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From the source into the accelerator complex : first stage of accelerating particles

From the source into the accelerator complex : first stage of accelerating particles . Alessandra Lombardi Cern BE/ABP. 4 days …. Day 1 : Introduction lexicon and general physics Day 2 : Low Energy Beam Transport (LEBT) Day 3 : Pre-injectors Day 4 : Radio Frequency Quadrupole.

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From the source into the accelerator complex : first stage of accelerating particles

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  1. From the source into the accelerator complex : first stage of accelerating particles Alessandra Lombardi Cern BE/ABP

  2. 4 days …. • Day 1 : • Introduction lexicon and general physics • Day 2 : • Low Energy Beam Transport (LEBT) • Day 3 : • Pre-injectors • Day 4 : • Radio Frequency Quadrupole

  3. Radio Frequency Quadrupole

  4. Radio Frequency Quadrupole

  5. Radio Frequency Quadrupole cavity loaded with 4 electrodes TE210 mode

  6. RFQ Structures

  7. four vane-structure • capacitance between vanetips, inductance in the intervane space • each vane is a resonator • frequency depends on cylinder dimensions (good at freq. of the order of 200MHz, at lower frequency the diameter of the tank becomes too big) • vane tip are machined by a computer controlled milling machine. • need stabilization (problem of mixing with dipole modeTE110)

  8. four rod-structure • capacitance between rods, inductance with holding bars • each cell is a resonator • cavity dimensions are independent from the frequency, • easy to machine (lathe) • problems with end cells, less efficient than 4-vane due to strong current in the holding bars

  9. CNAO RFQ

  10. How an RFQ works 1. Four electrodes (vanes) between which we excite an RF Quadrupole mode (TE210)  Electric focusing channel, alternating gradient with the period of the RF. Note that electric focusing does not depend on the velocity (ideal at low b!) 2. The vanes have a longitudinal modulation with period = bl this creates a longitudinal component of the electric field. The modulation corresponds exactly to a series of RF gaps and provides acceleration and bunching. − + Opposite vanes (180º) Adjacent vanes (90º) 10

  11. - - + + + + - - transverse field in an RFQ alternating gradient focussing structure with period length  (in half RF period the particles have travelled a length /2 )

  12. looking into an RFQ… Looking from the RF port into the new CERN RFQ (Linac4, 2011) 12

  13. acceleration in RFQ longitudinal modulation on the electrodes creates a longitudinal component in the TE mode

  14. acceleration in an RFQ modulation X aperture aperture

  15. important parameters of the RFQ Transverse field distortion due to modulation (=1 for un-modulated electrodes) limited by sparking type of particle Accelerating efficiency : fraction of the field deviated in the longitudinal direction (=0 for un-modulated electrodes) transit time factor cell length

  16. .....and their relation focusing efficiency accelerating efficiency a=bore radius, ,=relativistic parameters, c=speed of light, f= rf frequency, I0,1=zero,first order Bessel function, k=wave number, =wavelength, m=electrode modulation, m0=rest q=charge, r= average transverse beam dimension, r0=average bore, V=vane voltage

  17. Beam dynamics design (very first approach) The beam dynamics in an RFQ determined by the geometrical parameter of the electrode structure Aperture : determines the focusing strenght and the acceptance. Depth of the modulation : determines the field availible for acceleration Distance between the peaks and the trough of the modulation determines the synchronicity between the field and the particles

  18. Electrode structure modulation X aperture aperture

  19. Transverse plane-focusing • quadrupole focusing (1) • RF defocusing ( modulation ) (2) • space charge defocusing (3) Z0 is the free-space impedance (376.73 Ohm), I is the beam current, f(p) is a geometrical factor p is the ratio of the transvese beam dimensions, r is the average transverse beam dimension, b the longitudinal . (1) (2) (3)

  20. Longitudinal plane-bunching Smootly change the velocity profile of the beam without changing its average energy

  21. Longitudinal plane-acceleration use the rising part of the RF : receive less acceleration, late particles more (PHASE FOCUSING)

  22. RFQ sections

  23. High intensity vs. low intensity LOW INTENSITY RFQS CAN BE MADE SHORTER THAN THE CORRESPONDING HIGH INTENSITY ONES

  24. HIGH INTENSITY RFQ2 ( 200 mA protons)

  25. LOW INTENSITY LEAD ION RFQ (100 A )

  26. RFQ • The resonating mode of the cavity is a focusing mode • Alternating the voltage on the electrodes produces an alternating focusing channel • A longitudinal modulation of the electrodes produces a field in the direction of propagation of the beam which bunches and accelerates the beam • Both the focusing as well as the bunching and acceleration are performed by the RF field • The RFQ is the only linear accelerator that can accept a low energy CONTINOUS beam of particles • 1970 Kapchinskij and Teplyakov propose the idea of the radiofrequency quadrupole ( I. M. Kapchinskii and V. A. Teplvakov, Prib.Tekh. Eksp. No. 2, 19 (1970))

  27. The RFQ RFQ = Focusing channel + bunching + acceleration The Radio Frequency Quadrupole (RFQ) is the first accelerator in Linac4 (3m, up to 3 MeV). It focuses bunches and accelerates the beam in a quadrupole channel inside an RF resonator. 3 MeV because below the activation threshold

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