Superconducting Cyclotron Beam Development Section


Superconducting cyclotron is a charge particle accelerator, which is generally used to accelerate heavy ions up from Fermi energy to a few hundred MeV/u. The main component of a compact superconducting cyclotron is a pill-box type dipole magnet, with a cylindrical iron structure energized by superconducting Nb-Ti coil, operating at liquid helium temperature (4.2 K). It produces high magnetic field (from 3 to 5 Tesla) to bend the charge particle beams in a near-circular orbit, which spirals out as energy of the beam increases. The energy is given to the beam by radiofrequency electric field produced by RF resonating cavities and electrodes, conventionally called as ‘dees’. At the outermost orbit the beam is pulled out of the cyclotron by electrostatic deflectors and magnetic channels. Being a compact, economic and versatile tool, the superconducting cyclotron finds applications both as machines for nuclear physics research, where flexibility of the machine in terms of various ions and ion-energies are important, and as fixed energy, fixed ion medical cyclotrons.

The superconducting cyclotron has four major components: the magnet iron structure, the cryostat housing the superconducting coils, the trim coils, and the radio-frequency (RF) system of which the three dees are a part.

Superconducting Cyclotron Structure:

Magnet Iron Structure:

The magnet iron comprising of two poles, the lower and the upper, and the return yoke is a pill-box type of structure weighing about 80 Tons. Each of its 0.654 meter radius poles has three spiral hills, each covering an angle of 46° at the outer radii. The average spiral constant is (1/33.02) rad/cm. The poles having hill-valley sectors are installed on two end-plates (top and bottom) that form a part of the return yoke. The gap between the upper and lower hills is 64 mm. The pole gap is much larger (varies radially in three steps, details given in chapter 3) in the three spiral valley regions situated between adjascent hills. This hill-valley structures create the necessary azimuthal variation of the magnetic field. The cylindrical iron return yoke extends from 1.066 meter radius to 1.524 meter radius. The whole structure has a median plane reflection symmetry. The magnet iron frame is installed on pier supporting system, leveled within an accuracy of 800 μm.

Magnet Yoke SCC Magnet with Cryostat

Superconducting Coil and Cryostat:

The cryostat sitting in the annular space (0.654 meter radius to 1.066 meter radius),between the pole and the return yoke houses the superconducting coils, usually referred to as main coils. The superconducting coils are wound on a bobbin made from stainless steel (SS316L). The coils are made of NbTi multifilament composite superconducting cable consisting of 500 filaments of 40 micron diameter embedded in copper matrix. There are two independently powered coils, namely the alpha coil and the beta coil. The liquid helium chamber is created by welding an annular SS sheet to the top and bottom edges of the bobbin all along the periphery ensuring space for enough quantity of liquid helium necessary to cool down the alpha and the beta coils to 4.2 K and for cryogenic stability. The liquid helium chamber is wrapped with several layers of multi-layer insulation (MLI) sheets and outside it there is liquid nitrogen (LN) cooled thermal shield made of copper sheet. There are several layers of MLI wrappings outside the LN-shield also. The entire coil assembly (the liquid helium chamber and the Cu thermal shield wrapped with MLI layers) is then inserted into the cryostat vacuum chamber (coil-tank).The bobbin is kept suspended inside the coil-tank with the help of nine glass-epoxy support-links. Positioning of the bobbin/coil is done by adjusting the tension in these nine support links. There are 20 radial penetrations welded on the outer surface of coil-tank at the median plane. These radial penetrations are used for inserting the drives for electrostatic deflectors and magnetic channels, beam diagnostic elements etc. Cryogenic lines and the power feedthroughs are connected from the top. To access the cyclotron beam chamber one needs to raise the upper half of the magnet.

