B. Marchetti R. Assmann, U. Dorda, J. Grebenyuk, Y. Nie, J. Zhu Acknowledgements: C. Behrens, R. Brinkmann, K. Flöttmann, M. Hüning, H. Schlarb M. Ferrario, A. Bacci Status of the design of ARES (Accelerator Research Experiment at Sinbad)

ARES layout 12.9m 27.3m 18.3m REGAE-like RF gun GHz Beam final energy~5MeV LINAC II –like Traveling wave GHz To be used as RF compressor LINAC II –like Traveling wave GHz Total av. Gradient~80MV/100MV Gun solenoid TW solenoids (preliminary design) Xband linearizer Standing wave GHz Total av. Gradient~5MV LINAC II –like Traveling wave GHz Total av. Gradient~80MV/100MV Matching section + bunch compressor (containing a slit) R56= 0-30 mm Experiment area Diagnostic section Xband TDS GHz Total transverse voltage > 40MV

Beam parameters Goal parameters for external injection into plasma: E-bunch energy 100 MeV E-bunch length ≤ 1fs Arrival time jitter ≤ 10 fs Transverse position jitter ≤ few μm Energy upgrade: E-bunch energy MeV Other e- beam parameters: Charge: pC Energy spread: % Transverse emittance < 0.5 mm*mrad

POSSIBLE COMPRESSION SETUPS

Mode 1– pure velocity bunching compression 5 MeV COMPRESSION + ACCELERATION ACCELERATION MeV MeV Final bunch length Final peak current Low charge ( pC) 1-7 fs ̴ 100 A Intermediate charge (20-50 pC) fs ̴ 1000 A High charge ( pC) < 1 ps> 1000 A

Mode 1– pure velocity bunching compression 5 MeV COMPRESSION + ACCELERATION ACCELERATION MeV MeV Final bunch length Final peak current Low charge ( pC) 1-7 fs ̴ 100 A Intermediate charge (20-50 pC) fs ̴ 1000 A High charge ( pC) < 1 ps> 1000 A In this presentation some preliminary simulations

Advantages: ◦ Relatively simple ◦ High currents can be reached Limits: ◦ 1 fs FWHM bunch length for Q= pC… Possible? ◦ Tolerances 5 MeV COMPRESSION + ACCELERATION ACCELERATION MeV MeV Mode 1– pure velocity bunching compression

Mode 1b– hybrid compression: velocity bunching + chicane 5 MeV COMPRESSION + ACCELERATION ACCELERATION MeV MeV COMPENSATION OF THE SPACE CHARGE CHIRP OCCURRED DURING THE TRANSPORT Advantages: ◦ The beam can be re-compressed right in front of the target Limits: ◦ Bunch compressor with low charge: challenging. Not studied yet!

Mode 2– pure magnetic compression with cut 5 MeV ACCELERATION+ CHIRP 50 MeV CUT OF THE BEAM IN THE DISPERSIVE ARM

Preliminary! Slides by Jun Zhu

Preliminary! Slides by Jun Zhu

Preliminary! Slides by Jun Zhu

Mode 2– pure magnetic compression with cut 5 MeV ACCELERATION+ CHIRP 50 MeV CUT OF THE BEAM IN THE DISPERSIVE ARM Advantages: ◦ Most promising method to get bunches shorter than 1 fs ◦ Easy transport in the BC: the beam has high charge before the cut Limits: ◦ Destructive method: limit on the peak current achievable set by wake-fields. ACCELERATION+ CHIRP

Mode 3– compression of a train of bunches Laser on cathode

Mode 3– compression of a train of bunches E-bunch at the gun exit

Mode 3– compression of a train of bunches E-bunch at the linac exit See theoretical and experimental studies done at SPARC (INFN LnF, ITALY)

Mode 3– compression of a train of bunches

… Alternative way…

COMPRESSION USING PURE VELOCITY BUNCHING

Transverse beam envelope equation: Matching condition delivering constant envelope in the accelerator: (assumption I=I 0 * γ/ γ 0 ) PRL 104, (2010) k= e*B sol /(mc) γ΄~2E acc I A =17 kA SC External focusing Emittance pressure Valid for an ellipsoidal bunch distribution with a constant volume density charge. Adiabatic damping term In the transverse plane we want to control the emittance oscillations Mismatches between the space charge correlated forces and the external focusing gradient produce slice envelope oscillations that cause normalize emittance oscillations.

PRSTAB 8, (2005) NIM A 740 (2014) β normalized velocity γ 0 ΄=eE acc /(m 0 c 2 ) Longitudinal laminarity parameter: In the longitudinal plane we want: The RF longitudinal focusing to be stronger than the longitudinal space charge force at the beginning of the compression The longitudinal space charge force to be strong enough to prevent slice crossover in the last part of the compression (i.e. we want to preserve the longitudinal laminarity of the beam) Longitudinal beam envelope equation: RF compression SC Emittance pressure The strength of the longitudinal space charge can be modulated by tuning the transverse spot size.

Preliminary simulations Gaussian longitudinal laser profile – not optimized at all No linearizing cavity

Q=0.5pC E=110 MeV DeltaE/E=1.4% zFWHM ~ 5 fs C.S parameters at the linac exit: Alpha=1 Beta=0.15 m

Q=0.5pC E=110 MeV DeltaE/E=1% zFWHM ~ 4.8 fs C.S parameters: Alpha=14 Beta=17 m

Q=0.5pC E=110 MeV DeltaE/E=0.8% zFWHM ~ 2.8 fs C.S parameters: Alpha=-163 Beta= 3488 m

DIAGNOSTICS - TDS

TDS Resolution Beam parameters: Ek=100*10^6; % eV nemitty=0.1*10^(-6); % pi*m*rad % Optics parameters betays0=[0.1,1,5,10,50]; % beta twiss inside the cavity in m Dphiy=pi/2; % phase advance between the cavity and the screen in rad % RF cavity parameters Vy= [5:5:50]*10^6; % peak deflection voltage V freq= *10^9; % Hz