# Cyclotrons Yasser Nour ElDin Mohammed Group 2 Accelerators

• Slides: 20

Cyclotrons Yasser Nour El-Din Mohammed Group 2 - Accelerators and Applications Supervisors B. N. Gikal O. B. Borisov Flerov Laboratory of Nuclear Reactions Cairo University Electrical Engineering – Faculty of Engineering

Outlines • • • Introduction. AVF Cyclotrons U 400, U 400 M. Resonators and Dees Beam Extraction. Cairo University Electrical Engineering – Faculty of Engineering 2

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

Classical Cyclotron Cairo University Electrical Engineering – Faculty of Engineering 4

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: Cairo University Electrical Engineering – Faculty of Engineering -1 < k < 0 where: 5

Problem of Classical Cyclotron – cont. One solution is : Azimuthally varying field (AVF) Cairo University Electrical Engineering – Faculty of Engineering 6

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 Cairo University Electrical Engineering – Faculty of Engineering 7

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

U 400 Straight Sector Cyclotron Parameters of U 400 cyclotron Rpole = 200 cm 4 sectors (45 o straight boundaries) Bo =1. 9 - 2. 14 T W = 0. 5 - 20 Me. V/n Dee Voltage = 80 k. V Freq. Range = 5. 48 – 12. 2 Mhz Cairo University Electrical Engineering – Faculty of Engineering 9

U 400 M Spiral Sector Cyclotron Parameters of U 400 M cyclotron Rpole = 200 cm 4 sectors (45 o , max spiral angle ~ 40 o) Bo =1. 5 - 1. 9 T A/Z = 2 - 5 W = 6 - 100 Me. V/n Cairo University Electrical Engineering – Faculty of Engineering 10

Cyclotron System Power Supply Ion Source Control System RF Resonator Accelerating Structure (Dees) Magnetic Field Cairo University Electrical Engineering – Faculty of Engineering Beam Extraction Vacuum system 11

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. Cairo University Electrical Engineering – Faculty of Engineering 12

RF resonators and Dees Dee positioned in 2 Vally regions. Cairo University Electrical Engineering – Faculty of Engineering First half gap focusing Second half gap defocusing Total focusing effect in the gap (Phase focusing) 13

Cyclotron System Power Supply Ion Source Control System RF Resonator Accelerating Structure (Dees) Magnetic field Cairo University Electrical Engineering – Faculty of Engineering Beam Extraction Vacuum system 14

Beam Extraction – Charge Exchange Method Accelerated ion with charge Z 1 passes trough a thin carbon foil (40 -80 µg/cm 2). The charge ion after foil is equal Z 2 because of the electrons stripping. Z 2 / Z 1 – called ratio of chargeexchange. After stripping foil the ion with charge Z 2 have a more rigidty and can be extracted from the cyclotron. Cairo University Electrical Engineering – Faculty of Engineering 15

Extraction from U 400 Cyclotron Cairo University Electrical Engineering – Faculty of Engineering 16

Extraction from U 400 M Cyclotron Cairo University Electrical Engineering – Faculty of Engineering 17

Charge Stripping Foil Cairo University Electrical Engineering – Faculty of Engineering 18

Cyclotron System Power Supply Ion Source Control System RF Resonator Accelerating Structure (Dees) Magnetic field Cairo University Electrical Engineering – Faculty of Engineering Beam Extraction Vacuum system 19

Thanks Cairo University Electrical Engineering – Faculty of Engineering 20