Adolphsen, C E

Akasaka, N

Jobe, R Keith

Matsumoto, H

McCormick, D

Ross, M

Shintake, T

Slaton, T

Yoshida, M

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THE FIRST WAKEFIELD TESTON THE C-BAND CHOKE-MODE ACCELERATING STRUCTURET. Shintake, H. Matsumoto, N. Akasaka, M. Yoshida, KEK, Tsukuba, JapanC. Adolphsen, K. Jobe, D. McCormick, M. Ross, T. Slaton, SLAC, Stanford, CAAbstract A full-scale prototype C-band structure for the e+e- linearcollider has been built in which the dipole powers aredamped to suppress the long-range transverse wakefield.To verify that the damping works as expected, the C-bandstructure was tested in the Accelerator Structure SETup(ASSET) at SLAC. This paper presents results of thewakefield measurement with ASSET.1  INTRODUCTIONThe C-band scheme being considered at KEK for the 500GeV to 1 TeV scale linear collider employs multi-bunchbeam operation in a high accelerating gradient C-bandlinac [1]. If no any cares were taken in the acceleratingstructure, the high-Q transverse dipole mode, mainlyTM110 mode, would produce an enormous blow up thetransverse motion of the bunch train, resulting in loosingluminosity. In order to damp all the dipole modes inaccelerator cavities, the author proposed the choke-modecavity in 1992 [2]. In this scheme, the beam excited poweris eliminated by a microwave absorbing material in theaccelerator, thus the dipole modes are damped in each cellfor a wide frequency-range. The choke is used as a notch-filter, which traps the rf power within a cavity at theoperating frequency, thus a high accelerating field can begenerated.  The basic principle of the choke-mode cavity was provedusing an S-band model in 1994, where an electron beamwas accelerated at the field-gradient as high as 50 MV/mdriven by S-band power of 100 MW peak.   From 1996, we started design of C-band RF system, anddeveloped a full-scale prototype for the C-band linearcollider. The structure was installed in ASSET at SLAC,and its electrical performance was tested withelectron/positron bunches in December 1998.2  C-BAND STRUCTUREFigure 1 shows the developed C-band structure, which iscomposed of the regular section (89 choke-mode cells),input/output couplers attached at each end, and two RF-BPMs for beam position alignment at both end. Awakefield monitor is prepared on the center cell in orderto observe the beam induced HOM spectrum, anddetermine the beam position. To avoid unwantedtransverse kick due to asymmetric field, the double-feedcoupler using J-shaped waveguide is employed in theinput/output couplers [3]. The whole structure is enclosedin a cooling-water jacket made by stainless steel.2.1 Choke-Mode Cell Design   Figure 2 shows the cut-view of the C-band structure,including the wakefield monitor at the center cell. Thebeam-induced wakefield-power is damped in themicrowave absorbers made by SiC-ceramic, which ismechanically attached in the copper cell with a metal-spring-insert. The design detail is reported in ref. [3].   In order to maximise the damping performance for thelowest dipole mode, the volume of the SiC ring wasoptimised using computer simulation [4]. Three differentsizes of the SiC ring were used in the current design. Thewakefield of the whole accelerating structure wasapproximately estimated by averaging the wakefieldcalculated at equally spaced 12 points. It was expectedthat the wakefield would be damped below 0.5V/pC/m/mm at the second bunch at 2.8 nsec (0.84 m),which is lower than the limit of 0.7 V/pC/m/mm definedby the multi-bunch emittance dilution of 25% in the mainlinac.2.2  RF-BPMIn the C-band linear collider, it is necessary to align theaccelerating structure with respect to the beam in rathertight tolerances, a high-precision BPM will be attached atthe both end of the structure and the transverse positionswill be adjusted by fine movers. To limit the emittancedilution below 25% in the main linac, the structure shouldbe aligned with accuracy better than 30 µm and fabricatedwith straightness within 50 µm.Figure 1: C-band accelerating structure. 2 m long, 91 cell.0-7803-5573-3/99/$10.00@1999 IEEE. 3411Proceedings of the 1999 Particle Accelerator Conference, New York, 1999    For the C-band accelerator alignment, a new type RF-BPM was developed as shown in Fig. 3, which useslongitudinal slots to cut the common-mode signal from thebeam-position signal. To achieve highly accurate BPM,especially to avoid unwanted shift of the electrical center(zero point), elimination of the common-mode power isthe most important key [5]. In this new design, reductionof common mode by –40 dB or better can be made.3  ASSET BEAM TEST3.1 Wakefield Signal SpectrumIn order to monitor the microwave signal generated by thebeam, four pick-up antennas were prepared on the centercell as shown in Fig. 2. The antenna is the straight metalrod, which forms a closed loop together with the groundplane, and couples with the magnetic field behind the SiCabsorber.  Figure 4 shows the measured spectrum for thefrequency range between 5 to 25 GHz. The sharp peaks at5.7 GHz are the leakage power from the beam exciteddominant accelerating mode. The next peak at 7.8 GHz isthe lowest dipole mode (TM110 mode). The measured Q-factor was 36, which is slightly higher than the expectedvalue of 30 estimated with the computer simulation. Asshown in Fig. 2, the microwave absorber in the monitorcell has smaller dimension, thus the Q-factor takes highervalue than the regular cells. According to the computersimulation, the effective Q for the averaged wakefield isexpected as low as 12.    