ELI and other things A. D’Elia 1

C-BAND STRUCTURES FOR MULTI-BUNCH RF LINACS: ELI_NP PROPOSAL Bunch charge 250 pC Number of bunches 40 Bunch distance 15 ns C-band average accelerating gradient 35 MV/m Norm. emittance 0.4 mm  mrad Bunch length <300  m RF rep Rate 100 In the context of the ELI-NP Research Infrastructure, to be built at Magurele (Bucharest, Romania), an advanced Source of Gamma-ray photons is planned, capable to produce beams of mono-chromatic and high spectral density gamma photons. The Gamma Beam System is based on a Compton back-scattering source. Its main specifications are: photon energy tunable in the range 1-20 MeV, rms bandwidth smaller than 0.3% and spectral density larger than 10 4 photons/sec.eV, with source spot sizes smaller than 50 microns. 14 C-band cavities 85 cells each (1.7 m long) S-Band From David Alesini, RF Structure Development Meeting, July 18 th 2012 Full talk is here:

BBU Results Hypothesis: 1.Initial condition at linac injection equal for all bunches 2.Constant  -function 3.all transverse wakes that decays with the quality factor of the mode 4.One single mode trapped in each cell R T /Q26 Ω Q11000 f res GHz W T (  f 2 ) 245 V/m/pC w T =W T /L (  f 3 ) 14 kV/m 2 /pC tracking Tracking & Mosnier From David Alesini, RF Structure Development Meeting, July 18 th 2012 Full talk is here:

Advantages 1. Strong damping of all modes above waveguide cut-off 2. Possibility of tuning the cells 3. Good cooling possibility Disadvantages 1.Machining: need a 3D milling machine 2.Multipole field components (octupole) but not critical at least for CLIC Advantages 1.Easy machining of cells (turning) 2.2D geometry: no multipole field components Disadvantages 1. Critical e.m. design: notch filter can reflect also other modes. 2. Not possible to tune the structure 4. Cooling at 100 Hz, long pulse length (?) Damping of dipole modes choice Dipoles modes propagate in the waveguide and dissipate into a load CLIC structures X-band, high gradient C-Band structures Spring-8 From David Alesini, RF Structure Development Meeting, July 18 th 2012 Full talk is here:

ELI Damped structure: parameters PARAMETERVALUE TypeTW-constant impedance 85 cells Frequency (f RF )5.712 [GHz] Phase advance per cell 2  /3 Structure Length included couplers (L)1.7 m Iris aperture (a)6.5 mm group velocity (v g /c):0.022 Quality factor (Q)8830 Field attenuation (  ) 0.31 [1/m] series impedance (Z) 45 [M  /m 2 ] Shunt impedance per unit length (r) 72 [M  /m] Filling time (  ) 230 [ns] Power E acc =35 MV/m27 [MW] E s peak /E acc 2.1 H s peak /E acc 4.3  [A/V] Minimum RF pulse length (  IMP )0.85 [  s] Output power 0.39  P in E P IN =40 MW34 MV/m Pulsed P IN =40 MW9 o C Accelerating field unbalance (E IN, E OUT )42.4MV/m, MV/m Average dissipated 100 Hz, P IN =40 MW2 [kW] From David Alesini, RF Structure Development Meeting, July 18 th 2012 Full talk is here:

GdFidL Simulations (1/3) -Several simulations have been done assuming perfect matches waveguides 20 cells + 2 couplers  =5mm Mesh_step=500  m From David Alesini, RF Structure Development Meeting, July 18 th 2012 Full talk is here:

Comments 7 My comment is that they are using a bazooka to kill a fly and Walter added “using also wrong bullets”. From our side we propose two possible alternatives: Purely Detuned structure (no damping): – Cons: higher wake, but to be understood what is the limit – Pro: very chip structure (according to Igor about a factor 2 of reduction in price) – Pro: possibility of reaching very high precision machining (this will impact on the cost) with the possibility of simplifying or better avoiding tuning – Pro: No RF loads DDS – Pro: wake is very low, with the possibility of further optimization in terms of power consuption and/or machining semplification – Pro: Diagnostic for free, but if this is important has to be discussed with beam dynamics people; – Pro/Cons: machining is more complicated with respect to the purely detuned structure, but rather similar with respect waveguide damping; considering the reduction of the load it should be Pro with respect to waveguide damping (to be checked, anyway)

-With 16 GHz mode -Without 16 GHz mode 1 st Option: Pure detuning 8 a t1.0 b [mm] f [GHz] Q(Cu) vg/c [%] r’/Q [LinacΩ/m] r’ [MΩ/m] Es/Ea Hs/Ea [mA/V] Thanks to Vasim 21 Cells 21x4 Cells Wake limit

2 nd Option: DDS 9 WGW SlotW WGH SlotH Htot ParametersFirst CellMid CellLast Cell a (mm) L (mm) t (mm)321 eps222 b (mm) WGW (mm)12 WGH (mm)10 SlotW (mm)777 SlotH (mm) InSlot (mm)222 Htot (mm) fsyn (GHz)    Av. Cross (MHz) Vg (%c) R/Q (k  /m) Monopole=5.172GHz (2  /3 phase advance) 1 st Dipole band~500MHz NB: this is not optimized in terms of input power, however for first cell with this design we get Pin~33MW to get Eacc=35MV/m

2 nd Option: DDS 10 2  ~110MHz  F~500MHz Notice: the wake produced by the simple rectangle might be already enough

GdfidL-26 Cells Linear Tapering 11 Wake limit

Direct-Indirect calculation of transverse kick (TM11) 12 Indirect V/pC/mm/m - Offset mm 0.5 Direct V/pC/mm/m Indirect V/pC/mm/m Direct V/pC/mm/m Indirect V/pC/mm/m Direct V/pC/mm/m Ez Z*HxEy Ez Z*Hx Ey Ez Z*Hx Ey

Direct V  13  = 0.5mm =0=0   V  = V  = Are the transverse fields, in a close region around the beam axis, radially independent in TM11?

Preliminary conclusions Next time I will put down all the formulas and I will explain better how to do direct-indirect calculation (it is not so straightforward for transverse kick) With TM11 it works consistently fine With TE mode it works but I need to fix some final detail Preliminary on TE I can say presently: – Direct calculation seems giving consistent results as found in CST/GdfidL – Indirect calculation must be amended with the proper formula – Very important: one cell gives anyway wrong results, two or more cells are needed to get correct values 14