The superconducting coil and liquid nitrogen cooled thermal shield The cryostat assembly with median plane ports

Trim coils:

There are 13 trim-coils wound around each spiral pole-tips, below and above median plane. All these 78 trim coils are made of water-cooled copper conductor. The current leads of these trim coils come out through cicular holes in the pole-base and pole-cap. The upper and lower pairs of a particular trim-coil in all three sectors, i.e., six trim coils at a particular radius are connected in series. Trim coil 1 and 13 can be used to produce harmonic fields other than their natural 3N harmonics (N=3) and average field contribution. There is a circular trim coil at the center of the magnet, called trim-coil #0, mounted on the central-plug.The trim coils are mounted on the pole tip and then vacuum impregnated with epoxy resin.

Radio-Frequency System and Dees:

The radiofrequency system comprises of three half-wave co-axial cavities made of copper, placed axially (vertically) with an angular distance of 120° between them. In this structure, the dees and the dee stems act as the inner conductor and the liner on the pole and the hexagonal panels as the outer conductor. The half-wave cavities are actually combinations of two quarter wave cavities, one coming from the top and the other from the bottom and symmetrically placed around the cyclotron median plane. The quarter wave transmission lines’ inner conductors are terminated in three spiral Dees, each of 60° aziumthal width, situated in the three spiral valley regions. Portions of the cavities that extend out of the magnet iron in the axial direction above and below are separated from the cyclotron vacuum chamber by cylindrical ceramic windows. When the particle revolution frequency and the RF frequency match, the ions get accelerated once while entering a dee and again while leaving the same dee. The RF system is designed for a maximum dee voltage of about 100 kV and for a frequency range from 9 to 27 MHz that is achieved by moving a sliding short provided in each of the six quarter wave cavities.


Publication

Recent Publications in Journals:

1. Malay Kanti Dey, Anjan Dutta Gupta, Alok Chakrabarti, “Novel Compact Superconducting Cyclotron for Medical Applications”, Physical Review Special Topics-Accelerators and Beams, 16, 040101 (2013).

2. Tanushyam Bhattacharjee, Malay Kanti Dey, Partha Dhara, Suvodeep Roy, Jayanta Debnath, Rajendra Balakrishna Bhole, Atanu Dutta, S Pal, Amitava Roy, Gautam Pal and Alok Chakrabarti, “Development of a fast scintillator based beam phase measurement system for compact superconducting cyclotrons”, Review of Scientific Instruments, 84, 053303 (2013).

3. Malay Kanti Dey, Anjan Dutta Gupta and Alok Chakrabarti, “Design of ultra-light superconducting proton cyclotron for production of isotopes for medical applications”, Proceedings of the 20th International Conference on Cyclotrons and Their Applications, 16-20 September 2013, TRIUMF, Vancouver, Canada.

4. Malay Kanti Dey, Rakesh Kumar Bhandari, et. al. “Beam Tuning in Kolkata Superconducting Cyclotron”, Proceedings of the 19th International Conference on Cyclotrons and Their Applications, 6-9 October 2010, Lanzhou, China.

5. Malay Kanti Dey, Rakesh Kumar Bhandari et al., “Beam Injection System of the Kolkata Super-conducting Cyclotron”: Proceedings of the 18th International Conference on Cyclotrons and Their Applications 2007, Oct. 1-5, 2007, Giardini Naxos, Italy, page 346.

6. Malay Kanti Dey, Rakesh Kumar Bhandari, et. al., “Coil Centering of the Kolkata Superconducting Cyclotron Magnet”: Proceedings of the 18th Int. Conf. on Cyclotrons and Their Applications 2007, Oct. 1-5, 2007, Giardini Naxos, Italy, page 438.

7. Chaturanan Mallik, Malay Kanti Dey, Rakesh Kumar Bhandari, et al., “Magnetic Field Mapping of Kolkata Superconducting Cyclotron”: Proceedings of the 18th Int.Conf. on Cyclotrons and Their Applications 2007, Oct. 1-5, 2007, Giardini Naxos, Italy, page 435.

    

Staff
Malay Kanty Dey Santanu Paul Atanu Dutta Uttam Bhunia Jedidiah Pradhan
Md. Zamal A.Naser Vinay Singh Chiranjib Das Ankur Agrawal Sourav Shit
Vikas Tiwari Sunil Kumar Anwar Ali Amar Nath
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