Most of all peaks at higher frequency bands wereidentified with the simulated spectrum. Exception was afew sharp peaks around 20 - 24 GHz, which were notpredicted at design time. The same peaks appeared as along tail in the measured wakefield as discussed later.   In Fig. 4, the peak at 15 GHz seems to be the highestone, but its power is much lower than the dominant dipolemode at 7.8 GHz. This is due to the characteristic of thepick-up. Since the loop-coupler picks up the time-variation of the magnetic flux:BSdtdV ω=Φ∝ / ,A higher frequency component generates a higher outputsignal for the same intensity of magnetic field.3.2 RF-BPM TestFigure 5 shows the circuit diagram of the RF-BPM. The rfsignal from two RF-BPMs, the wakefield monitor and thereference cavity are processed with the band-pass filters(7854 MHz, 3% BW), and down converted to 310MHz signal, finally sampled with 4-ch digital scope(Tektronix TDS684B, 5Gs, 1GBW). The referencesignal is used to find the beam timing and the beamphase. The wakefield signal from two pick-upantennas at the center BPM are combined in a 180-deg. hybrid to eliminate the common mode power.    Figure 6 is the correlation plot for three BPMs. Thezero-cross point gives the vertical offset of the centerBPM with respect to the mean center of the upstreamand downstream BPMs. It was +63 µm. By takingaccount the calibration data, the straightness becomes5.020.03.08.041.04.036.014.03.0ICF 34SUS304ICF 70Signal OutputType-NC-band AcceleratingStructureBPM CavityTM110-mode7854 MHzCoupling SlotCoupling AntennaØ90Figure 3:  RF-BPM with common-mode less design.44 1519.7l/4l/4SiC RingElectro-plated CopperWakefield AbsorberTrappedAccelerating ModeChoke FilterBEAM Pickup Antenna x 4Wakefield MonitorCenter CellFigure 2: A cut view of the C-band choke-mode structure with a wakefield monitor.Fig. 4  Measured wakefield spectrum with a horizontal antenna.3412Proceedings of the 1999 Particle Accelerator Conference, New York, 199949 µm. This is just border of design tolerance. On theother hand, the straightness mechanically measured fromoutside was 90 µm. We need further study beforeconcluding these test results, such as measurement of thestraightness using laser-beam from inside of the structure.The observed spatial resolution was about 8 µm, whichwas limited by high digital noise in the sampling-scope. 3.3 Wakefield MeasurementWe measured the wakefield generated in the C-bandstructure with the same method as done in the X-bandstructure R&D. The layout of the ASSET and detailedprocedure are found in elsewhere [6].    We injected the positron beam first as a drive bunch,which extracted from the South Damping Ring. Thebunch passed through the C-band structure and was thensteered into a dump. Right after the positron, the electronbunch was injected from the North Damping Ring and thedeflection angle due to the wakefield was measured withthe BPMs in the linac. The measured data is shown in Fig.7. The measured wakefield damps according to theexpected waveform (solid line) up to 1.6 nsec (0.5 m)from about 15 to 1 V/pC/m/mm. However, after thispoint, the measured wakefield showed a high frequencyoscillation with an amplitude of 0.8~1.0 V/pC/m/mm.This is just the border-line of the 25% emittance dilution.Applying FFT, we found that the oscillation correspondsto the spectrum peak at 20, 23 GHz. With computersimulations, it was found that in some condition the modecan be trapped in the cavity at very high-frequency, whosefield pattern has a node-point at the slot location, thus nopower can leak out to the damper. This was caused by ourdesign change. To improve the shunt-impedance by about10%, we changed the disk thickness from 4 to 3 mm.However, the mode pattern in the cavity moved in z-direction, and occasionally created a node at the couplingslot. To solve this, we will shift z-location of the dampingslot by 2 mm. With careful simulations, it was shown thatthe trapped mode could be eliminated. The new model isunder design.4 ACKNOWLEDGEMENT We would like to thank Prof. David Burke for hisencouragement, and Charles Yoneda, Leo Giannini, LeifEriksson, Tony R. King, M. W. Perry, J. R. McDougal fortheir help in structure installation.5 REFERENCES[1] T. Shintake et al. ,”C-band RF Main Linac System for e+e- LinearCollider at 500 GeV to 1 TeV C.M. Energy”, EPAC96, KEK Preprint96-68, and K. Yokoya et al., “C-band Linear Collider with C.M.Energy 500 GeV to 1 TeV”[2] T. Shintake, “The Choke Mode Cavity”, Jpn. J. Appl. Phys. Vol.31,pp. L1567-L1570, Part2, No.11A, 1992[3] H. Matsumoto, T. Shintake and N. Akasaka, “Fabrication of the C-band (5712 MHz) Choke-Mode Type Damped Accelerator Structure,LINAC98, KEK Pre-print 98-143.[4] N. Akasaka T. Shintake and H. Matsumoto, “Optimisation onWakefield Damping in C-band Accelerating Structure”, LINAC98,August 23-28, Chicago, IL, KEK Pre-print 98-142[5] T. Shintake, “Development of Nanometer Resolution RF-BPMs”,HEACC98, Sept 1998, Dubna, Russia, KEK Pre-print 98-188.[6] C. Adolphsen, et al., “Measurement of Wakefield Suppression in aDetuned X-band Accelerator Structure”, LINAC94 Proc., p481Fig. 5  RF-BPM detector circuit.-10-50510150 0.2 0.4 0.6 0.8 1distance(m)wakefield(V/pC/m/mm)↓2nd BunchEnergy ∼1.2GeVDrive e+ ∼1nCWitness e- ∼1nCOffset ∆y ±2mmUsed BPMs  ∼20unitsAveraging  16 shots/pointActive Str.Length 1.791mFig. 7 Measured (circle) and expected (solid line)wakefield.-0.8-0.6-0.4-0.200.20.40.60.8-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8(Ych1+Ych3)/2  (mm)Y ch2  (mm)+63µmFig. 6 Correlation plot on three BPM readout.3413Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

The First Wakefield Test On The C-band Choke-mode Accelerating Structure

The First Wakefield Test On The C-band Choke-mode Accelerating Structure

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