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FIG. 5.1: Successive frames in time (progressing from left to right) from a movie of the synchrotron light monitor in the second endloop at the onset of BBU. 5.1 Overview While the remainder of this chapter is dedicated to describing the details of the quantitative measurements, Fig. 5.1 illustrates a qualitative characterization of BBU. Figure 5.1 shows a series of frames from a recording of a synchrotron light monitor located in the second recirculation arc of the FEL Driver. During the recording, the average beam current was being slowly increased until beam breakup developed. The instability clearly manifests itself as vertical growth which continues until beam losses become large enough to trip the machine off. The time elapsed from the first to the last frame of Fig. 5.1 is approximately 0.25 s. From the point of view of a machine operator in the control room, the only indication that the operating current is approaching the threshold current are obser- vations of the SLM image growing as depicted in Fig. 5.1. Additional characteristics of the presence of BBU - from an operator’s perspective - are single, hard machine trips at one particular beam loss monitor (BLM) location. In most instances, these BLM trips occurred in the 5F region of the Driver where the recirculated beam is re-injected through the linac and the beam envelopes are largest. Clearly a more quantitative method is needed to confirm that a machine trip is due to BBU. Additional measurements are required to ascertain which cavity contains the dangerous HOM and to determine the frequency of the mode. Fur- 113 thermore, adequately benchmarking BBU codes requires accurately measuring the
threshold current - preferably with several different methods for consistency. All these measurements require the ability to measure one of the signatures of BBU, namely the HOM power. 5.2 HOM Power Measuring the HOM power proved to be ideal in regards to studying BBU and was achieved using Schottky diodes [77]. Several attempts to measure the response of BPM striplines, to see the exponentially growing displacement due to the instability, were unsuccessful. Ultimately, the HOM power is easier to monitor and provides a signal that is robust enough to make a number of independent measurements as discussed in Sections 5.4.3 and 5.4.4. The key element in all these measurements is the fact that the cavities in the zone 3 cryomodule, unlike previous CEBAF 5-cell cavities, use DESY-like coaxial HOM couplers [75]. Cables connected to the HOM ports are loaded on 50 Ω resistors. In order to monitor the HOM power a small portion of the signal from each HOM port is directed to a Schottky diode by a −20 dB directional coupler. The output of each diode is connected to a separate oscilloscope channel. This allows the HOM power to be individually monitored from each of the 8 cavities. The Schottky diode assembly is shown in Fig. 5.2. 5.2.1 Schottky Diodes A Schottky diode acts as a rectifier, converting an AC waveform to a DC waveform. Schottky diodes have the added advantage over conventional PN junction diodes in that they work well at high frequency. The diodes used for BBU studies were manufactured by Herotek (model DZM124NB) and work across a frequency range from 10 MHz to 12.4 GHz [78]. 114
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STUDIES OF ENERGY RECOVERY LINACS A
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DEDICATION To my wife Danielle and
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2 CEBAF with Energy Recovery . . .
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5.2 HOM Power . . . . . . . . . . .
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ACKNOWLEDGMENTS First and foremost
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6.1 Summary of the measured effects
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2.10 Illustration of quadrupole sca
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5.1 Successive frames in time (prog
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6.8 A plot of 1/Qeff versus average
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ABSTRACT An energy recovering linac
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CHAPTER 1 Introduction An increasin
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FIG. 1.1: Schematic of a generic li
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FIG. 1.2: A CEBAF 5-cell cavity wit
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The solution to Eq. (1.3) is U(t) =
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y reducing the impedance of HOMs, a
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Despite its success, this method of
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design parameters, most notably ach
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1.4.2 Machine Optics The second cat
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analytic model elucidates many impo
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CHAPTER 2 CEBAF with Energy Recover
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FIG. 2.1: Energy versus average cur
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FIG. 2.3: Additional hardware insta
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FIG. 2.4: A picture of the energy r
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dipoles and beam diagnostics such a
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FIG. 2.7: Horizontal (red) and vert
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FIG. 2.8: Illustration of the cryom
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linac and θNL is the RF phase. The
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2.4 Transverse Emittance One of the
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where σ2 is the rms beam size meas
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eams. The effects of varying the qu
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FIG. 2.12: A typical wire scan near
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quadratic fit and a multiple regres
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ting the data is difficult. Without
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primary source of error is measurin
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identified, although the phase dela
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TABLE 2.3: Comparison of Twiss para
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the results of the fits. The vertic
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FIG. 2.18: Schematic illustrating t
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FIG. 2.19: The GASK signal measured
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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
Page 97 and 98: Under the constraint that each orde
Page 99 and 100: form of beam breakup not only occur
Page 101 and 102: 4.1 The Pillbox Cavity Although the
Page 103 and 104: FIG. 4.2: Electric field (red) and
Page 105 and 106: where the full 4×4 transfer matrix
Page 107 and 108: The threshold is inversely proporti
Page 109 and 110: 4.3 BBU Simulation Codes: Particle
Page 111 and 112: 6. The second pass beam bunch then
Page 113 and 114: which excites it. The BBU instabili
Page 115 and 116: Equation (4.41) is a dispersion rel
Page 117 and 118: FIG. 4.4: Output from MATBBU showin
Page 119 and 120: FIG. 4.5: Setup for measuring cavit
Page 121 and 122: Consequently, depending on the exte
Page 123 and 124: The projection of the beam displace
Page 125 and 126: TABLE 4.1: Experimental measurement
Page 127 and 128: FIG. 4.10: A plot showing the effec
Page 129 and 130: these cryomodules. Modes from these
Page 131: CHAPTER 5 Experimental Measurements
Page 135 and 136: occurred at approximately 2 mA of a
Page 137 and 138: FIG. 5.5: FFT of a pure 2106.007 MH
Page 139 and 140: FIG. 5.6: Illustration to show the
Page 141 and 142: 5.4 Measuring the Threshold Current
Page 143 and 144: for the HOM-beam system and is deri
Page 145 and 146: FIG. 5.10: Schematic of the experim
Page 147 and 148: FIG. 5.12: A plot of 1/Qeff versus
Page 149 and 150: 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
Page 155 and 156: the beam’s response in regions wh
Page 157 and 158: CHAPTER 6 BBU Suppression: Beam Opt
Page 159 and 160: FIG. 6.1: Schematic of a FODO cell
Page 161 and 162: plane [85]. Equations (6.7) and (6.
Page 163 and 164: 6.2.3 Discussion The method of poin
Page 165 and 166: FIG. 6.3: Beam envelopes (horizonta
Page 167 and 168: FIG. 6.6: Beam position monitor rea
Page 169 and 170: FIG. 6.8: A plot of 1/Qeff versus a
Page 171 and 172: ⎛ ⎞ ⎜ ⎝ 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
Page 179 and 180: 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
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FIG. C.5: Impedance and frequency o
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BIBLIOGRAPHY [1] M. Tigner, Nuovo C
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[22] L. Merminga, in Proceedings of
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[50] C. Hernandez-Garcia et al., in
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[79] L. Merminga et al., in Proceed
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VITA Christopher D. Tennant Christo
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