2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Introduction to the CLIC Power Extraction and Transfer Structure (PETS) Design. I. Syratchev

2nd CLIC Advisory Committee (CLIC-ACE), CERN January TeV CLIC Decelerator sector CLIC module The CLIC Power Extraction and Transfer Structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the impedance of the periodically loaded waveguide and generate RF power for the main linac accelerating structure. The power produced by the bunched (  0 ) beam in a constant impedance structure: Design input parametersPETS design P – RF power, determined by the accelerating structure needs and the module layout. I – Drive beam current L – Active length of the PETS F b – single bunch form factor (≈ 1) Introduction

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Combination factor For the given drive beam initial and final energies, the drive beam current is almost uniquely defined by the accelerating structure performance and DB generation arithmetic: Drive beam energy DB extraction efficiency (~0.9) RF transfer efficiency (~0.94) Accelerating structure Number of decelerating sectors in the linac: (Limited by radiation losses in combiner rings) #1. Drive beam current #2. PETS Power and length L T =Quad+BPM L PETS x m L Acc. Str. x n = L unit P PETS = P Acc. Str. x n/m (n/m-should be even number) Drive beam Main beam CLIC sub-unit layout: The equation for the power production now can be rewritten in terms of the PETS parameters (for simplicity the extraction length is taken = 0): #3. PETS aperture In the periodical structure, for the fixed iris thickness and phase advance: where a- radius of the structure and G – geometrical parameter. Now the PETS aperture can expressed as: The CLIC sub-unit general issues: -The CLIC sub-unit length is defined by the decelerator FODO lattice period. In turn, it is tuned to be equal to the even number of the accelerating structures length. -The space reserved for the PETS should satisfy: (L PETS x m)+L T ≤ L unit to ensure the highest effective gradient. - For any sub-unit layout chosen (m), the PETS active length should be maximized to reduce the impedance and thus to provide more stable beam transportation. Introduction

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 #4 RF power extraction (aperture) #3. PETS aperture (continued) Following the CLIC accelerating structure development (CLIC_G), simulations of the beam dynamics in the DB decelerator and the design considerations for the DB quadrupole and BPM, the following parameters were used to finalize the PETS design: Accelerating structure length: 0.25 m RF power per structure: 63.8 MW Transfer efficiency: 0.94 Sub-unit Length = 4xL structure: 1.0 m Quad. +BPM length: 0.4 m Combination factor (Nc) 12 Drive beam current: 100 A Accordingly, the tree possible sub-unit layouts can be envisaged: m = 1,2 and 4, and the PETS active length: m=1 m=2 m=4 Introduction PETS active length, m Extractor length ~ 3λ a/λ TE 11 TM 01 TM 02 a/λ PETS apertures range Extraction method ClassicalChoke-typeQuasi-optical Beam aperture0.49 TE 12 b>a 0.75 b>a Extraction length ~0.5 λ~3 λ~4 λ> 5 λ Design specifics two feed Wrap around Four feed Double choke The choice of the PETS aperture to a great extent, strictly determine the type of the RF power extraction method. The only constrain should be applied to the extractor aperture: b ≥ a. For the range of the specified PETS apertures, the power extraction length should be ~ 3λ. Now the PETS active lengths can be re-iterated:

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 PETS active length, m E surf. MV/m Introduction #5 RF constrains (aperture) Grudiev A PETS active length, m 20% Margin Pt 1/3 /C CLIC_G target PETS target X-75 klystron #6 Beam stability (aperture) The transfers wakes can significantly distort the drive beam and cause heavy beam losses. The wake potential in a finite length structure with a high group velocity is: The best scenario to suppress wake field effects will be to reduce the wake amplitude and to increase the group velocity. In a periodical structure the wake effect ~ a -3 x L and the group velocity ~ a. a -3 x L x m PETS active length, m 1-  PETS Accelerating structure x4 135 MW 250 ns Quad Drive beam Main beam 100 A 2.4 GeV  0.24 GeV (0.87 km) Deceleration: ~6MV/m 1.2 A 9 GeV  1500 GeV (21 km) Acceleration: 100 MV/m Finally, as a result of multiple compromises, the PETS aperture a/λ = 0.46 was chosen.

