STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA STUDIES OF ENERGY RECOVERY LINACS AT ... - CASA
ever, linacs are limited to accelerating small amounts of average beam current due to the prohibitively expensive radio-frequency (RF) power required. An energy recovering linac (ERL) is a powerful alternative accelerator concept which combines the desirable characteristics of both storage rings and linacs, by having the potential to accelerate hundreds of milliamperes of average current to several giga-electron volts in energy while maintaining excellent beam quality. 1.1 Energy Recovering Linear Accelerators The idea of energy recovery was first proposed in 1965 for use in a collider [1]. While such a collider has yet to be realized, within the last decade energy recovery has found a niche in drivers for light sources. A schematic for a generic ERL based light source is given in Fig. 1.1. Electrons are generated in a high brightness injector, accelerated through a linac and then transported to a region where the desired radiation is generated (e.g. an undulator or a wiggler). After performing their intended purpose, the electrons are returned to the linac 180 ◦ out of phase with respect to the RF accelerating field for energy recovery. At the exit of the linac, the energy of the decelerated beam is approximately equal to the injection energy and the beam is directed to a beam dump. In ERLs the decelerated beam cancels the beam loading effects of the accelerated beam. Therefore ERLs can, in principle, accelerate very high average currents with only modest amounts of RF power. Because the net RF current seen in the linac is negligible, high average currents can be accelerated economically. Furthermore, since the electron beam only exists in the accelerator for a short time (typically two passes), the equilibrium that is unavoidable in a storage ring does not have time to develop. Thus the beam quality in an ERL is determined, to a large extent, by the injector. This combination of 3
FIG. 1.1: Schematic of a generic light source based on an energy recovering linac driver. high average current capability and high beam quality make ERLs attractive as, among other things, drivers for oscillator FELs and synchrotron light sources. Another advantage of ERLs results from the fact that the energy recovered beam loses energy as it gets decelerated and is dumped at an energy close to its injection energy. Thus the beam dump design is simplified because the energy of the beam is reduced by a factor of (Emax/Einj) where Emax is the energy of the beam before energy recovery and Einj is the injection energy. Energy recovering linacs are not without their challenges, however. One of the most severe limitations to ERL performance is a form of regenerative beam breakup (BBU), called multipass, multibunch BBU, and is the primary subject of this dissertation. The mechanism for BBU begins when a beam bunch passes through an RF cavity off-axis, thereby exciting dipole higher-order modes (HOMs). The magnetic field of an excited mode deflects following bunches traveling through the cavity. Depending on the details of the machine optics, the deflection produced by the 4
- 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: CHAPTER 1 Introduction An increasin
- 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
FIG. 1.1: Schematic of a generic light source based on an energy recovering linac driver.<br />
high average current capability and high beam quality make ERLs attractive as,<br />
among other things, drivers for oscillator FELs and synchrotron light sources.<br />
Another advantage of ERLs results from the fact that the energy recovered<br />
beam loses energy as it gets decelerated and is dumped at an energy close to its<br />
injection energy. Thus the beam dump design is simplified because the energy of the<br />
beam is reduced by a factor of (Emax/Einj) where Emax is the energy of the beam<br />
before energy recovery and Einj is the injection energy.<br />
Energy recovering linacs are not without their challenges, however. One of<br />
the most severe limitations to ERL performance is a form of regenerative beam<br />
breakup (BBU), called multipass, multibunch BBU, and is the primary subject of<br />
this dissertation.<br />
The mechanism for BBU begins when a beam bunch passes through an RF<br />
cavity off-axis, thereby exciting dipole higher-order modes (HOMs). The magnetic<br />
field of an excited mode deflects following bunches traveling through the cavity.<br />
Depending on the details of the machine optics, the deflection produced by the<br />
4
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