pelletron accelerator. Inside the terminal the ions pass through a thin carbon foil or a small volume of a gas, where they lose electrons and acquire a high positive charge. The average charge of the ion depends upon the type of ion and the terminal voltage. The resulting positive ions now enter the second or high energy stage of acceleration where the positive voltage of the terminal acts repulsively on the positive ions. In this way, the final energy of the ion that has acquired a positive charge of n units will be (n +1)V, where V is the terminal voltage (maximum of 14 MV). For example, the final energy at the maximum terminal voltage for 12C6+ is 98 MeV.  Beams ranging from protons to Uranium can be accelerated.  In addition to continuous beams, pulsed beams of ~1 ns width separated by about 100 ns to 1.6 ms have also been made available by installing a double harmonic buncher in the low energy section [2]. For some atomic physics experiments a post accelerator foil strippe [3] is installed to provide fully stripped beams up to Cl.

Five beam lines are laid in the main beam hall .The beam transport system on these lines is remotely controlled using a CAMAC system and integrated with the indigenously developed PC based control system of the main accelerator [4]. The 300 N beam line is mainly used for irradiation. Besides the routine operation and maintenance of the accelerator a major effort of the machine staff has been made towards indigenous development of accelerator components and facilities.

Some of the major developments at the accelerator laboratory are charging chain safety, shorting rod guide, emergency power setup, UPS, roots pump and power distribution centres. It is found that the pellet chain elongates due to creep, which can result in a chain failure. A micro-switch and position read back has been designed and installed in order to prevent chain break. It was noticed that the shorting rod system at the low energy often failed due to improper alignment. A new guide system was designed and installed such that the rod follows a prescribed path thus eliminating possibility of failure. A charging chain test facility was constructed in order to carry out electrical and mechanical tests on indigenously developed pellet chains.  Maintaining good vacuum in the entire beam line is very much required. Most of the vacuum pumps are now connected with emergency power (diesel generator set) so that during main power failure, emergency power takes over within 20 secs. The generator set also supplies to the computer and CAMAC systems in the control room and to other essential services.

An ion source test bench facility  has been set up for the development of the negative ion sources and sputter cathode samples to obtain higher intensities of negative ion species throughout the periodic table with good quality emittance. The facility consists of an ion source modular assembly, a cylindrical Einzel lens, an accelerating tube (75 KV), a double focusing 90o analyzing magnet having mean bending radius of 50 cm, and a mass energy product of about 8 amu-MeV. Beam diagnostic elements like BPM (Beam Profile Monitor), Faraday cup, horizontally adjusted single slits and steerers are also included in the test stand.

The target laboratory has been set up which not only provides the stripper foils for the accelerator but also caters to the important requirement of preparing targets for the users. Besides the DC glow discharge set up for producing the thin carbon foils of thickness 3-5 mg/cm2 used as stripper foils, other facilities in this laboratory include evaporation set up using resistance and electron beam heating, a carbon arc which provides carbon foils as backing for thin targets, a centrifugal deposition apparatus and a rolling mill.

In order to test/calibrate  accelerator devices  (e.g. vacuum electronics, power supplies, device controllers, fiber optic data telemetry, terminal potential stabilizer system etc.) either in the laboratory or in-situ, modules  like  Glassman high voltage test module, nanoamp log amplifier test module, Faraday cup controller test module  etc. have been developed to generate simulated signals.

The Pelletron Accelerator produces radiation when the ion beam is extracted, and the accelerator tower and beam hall are radiation prone areas. For monitoring the radiation levels, neutron and gamma ray radiation monitors are installed at key places along the accelerator, inside and outside, at several landings and approaches of the accelerator, as well as inside and outside the beam hall. A search and secure interlock system is installed for the personal safety of the operation staff. This system is interlocked with the main accelerator control system. Before the accelerator is started the whole area is searched and locked. An additional radiation alarm system consisting of an audio visual alarm and emergency scram buttons is installed in the beam hall for the safety of personnel working there.

 References

  1. K.G. Prasad Nucl. Instrum. Meth. B40/41, 916 (1989).
  2. H.C. Jain, U.T. Raheja, P.J. Bhalerao, V.M. Datar and M.Y. Vaze, DAE Symp. Nucl. Phys. 34B, 383 (1991), Indian Journal of Physics
  3. S.D. Narvekar, R.R. Hosangdi, L.C. Tribedi, R.G. Pillay, K.G. Prasad and P.N. Tandon, Pramana J. Phys. 39, 79 (1992).
  4. S.A. Kori, M.M. Date, S.K. Kataria, R.P. Thakkar, R.R. Hosangdi, S.K. Sarkar and S.K.Mitra, Int. Conf. on Data Acquisition and Control of Accelerators, VECC, Calcutta, 155 (1991).

 

Contact  Dr.A.K.Gupta  for further information

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