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Introduction Frequency, GHz Radial Inclined Flat (actual choice) #7 Phase advance #8 Iris thickness #9 PETS cross-section mm 2.0 mm Iris 2.0 mm Phase/cell The lower phase advances and thinner irises favor the beam stability Frequency, GHz Transverse Impedance  Frequency = GHz  Aperture = 23 mm  Active length = (34 cells)  Period = mm (90 0 /cell)  Iris thickness = 2 mm  Slot width = 2.2 mm  R/Q = 2222 Ω/m  V group= 0.459C  Q = 7200  Pt 1/3 /C = 13.4  E surf. (135 MW)= 56 MV/m  H surf. (135 MW) = 0.08 MA/m (ΔT max (240 ns, Cu) = 1.8 C 0 ) In its final configuration, PETS comprises eight octants separated by the damping slots. Each slot is equipped with HOM damping loads. This arrangement follows the need to provide strong damping of the transverse modes. PETS parameters:

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 E max (135 MW)=56 MV/m H max (135 MW)=0.08 MA/m PETS regular iris shape and matching cell To reduce the surface field concentration in the presence of the damping slot, the special profiling of the iris was introduced. As a result, compared to the circularly symmetric geometry, the 20% field amplification was achieved. The special matching cell was designed to provide the most compact and efficient connection between the regular PETS part and the RF power extractor. PETS machining test bar Special matching cell Electric field RF power density

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 E max (135 MW)=41.7 MV/mH max (135 MW)=0.07 MA/m PETS RF power extractor reflection isolation HFSS simulations

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 The PETS tolerances issues P G t/2 Fabrication errors, micron Power, norm. Pt G Fabrication and assembly errors can detune the PETS synchronous frequency and thus affect the power production: 2R Assembly errors (R), micron PETS machining test bar fabricated in IMP (Italy) Metrology results translated into the frequency error: If the hypothetical PETS will be constructed of such 8 identical bars, the expected power losses will be 0.03%. The fabrication accuracy of ± 20 μm is sufficient enough and can be achieved with a conventional 3D milling machine.

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 OFF: 20 msec ON: 20 sec RF power contour plot Coordinate along the PETS ON OFF Wedge radial position, mm Power norm. Phase, degree RF phase Output power Max.fields amplitudes on the wedge surface Wedge radial position, mm E, MW/mH, MA/m Wedge radial position, mm Power dissipated at the slot extremity Power MW Wedge radial position, mm Main beam acceleration Gradient, norm. OFF position Gradient (PETSONOV)Local termination of the RF power production in the PETS The net RF power generated by the beam at the end of the constant impedance structure will be zero if the structure synchronous frequency is detuned by the amount ±2βc/(1-β)L, where β – Vg /c and L - length of the structure. We have found that such a strong detuning can be achieved by inserting four thin wedges through four of the eight damping slots.

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 Distance, m (log) Wt, V/pC/m/mm (log) HFSS Frequency, GHz Re (Zt) V/A/m/mm (log) Reflected wave Wake model (single mode from HFSS) GDFIDL PLACET – the tool to simulate beam dynamics With slot No slot |E| HE EH PETS transverse wake. Introduction. 12 HFSS GDFIDL a/λ=0.46 3σ beam envelope Quadrupole number Computing tools: Moderate damping: Q(1-  ) ~ 20 -To provide the drive beam transportation without looses, heavy damping of the transverse HOW ( Q(1-  ) < 10 ) is absolutely needed. - In periodical structures with high group velocities, the frequency of a dangerous transverse mode is rather close to the operating one. The only way to damp it is to use its symmetry properties. To do this, only longitudinal slots can be used. In the presence of the longitudinal slots, the transverse mode field pattern is dramatically distorted so that a considerable amount of energy is now stored in the slots. The new, TEM-like nature of the mode significantly increases the group velocity, in our case from 0.42c to almost 0.7c.

2nd CLIC Advisory Committee (CLIC-ACE), CERN January 2008 PETS transverse wake damping and beam stability With introduction of the lossy dielectric material close to the slot opening, the situation improves further. The proper choice of the load configuration with respect to the material properties makes it possible to couple the slot mode to a number of heavily loaded modes in dielectric. This gives a tool to construct the broad wakefields impedance. The transverse wakepotential simulations in a complete PETS geometry (GDFIDL). Snapshot of the electric field pattern at the moment when the bunch left the structure RF load Dielectric with moderate losses The computer code PLACET was used to analyze the beam dynamic along the decelerator in the presence of strong deceleration and calculated wakefields. The results of the simulation clearly indicate that the suppression of the transverse wakefields obtained, is strong enough to guarantee the beam transportation without losses With HOM Without HOM Q effective ~ 2 Dielectric with heavy losses