Design and RF Measurements of an X-band Accelerating Structure for the Sparc Project

Transcription

1 Design and RF Measurements of an X-band Accelerating Structure for the Sparc Project INFN-LNF ; UNIVERSITY OF ROME LA SAPIENZA ; INFN - MI Presented by BRUNO SPATARO Erice, Sicily, October 9-14; 2005

2 SALAF (Strutture Acceleranti Lineari ad Alta Frequenza) is the INFN r&d programm on multicell resonating structures operating at X-band (10 12 GHz). the MOTIVATION. To use in high brilliance photo-injectors (SPARC-phase-2) to compensate for the beam longitudinal phase-space distortion, enhanced by the bunch compression of the acceleration process To gain know-how in vacuum microwave technologies RF GUN X-band structure RF compressor Traveling Wave accelerating structures

3 Correction of the P h a s e-space distortion! The 4th harmonic structure provides RF curvature local correction.! Beam longitudinal emittance hold within limited values.! Minimum bunch length achieved. without 4th HARMONICS before compression after compression time evolution of the beam phase-space with RF compression + 4th Harmonics

4 SUMMARY of the ACTIVITY ElectroMagnetic Design Travelling or Standing Wave structure? Mode of operation! or!/2? Hardware Design! Construction of a GHz Cu prototype! RF characterization! Development and test of a real model:! precise machining! brazing! vacuum tests! RF tests

5 E.M. DESIGN Recent experiences (NLC) have been made at SLAC and KEK Travelling W a v e X-band structures. RF IN I N fully matched TW acc. section RF OUT BEAM LOAD MultiMegawatts peak power tests, carried out at SLAC, had been showing frequent internal discharges caused by field emission. BEAM OUT OUT

6 TW structures! the fully matched condition of TW structures helps the RF generator in feeding the resonator under discharge. and sustains the power flow throughout it.! hence, the inner surface may get damaged.! the design accelerating gradient (60 70 MV/m) may not be achieved routinely. To overcome the problem, instead of TW units, Standing Wave structures can be used. Internal discharges cause strong cavity detuning and generator mismatching. RF POWER FEEDING! IMPOSSIBLE under discharge

7 Standing Wave structures the Standing Wave accelerating section is a diskloaded guide with the output port shorted. reverse power RF IN forward power Input RF matching is only possible if the RF power distribution is unperturbed. Field emission discharges detune the SW cavity by several bandwidths. Risk of damaging the structure is reduced. reverse power the Standing Wave section iis considered the best structure to to achieve reliability and high field operatio at at X-band

8 R &D ai ms to an al yz e in detai ls. ax ia ll y co upled! a nd!/2 mod e SW s tr uc tur es a nd 2!/3 and 5!/6 mod e TW st ru ctu re s Important R&D goals are. To study the accelerating structure sensitivity vs mechanical tolerances and assembling errors To investigate the effects of the power dissipation on the general performances of the structure.

9 E.M. design guide-lines of 11 GHz accelerating structures!!!!!!! To get high accelerating field per unit-length t o shorten the structure To get high shunt impedance to reduce the need of RF pow To get the lowest E p / E 0 and B p / E 0 ratios to minimize dark c achieve high break-down free field gradient and low thermal effect To get high ratio E 02 /W to optimize the efficiency of the st To re d u c e accelerating structure sensitivity vs mechanical tolerances and assembling errors by increasing the group velocity (i.e. filling time) To reduce the parasitic mode content which affect the bea dynamics To shape the structure internal profile in order to avoid multipactoring discharges

10 Study and simulation of a 9-cell!-mode X-band struc Simulated structure with no coupling tubes r = mm p = mm h = 2 mm r 2 = 4 mm p h r 2! Symmetry planes r 2 /" = 0.15 Simulated structure with coupling tubes p = mm h= 2 mm r 2 = 4 mm r = mm (End Cell) r = mm (Central Cells) p h r 2 r = mm (End Cell) r 1 = 1mm

11 some basic expressions of the disk-loaded waveguide Analytical expression of the dispersion curve: " # ! = " % (" $ " 0 ) % (1 $ cos! ) Coupling coefficient: K = (_! -_ 0 ) / _!/2 K = 2.42 % Analytical relation of the coupling coefficient: (based upon geometric parameters) Coupling coefficient: with k + 3# J 2 1 K 4a 3 = (2.405) b!# $! 0 $ " h! 2 l! / 2 << 1, ke and " = * ( ) h = 2 mm K = 2.27 % ' % a & 2! $ 2 c very similar values 2

12 simulation of 9-cell!-mode. structure with mirrors Frequency [MHz] Mode [!] DISPERSION CURVE with and without beam-tubes f_mirrors f_analytical (!) f_with_tubes / / / / / / / Frequency [MHz] structure with tubes Mode [p] mode1,!, mode3 Mode / / / / / /3 h = 2 mm / /

