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1.4 Fundamental ERL Challenges While ERLs exhibit tremendous potential, there also exist many formidable challenges. Generally speaking, these challenges can be grouped into three cate- gories: the injector, machine optics, and superconducting RF [22]. A brief introduction to some of the most important issues and challenges are discussed below, with particular attention towards applications to light sources. Issues specific to the Jefferson Laboratory FEL Driver will be addressed more fully in Chapter 3. 1.4.1 Injector The injector includes the gun and an accelerating, or booster, section. The injector is a vital component of an ERL because it determines, to a large extent, the beam quality that can be achieved. There has been much debate with regard to the type of gun best suited for ERL applications. Options include DC, normal conducting RF and superconducting RF guns [23]. While persuasive arguments can be made for each, regardless of the technology chosen, the gun must be able to provide a high brightness, high average current, cw electron beam. The most mature technology for cw applications is the DC gun which is used at both CEBAF and the FEL at Jefferson Laboratory. The FEL gun has delivered up to 9 mA of cw beam at a repetition rate of 74.85 MHz [24]. Extending the capability of a DC gun to produce a cw electron beam on the order of 100 mA will require increasing the cathode’s quantum efficiency and lifetime and designing a suitable drive laser system. Once the electron beam is extracted from the cathode, the challenge will be to generate, and then maintain, a small beam emittance. 15
1.4.2 Machine Optics The second category of challenges is machine optics which requires proper man- agement of the 6-dimensional beam phase space throughout the machine. There are three primary regions of interest: the linac optics, the recirculation optics and the merger optics. The linac optics requires a design that cleanly transports two co-propagating beams of different energy. The recirculation optics is vital in maintaining the beam quality delivered to the insertion device (accelerating beam) and then to the beam dump (decelerated, energy recovered beam). Finally, the merger section, where the low energy beam from the injector is merged with the high energy recirculated beam, must be carefully designed to avoid beam degradation. 1.4.3 Superconducting RF There exist many challenges with regard to SRF technology, including maxi- mizing the cryogenic efficiency, maintaining precise control of cavity fields in the presence of microphonics and Lorentz force detuning, achieving strong HOM damping and efficiently extracting HOM power [25]. The issue of HOM damping is con- sidered specifically as insufficiently damped HOMs lead to BBU - one of the most severe performance limitations of ERLs. While high Qo and QL can be achieved for the fundamental mode in SRF cavities, an unfortunate consequence is the presence of HOMs with very high Qs as well. This requires strong HOM damping to avoid beam instabilities. Recirculating linacs, and ERLs in particular, are more susceptible to these instabilities because they can support currents approaching, or exceeding, the threshold current. The instability of greatest concern is transverse, multipass, multibunch beam breakup [26]. This form of BBU was first observed in 1977 at the Stanford SCA [11] and later that year at the University of Illinois’ MUSL-2 (Microtron Using a 16
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: design parameters, most notably ach
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
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
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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
Page 103 and 104:
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
Page 151 and 152:
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
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
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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
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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