STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

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implementation of energy recovery has been most active. The first demonstration of energy recovery occurred at Chalk River Nuclear Laboratories in 1977 using a two-pass reflexotron [6]. In a reflexotron the beam passes through an accelerating structure and is returned through the structure in the opposite direction by a 180 ◦ reflecting magnet. By changing the distance of the reflecting magnet from the accelerating structure, the phase of the beam relative to the accelerating field can be made to generate either energy doubling or energy deceleration and recovery. Using this method, output energies between 5 MeV (with energy recovery) and 25 MeV (with energy doubling) were achieved. In 1985 a 400 MeV electron beam was energy recovered to 23 MeV at the MIT- Bates Linac as part of an experiment to operate the recirculation system under a variety of conditions [7]. A unique three pass beam operation scenario was also demonstrated by producing a re-injection phase of 90 ◦ relative to the accelerating field with the recirculator. In this way, the second pass beam traveled through the linac without feeling any acceleration. A third pass beam was re-injected into the linac with a 180 ◦ phase difference relative to the accelerating field and energy recovered. Beam transmission was poor on the third pass however, due to the large energy spread acquired. In 1986, Stanford University’s Superconducting Accelerator (SCA) energy recovered 150 µA of average beam current from 55 MeV to 5 MeV [8]. This experiment was significant in that it marked the first time energy recovery had been demonstrated in a superconducting RF environment. At about the same time, the free electron laser at Los Alamos National Labora- tory demonstrated energy recovery in a unique configuration where the decelerated beam deposited energy in a different cavity from which it was accelerated [9]. This scheme represents a departure from the previous examples of “same-cell” energy recovery. Using this setup, they successfully energy recovered 21 MeV to 5 MeV. 11
Despite its success, this method of energy recovery has not been used since. The subsequent material in this dissertation is focused solely on accelerators utilizing same-cell energy recovery in superconducting RF cavities. 1.3.1 Jefferson Laboratory and ERLs Over the course of 12 years, from 1993 to 2005, Jefferson Laboratory successfully demonstrated same-cell energy recovery in four different accelerators. Due in large part to the success of the IR FEL Demo in the mid 1990’s, there has been a renewed interest in ERLs as drivers for applications ranging from electron-ion colliders, to electron coolers, to light sources and FELs. As discussed in Section 1.2, combining the principle of energy recovery with SRF cavities leads to an accelerator capable of generating an intense beam with ex- cellent beam qualities in an efficient and economical manner. Initial experience with SRF cavities, however, presented formidable challenges. In the early 1970s, when Stanford University began operation of the SCA, multipactoring in the SRF cavities severely limited the gradients and consequently the final beam energy. To overcome this obstacle, transport elements were installed to recirculate the beam multiple times through the linac [10, 11]. When the beam was recirculated, insufficiently damped HOMs caused beam breakup, thereby limiting the achievable average beam current. Thus, despite the great potential of SRF cavities, the first accelerator to implement SRF technology was limited in beam energy (due to multipactoring) and average beam current (due to BBU). When in 1985 it was proposed to build a 4 GeV electron accelerator for nuclear physics based on SRF technology at Jefferson Laboratory, a great effort was made to address the issues of implementing SRF technology on such a large scale [12]. By this time Cornell University had designed a cavity using an elliptical cell shape which 12
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: y reducing the impedance of HOMs, a
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 and 44: FIG. 2.3: Additional hardware insta
Page 45 and 46: FIG. 2.4: A picture of the energy r
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
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accelerating gradient at the front
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eason for making the endloops achro
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FIG. 3.7: Illustration of path leng
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3.5 Longitudinal Dynamics This sect
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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
Page 145 and 146:
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
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FIG. 6.8: A plot of 1/Qeff versus a
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⎛ ⎞ ⎜ ⎝ 0 0 0 0 0 −1/K 0
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FIG. 6.11: A plot of 1/Qeff versus
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FIG. 6.12: Threshold current for no
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FIG. 6.14: Threshold current utiliz
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TABLE 6.1: Summary of the measured
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CHAPTER 7 BBU Suppression: Feedback
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FIG. 7.1: A schematic of the feedba
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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
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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
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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
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[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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