13 Ez simulation of 9-cell!-mode ! Z (cm) With beam-tubes and constant cavity radius no flatness of the on-axis longitudinal E-field Ez 60 Ez (MV/m) ! Z (cm) With beam-tubes and reduced end-cells radius flatness of the on-axis longitudinal E-field

14 simulation of 9-cell!-mode X-band structure with h = 3 mm f_mirrors f_analytical (!) f_with_tubes "_mirrors "_Analytical(!) "_with_tubes h =3 mm Mode Modo,!, Modo3 Coupling coefficient: k,!, k $ 10 2 K = 1.76 % (with h = 3 mm the thickness iris) structure with mirrors Frequency Mode [p] [MHz] / / / / / / / structure with tubes Frequency Mode [p] [MHz] / / / / / / / /

15 simulation of 9-cell!-mode.. Comparison of dispersion curves for h = 2 and 3 mm thickness frequencies_2mm f_2mm( x) frequenze_3mm f_3mm( x) K=1.76% h K=2.42% mode_2mm, x, mode_3mm, x

16 simulation of 9-cell!-mode.. Thermal flux and temperature fields of the boundary region of a copper structure. Study performed with the ANSYS-code (TM).

17 simulation of 9-cell!-mode.. Comparison of Standing-W a v e X-BAND Structures with different disktickness RF parameter list calculated with SUPERFISH, OSCAR2D and ABCI codes

18 1 2 3D coupler design (HFSS) The radius of the central coupling cell has b e e n retuned to compensate for the perturbation induced by t h e coupling hole; mm 4 mm The waveguide input port is connected to an X Band standard waveguide by a tapered section of 200 mm. 4 mm 6.98 mm 1.6 mm

19 Results Field flatness after retuning the central cell radius Nearest mode excited by the coupler (3/4!) Accelerating mode S11=0.06#$=1.12

20 Tapered section The wa ve g uide input port is connected to an X Band standard waveguide with a 200 mm long t a pe re d section. The re flection co effic ien t (ob ta ined wit h HF SS) of th e tape re d sectio n is of the or de r of mm 4 mm mm mm Standard X BAND waveguide 200 mm Waveguide at the coupler input port

21 Tapered section 200 mm HFSS project

22 CONSTRUCTION of a!-mode STANDING-WAVE 11.4 GHz COPPER PROTOTYPE SINGLE ELEMENTS RF INPUT SLOT ASSEMBLED X-BAND MODEL TAPERED COUPLER TIGHTENING BARS TUNING RODS

23 F CHARACTERIZATION OF THE X-BAND STANDING-WAVE COPPER MODE RF MEASUREMENT SET-UP PULLEY STEP- MOTOR BEAD-PULL MEASUREMENT SET-UP

24 E 2 / E M!-mode ACCELERATING ELECTRIC FIELD BEHAVIOR AFTER the 9-CELL TUNING THE FIELD FLATNESS IS < 1%!-mode BEAM AXIS ELECTRIC FIELD In green, the CONTRIBUTION of the WIRE and the BEAD GLUE

25 BEHAVIOUR OF OTHER LONGITUDINAL E-FIELD FUNDAMENTAL MODES 0-mode!/4-mode!/2-mode

26 DETECTION OF THE FUNDAMENTAL MODE RESONANCES WITH AN ANTENNA RF IN s11!-mode GHz NETWORK ANALYZER LONGITUDINAL INDUCED MODES

27 DETECTION OF THE FUNDAMENTAL MODE RESONANCES BY THE INPUT COUPLER NETWORK ANALYZER s11 RF IN INPUT COUPLER INDUCED MODES!-mode GHz

28 X-BAND COPPER PROTOTYPE MAIN PARAMETERS!-mode frequency Form factor r/q Unloaded Q External Q E-Field flatness Number of cells Structure length CONCLUSIONS GHz 9400 %/m (9165 ) 7960 (8413 ) 8000 < 1 % mm In red, the theoretical values FIRST R&D ACTIVITY on X-BAND STANDING WAVE DISK-LOADED STRUCTU STARTED SUCCESSFULLY. IMPORTANT KNOW-HOW ASPECTS of the E.M DESIGN and FABRICATION of X-BAND ACCELERATING SECTIONS HAVE BEEN ACQUIRED FUTURE ACTIVITY : CARRY OUT BRAZING PROCESS t o VERIFY the VACUUM PERFORMANCES of a CAVITY DEVELOPE a!/2-mode STRUCTURE TO CH E.M. SENSITIVITY vs FABRICATION ERROR