STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

More documents

Recommendations

Info

FIG. 5.2: The Schottky diode assembly showing the directional coupler, attenuator and Schottky diode. Each of the 16 Schottky diodes were calibrated by measuring the output voltage as a function of incident power using an RF signal generator at a frequency of 2000 MHz (the HOM signals of primary interest are around 2100 MHz, however the signal generator did not extend that far in frequency). The resulting data was fit with a polynomial up to second order in the voltage. 5.2.2 Observations of BBU A thorough experimental investigation of BBU commenced in early 2005. The nominal machine setup for the extent of the study was an 88 MeV configuration with decoupled transverse optics. The injector was set to provide 7.3 MeV electrons into the linac where the accelerating gradients were set such that zone 2, zone 3 and zone 4 provided 28.7 MeV, 15.1 MeV and 36.3 MeV of energy gain, respectively. Operating in cw mode, the average beam current was slowly increased until exponential growth of the HOM power was observed from cavity 7, which occurred 115 simultaneously with a machine trip caused by excessive beam losses. These trips
occurred at approximately 2 mA of average beam current. The process of slowly ramping up the current was repeated several times to ensure that the instability developed at the same current each time. With the FEL Upgrade Driver in a configuration to readily observe beam breakup, the mode causing the instability was identified (Section 5.3) and measurements of the threshold current were conducted (Section 5.4) to benchmark BBU simulation codes. 5.3 HOM Voltage Upon identifying cavity 7 as containing the unstable mode, the next measurement was identifying the frequency of the mode. To do this, the signals from the HOM coupler are split further after the −20 dB directional coupler, with one part connected to a Schottky diode to measure the power while the other part is sent di- rectly to an oscilloscope to measure the voltage. A schematic of this setup is shown in Fig. 5.3. If the oscilloscope is fast enough and the signal sufficiently sampled, the frequency of the offending mode can be extracted by Fourier analysis. A screen shot of the oscilloscope screen showing the HOM power and voltage during BBU is given in Fig. 5.4. Taking the FFT of the voltage signal reveals that the mode frequency is 2106.007 MHz (see Fig. 5.5). The results of the measurements show that with nominal, decoupled optics for an 88 MeV machine configuration, the most dangerous mode is at a frequency of 2106.007 MHz and located in cavity 7. This is in agreement with simulation results presented in Section 4.8.2. The 2106 MHz mode was considered one of the prime candidates for causing BBU because it has the second highest impedance, (R/Q)QL, of the 224 modes measured in zone 3. In fact, the highest impedance mode is also in cavity 7, at a frequency of 2116 MHz. However subsequent measurements, described 116 in Section 5.4.3, confirmed that this HOM was not a threat for causing BBU because
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 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
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 and 132: CHAPTER 5 Experimental Measurements
Page 133: threshold current - preferably with
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
Page 183 and 184: FIG. 7.1: A schematic of the feedba
Page 185 and 186:
FIG. 7.4: A coaxial 3-stub tuner us
Page 187 and 188:
All of these considerations are con
Page 189 and 190:
FIG. 7.6: Generic layout for a feed
Page 191 and 192:
in Section 4.2.1, however, in the p
Page 193 and 194:
FIG. 7.7: The threshold current as
Page 195 and 196:
FIG. 7.8: Threshold current versus
Page 197 and 198:
FIG. 7.10: The threshold current as
Page 199 and 200:
CHAPTER 8 Conclusions The work pres
Page 201 and 202:
le were experimentally measured. Du
Page 203 and 204:
APPENDIX A The Pillbox Cavity Start
Page 205 and 206:
FIG. A.1: A pillbox cavity exhibiti
Page 207 and 208:
Ez(ρ, φ) = ψ(ρ, φ) = E0Jm(γρ
Page 209 and 210:
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
show all

STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?