STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

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upon re-entry into the north linac, the beam was 180 ◦ out of phase with respect to the accelerating RF waveform and was decelerated to 556 MeV. After traversing arc 1 a second time the beam entered the south linac - still out of phase with the RF accelerating field - and was decelerated to 56 MeV at which point the energy recovered beam was deflected to a dump. Upon configuring the machine for energy recovery, measurements were performed to characterize the beam phase space at various points in the machine. These will be discussed in detail in Sections 2.4, 2.5 and 2.6. Once satisfactory measurements were obtained using the 56 MeV injection energy, the measurements were repeated for a lower injection energy of 20 MeV to study the parametric dependence on high maximum-to-injection energy ratios. While the modifications required to transform CEBAF into an ERL-based ac- celerator are relatively minor, the fact that CEBAF was not originally designed with the intention of performing energy recovery presents challenges. The following sections will discuss some of those issues and challenges. 2.2.1 Phase Delay Chicane and Beam Dump The purpose of the phase delay chicane is to provide a path length differential such that the beam enters the north linac on the second pass 180 ◦ out of phase with respect to the accelerating RF field. The wavelength corresponding to the fundamental frequency of 1497 MHz is given by λRF = c fRF 25 = 0.2 m (2.1) where c is the speed of light in vacuum. For energy recovery a path length differential of λRF /2 = 10 cm is required. The phase delay chicane consists of four dipole magnets and is achromatic, which means that the beam transport does not depend on beam momentum. Because the chicane is installed in a non-dispersive region, it
FIG. 2.4: A picture of the energy recovery phase delay chicane to the left of the nominal straight ahead CEBAF beamline. The chicane is comprised of four dipole magnets. is expected that no remnant dispersion will be generated by the dipoles. Figure 2.4 shows the chicane - installed in region 2L23 - as seen by the beam after exiting the last cryomodule of the south linac. The beam dump was moved from the injector region, where it was used in conjunction with a spectrometer to measure the injection energy into the linac, to region 2L22. Figure 2.5 shows the dump beamline instrumented with a wire-scanner, beam current monitor (BCM), beam position monitor (BPM) and optical transition radiation (OTR) monitor. The wire-scanner was used to measure beam profiles, the BPM provided information about the position of the beam, the BCM registered the beam current getting to the dump and the OTR monitor allowed confirmation that the beam was reaching the dump face through visual inspection. In addition to the beam dump, region 2L22 also contains a second, smaller chicane. The purpose of the chicane is to correct the high energy beam orbit which 26
Page 1 and 2: STUDIES OF ENERGY RECOVERY LINACS A
Page 3 and 4: DEDICATION To my wife Danielle and
Page 5 and 6: 2 CEBAF with Energy Recovery . . .
Page 7 and 8: 5.2 HOM Power . . . . . . . . . . .
Page 9 and 10: ACKNOWLEDGMENTS First and foremost
Page 11 and 12: 6.1 Summary of the measured effects
Page 13 and 14: 2.10 Illustration of quadrupole sca
Page 15 and 16: 5.1 Successive frames in time (prog
Page 17 and 18: 6.8 A plot of 1/Qeff versus average
Page 19 and 20: ABSTRACT An energy recovering linac
Page 21 and 22: CHAPTER 1 Introduction An increasin
Page 23 and 24: FIG. 1.1: Schematic of a generic li
Page 25 and 26: FIG. 1.2: A CEBAF 5-cell cavity wit
Page 27 and 28: The solution to Eq. (1.3) is U(t) =
Page 29 and 30: y reducing the impedance of HOMs, a
Page 31 and 32: Despite its success, this method of
Page 33 and 34: design parameters, most notably ach
Page 35 and 36: 1.4.2 Machine Optics The second cat
Page 37 and 38: analytic model elucidates many impo
Page 39 and 40: CHAPTER 2 CEBAF with Energy Recover
Page 41 and 42: FIG. 2.1: Energy versus average cur
Page 43: FIG. 2.3: Additional hardware insta
Page 47 and 48: dipoles and beam diagnostics such a
Page 49 and 50: FIG. 2.7: Horizontal (red) and vert
Page 51 and 52: FIG. 2.8: Illustration of the cryom
Page 53 and 54: linac and θNL is the RF phase. The
Page 55 and 56: 2.4 Transverse Emittance One of the
Page 57 and 58: where σ2 is the rms beam size meas
Page 59 and 60: eams. The effects of varying the qu
Page 61 and 62: FIG. 2.12: A typical wire scan near
Page 63 and 64: quadratic fit and a multiple regres
Page 65 and 66: ting the data is difficult. Without
Page 67 and 68: primary source of error is measurin
Page 69 and 70: identified, although the phase dela
Page 71 and 72: TABLE 2.3: Comparison of Twiss para
Page 73 and 74: the results of the fits. The vertic
Page 75 and 76: FIG. 2.18: Schematic illustrating t
Page 77 and 78: FIG. 2.19: The GASK signal measured
Page 79 and 80: FIG. 2.20: The measured normalized
Page 81 and 82: CHAPTER 3 The Jefferson Laboratory
Page 83 and 84: FIG. 3.1: Schematic of the 10 kW FE
Page 85 and 86: FIG. 3.2: Layout of the DC photocat
Page 87 and 88: accelerating gradient at the front
Page 89 and 90: eason for making the endloops achro
