Cyclotrons

1. Yasser Nour El-Din Mohammed Group 2 - Accelerators and Applications Supervisors B.N. Gikal O.B. Borisov Flerov Laboratory of Nuclear Reactions Cyclotrons

2. Outlines • Introduction. • AVF Cyclotrons. • Cyclotrons U400, U400M. • Resonators and Dees • Beam Extraction.

3. Introduction • Electric field used for acceleration. • Magnetic field used for bending/deflection. No energy change – Can't accelerate. • In homogeneous magnetic field B, the particle has a circular orbit of radius R. R “For constant q and m, and uniform B, the angular frequency ω remain constant independent of particle speed (Energy)”

4. Classical Cyclotron

5. Problem of Classical Cyclotron • Increasing energy and therefore increasing mass “m” (Relativistic particles) requires rising B as radius of beam circle increase to keep angular frequency (and so frequency of Dees voltage) constant. • But increasing magnetic field B with radius r will make axial instability. • Axial Stability Condition: -1 < k < 0 where:

6. Problem of Classical Cyclotron – cont. • One solution is : • Azimuthally varying field (AVF)

7. AVF Radial-sector Cyclotrons • Strong field Bh in the “hills” and weak field Bv in the “valleys” leads to a scalloped not circular orbit, which crosses the sector edges at a small “Thomas angle” κ. • Radial vr and azimuthal Bθ components yield an axial restoring focusing force Fz(θ). • As a result there is a positive edge-focusing effect when both entering and leaving a hill, each edge acting as a thin focusing lens with a focal length given by

8. AVF Spiral-sector Cyclotrons • D. Kerst 1956 suggested to use spiral sectors to increase axial focusing. • edge-crossing angles: k + ε :strong focusing lens. or k - ε : weak defocusing lens. • Focal Length becomes: Spiral angle ε

9. U400 Straight Sector Cyclotron • Parameters of U400 cyclotron • Rpole = 200 cm • 4 sectors (45ostraight boundaries) • Bo =1.9 - 2.14 T • W = 0.5 - 20 MeV/n • Dee Voltage = 80 kV • Freq. Range = 5.48 – 12.2 Mhz

10. U400M Spiral Sector Cyclotron • Parameters of U400M cyclotron • Rpole = 200 cm • 4 sectors (45o , max spiral angle ~ 40o) • Bo =1.5 - 1.9 T • A/Z = 2 - 5 • W = 6 - 100 MeV/n

11. Cyclotron System Power Supply Control System RF Resonator Beam Extraction Accelerating Structure (Dees) Ion Source Magnetic Field Vacuum system

12. RF Resonators and Dees • Resonator is the source of the accelerating electric field. • The Dees are (capacitive) electrodes, inserted into the magnet gap, usually in the ‘valley’ of the pole plates. • A shorted coaxial part (inductive part) extends to the outside of the magnet yoke, thus completing the structures to λ/4 resonators.

13. RF resonators and Dees Second half gap defocusing First half gap focusing Dee positioned in 2 Vally regions. Total focusing effect in the gap (Phase focusing)

14. Cyclotron System Power Supply Control System RF Resonator Beam Extraction Accelerating Structure (Dees) Ion Source Magnetic field Vacuum system

15. Beam Extraction – Charge Exchange Method • Accelerated ion with charge Z1 passes trough a thin carbon foil (40-80 µg/cm2). The charge ion after foil is equal Z2 because of the electrons stripping. • Z2 / Z1 – called ratio of charge-exchange. • After stripping foil the ion with charge Z2 have a more rigidty and can be extracted from the cyclotron.

16. Extraction from U400 Cyclotron

17. Extraction from U400M Cyclotron

18. Charge Stripping Foil

19. Cyclotron System Power Supply Control System RF Resonator Beam Extraction Accelerating Structure (Dees) Ion Source Magnetic field Vacuum system

20. Thanks