STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
and vertical betatron oscillations [31]. In standard CEBAF operation this effect is mitigated by the use of a magnetic skew quadrupole field between cryomodules to produce a compensating gradient integral. However, in CEBAF-ER operation, the sign of the induced skew quadrupole changes since the second pass beam is 180 ◦ out of phase with the first pass. Therefore, although the external skew quadrupoles can locally correct the coupling for a single pass through the linac, the effect of the coupling will double on the other pass. This effect makes it difficult operationally to propagate the beam through the machine. To alleviate the consequences of the coupling, a so-called “up-down” correction scheme was implemented in which the lower energy beams in each linac were cor- rected using skew quadrupoles. Although the coupling is not fully suppressed with this configuration, it was the most attractive solution based on simulations showing that the initial projected emittances would be recovered after energy recovery [32]. In addition to the fields in the HOM coupler, a transverse electric field gradient exists in the 5-cell cavity’s fundamental power coupler. This field gradient not only can transversely deflect the bunch centroid but can also differentially steer the head and tail of a bunch [33]. While the effects of centroid steering can be minimized using correctors, the differential steering of the electron bunch can lead to emittance growth and presents a more difficult problem. Similar to the HOM coupler skew quadrupole coupling, the dipole steering is a phase dependent effect. Unlike the coupling, the effect of the dipole steering depends strongly on the RF feed geometry. That is, the strength of the steering depends on whether the FPC is located at the downstream or upstream end of the cavity and whether the RF power is fed in from the left or right side (as seen by the beam). Therefore the magnitude of the effect can be minimized with an appropriate choice of RF feed geometry. Because cavities are joined in pairs and the FPCs placed at the center of each cavity pair, alternating the feed direction for each cavity is not feasible technically. 31
FIG. 2.8: Illustration of the cryomodule RF feed geometry used to minimize the FPC induced dipole kick. The RF feed geometry that minimizes the emittance dilution due to head-tail steer- ing, while remaining technically viable, is illustrated in Fig. 2.8 [33]. Within each cavity pair, the downstream FPC is followed by a cavity with an upstream FPC. The RF power to the two outer cavity pairs is fed from the same direction, while the two middle cavity pairs are fed in the opposite direction. Note that these SRF-induced effects are due to particular features of the CE- BAF 5-cell cavity design and do not represent fundamental limitations of the energy recovery process. In principle, a well designed SRF cavity can avoid these problems altogether. 2.2.4 Balancing Linac Energy An important step in configuring CEBAF for energy recovery was balancing the north and south linac energy gains to within the machine acceptance. This is to ensure that arc 1 can cleanly transport the two co-propagating, yet equal energy, beams. Operationally, the linac energies were balanced using a deceleration exper- iment, wherein beam was accelerated through the north linac, decelerated through the south linac and sent to the energy recovery beam dump [34]. Decelerating the beam through the south linac was accomplished by changing the RF ganged phases by 180 ◦ . The linac energies are balanced when the injected energy is equal to the 32
- 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: FIG. 2.7: Horizontal (red) and vert
- 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
FIG. 2.8: Illustration of the cryomodule RF feed geometry used to minimize the FPC<br />
induced dipole kick.<br />
The RF feed geometry that minimizes the emittance dilution due to head-tail steer-<br />
ing, while remaining technically viable, is illustrated in Fig. 2.8 [33]. Within each<br />
cavity pair, the downstream FPC is followed by a cavity with an upstream FPC.<br />
The RF power to the two outer cavity pairs is fed from the same direction, while<br />
the two middle cavity pairs are fed in the opposite direction.<br />
Note that these SRF-induced effects are due to particular features of the CE-<br />
BAF 5-cell cavity design and do not represent fundamental limitations of the energy<br />
recovery process. In principle, a well designed SRF cavity can avoid these problems<br />
altogether.<br />
2.2.4 Balancing Linac Energy<br />
An important step in configuring CEBAF for energy recovery was balancing<br />
the north and south linac energy gains to within the machine acceptance. This is<br />
to ensure that arc 1 can cleanly transport the two co-propagating, yet equal energy,<br />
beams. Operationally, the linac energies were balanced using a deceleration exper-<br />
iment, wherein beam was accelerated through the north linac, decelerated through<br />
the south linac and sent to the energy recovery beam dump [34]. Decelerating the<br />
beam through the south linac was accomplished by changing the RF ganged phases<br />
by 180 ◦ . The linac energies are balanced when the injected energy is equal to the<br />
32
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