Page 91 and 92: FIG. 3.7: Illustration of path leng
Page 93 and 94: 3.5 Longitudinal Dynamics This sect
Page 95 and 96:
FIG. 3.9: The effect of a thin focu
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Under the constraint that each orde
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form of beam breakup not only occur
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4.1 The Pillbox Cavity Although the
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FIG. 4.2: Electric field (red) and
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where the full 4×4 transfer matrix
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The threshold is inversely proporti
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4.3 BBU Simulation Codes: Particle
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6. The second pass beam bunch then
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which excites it. The BBU instabili
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Equation (4.41) is a dispersion rel
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FIG. 4.4: Output from MATBBU showin
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FIG. 4.5: Setup for measuring cavit
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Consequently, depending on the exte
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The projection of the beam displace
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TABLE 4.1: Experimental measurement
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FIG. 4.10: A plot showing the effec
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these cryomodules. Modes from these
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CHAPTER 5 Experimental Measurements
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threshold current - preferably with
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occurred at approximately 2 mA of a
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FIG. 5.5: FFT of a pure 2106.007 MH
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FIG. 5.6: Illustration to show the
Page 141 and 142:
5.4 Measuring the Threshold Current
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for the HOM-beam system and is deri
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FIG. 5.10: Schematic of the experim
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FIG. 5.12: A plot of 1/Qeff versus
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measured HOMs in zone 3, a BTF meas
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FIG. 5.16: HOM voltage measured fro
Page 153 and 154:
FIG. 5.18: A plot of the three valu
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the beam’s response in regions wh
Page 157 and 158:
CHAPTER 6 BBU Suppression: Beam Opt
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FIG. 6.1: Schematic of a FODO cell
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plane [85]. Equations (6.7) and (6.
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6.2.3 Discussion The method of poin
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FIG. 6.3: Beam envelopes (horizonta
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FIG. 6.6: Beam position monitor rea
Page 169 and 170:
FIG. 6.8: A plot of 1/Qeff versus a
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⎛ ⎞ ⎜ ⎝ 0 0 0 0 0 −1/K 0
Page 173 and 174:
FIG. 6.11: A plot of 1/Qeff versus
Page 175 and 176:
FIG. 6.12: Threshold current for no
Page 177 and 178:
FIG. 6.14: Threshold current utiliz
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TABLE 6.1: Summary of the measured
Page 181 and 182:
CHAPTER 7 BBU Suppression: Feedback
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FIG. 7.1: A schematic of the feedba
Page 185 and 186:
FIG. 7.4: A coaxial 3-stub tuner us
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All of these considerations are con
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FIG. 7.6: Generic layout for a feed
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in Section 4.2.1, however, in the p
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FIG. 7.7: The threshold current as
Page 195 and 196:
FIG. 7.8: Threshold current versus
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FIG. 7.10: The threshold current as
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CHAPTER 8 Conclusions The work pres
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le were experimentally measured. Du
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APPENDIX A The Pillbox Cavity Start
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FIG. A.1: A pillbox cavity exhibiti
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Ez(ρ, φ) = ψ(ρ, φ) = E0Jm(γρ
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FIG. B.1: Relationship of the S-par
Page 211 and 212:
FIG. C.1: Impedance and frequency o
Page 213 and 214:
FIG. C.5: Impedance and frequency o
Page 215 and 216:
BIBLIOGRAPHY [1] M. Tigner, Nuovo C
Page 217 and 218:
[22] L. Merminga, in Proceedings of
Page 219 and 220:
[50] C. Hernandez-Garcia et al., in
Page 221 and 222:
[79] L. Merminga et al., in Proceed
Page 223 and 224:
VITA Christopher D. Tennant Christo
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