JLAB-TN-09-046
10 August 2009
Use of Recirculation in Short-Wavelength FEL Drivers
D. Douglas and C. Tennant
Abstract
We discuss issues associated with the use of recirculation and energy recovery as cost-reduction measures in the design of short-wavelength FEL drivers. An example recirculation transport line is presented.
Introduction
Providing a CW drive beam for a short-wavelength FEL requires:
1. A high brightness electron source
2. An injector and injection line that preserve beam quality
3. A phase space management scenario using the beam provided by the injector; in particular, there must be a longitudinal matching scenario giving adequate bunch compression/peak current at the wiggler and appropriate provision for transverse matching
4. The ability to maintain beam brightness during the acceleration, transport and compression process by avoiding the impact of lattice aberrations (chromatic and geometric) and collective effects such as BBU, other wakefield/impedance effects (e.g. the microbunching instability (MBI), resistive wall, etc), space charge, and coherent synchrotron radiation (CSR).
These challenges have been/are being met in pulsed systems (LEUTL, VISA, FLASH, LCLS, FERMI) and there is a consensus that they can be met in a CW FEL driver with “linear” topology (i.e., without recirculation) such as WiFEL [1]. In this note, we discuss the implications of using recirculation and energy recovery as a cost-control measure in the design of driver systems. We detail additional challenges thereby introduced, and present a notional approach for addressing one of the issues – specifically, the problem of beam quality preservation during recirculation. This discussion will occur in the context of JLAMP – a proposed upgrade (to short wavelength) of the JLab IR/UV FEL.
Recirculation, Energy Recovery, Cost Control, and Beam Quality
Use of multi-pass acceleration is universally recognized as an effective cost-control measure in the design of SRF linacs. Energy recovery provides similar benefit inasmuch as it simplifies radiation control (by limiting beam energy and power at the beam dump) and alleviates RF power demands. This is true even in systems running modest (~ 1 mA) current if they operate at suffiently high gradient: a 7-cell 1497 MHz cavity at 20 MV/m accelerating 2 passes of 1 mA beam (roughly JLAMP parameters) will, for example, draw ~30 kW RF power without energy recovery, but (depending on the choice of QL) may draw only 1 to 2 kW with recovery. This represents a savings of ~1/4 MW RF drive– a cost reduction of order 2.5 M$ – per cryomodule.
Many issues are, however, introduced by the use of recirculation, including:
1. The need for an appropriate (beam-quality preserving) injection merger
2. The potential impact of additional beam transport length; in particular, the effect of wakefields, environmental impedances (with their potential to aggravate MBI), and space charge
3. Additional complexity in longitudinal matching
4. Use of common transport for multiple beams (during energy recovery)
5. Possible BBU limitations, and
6. The impact of lattice aberrations and CSR during recirculation.
Most of these issues appear to be tractable and/or are the focus of ongoing investigation in a number of projects. Merger design is a critical problem for ERL-based x-ray sources and the Navy INP FEL. Initial results from these efforts suggest however that this difficulty is manageable: a JLAMP-class machine can use higher injection energy than either an x-ray ERL or the INP inasmuch as the lower JLAMP current requires far less RF power, and, in addition, JLAMP uses much lower charge than the INP and only modestly higher charge than a conventional ERL.
Wakes/impedances have been investigated as part of the high-energy ERL design program [2], and appear to be controllable using methods proven in storage rings over the past few decades. Though the bunch charge involved is lower in a “big ERL" than in JLAMP, the bunch length is also shorter, yielding similar peak currents; path lengths involved in fact favor the JLAMP scenario as the machine is much smaller. Similar (and acceptable) impact on beams of more or less equivalent brightness is thus to be expected. Though a careful characterization and impedance policy will be required, the integrated effect of these phenomena should be adequately controlled.
Operation of a multipass ERL will require a rather more complex longitudinal matching scenario than that used during SRF ERL operation to date. We have limited experience with “one-and-a-half-pass” operation of the IR Demo [3], and preliminary analysis has yielded a reasonable solution for a 2-pass up/2-pass down JLAMP-class machine [4]. This will be discussed in more detail below. In this solution, an isochronous arc is used on the first (accelerating) pass and final (recovery) pass. This requires the use of common transport for the two beams; this method was successfully demonstrated with CEBAF-ER [5], wherein two 500 CW MeV beams (one accelerated, one recovered) were transported through a common beamline for several hundred meters. This requirement is therefore not expected to present fundamental limitations.
BBU has historically imposed serious limitation on SRF linac performance, but recent developments render it much less of a concern for JLAMP-class systems. This is due to the relatively low currents involved, better management of the instability by the transport lattice, and much better control of SRF cavity HOM spectra available using modern design and construction methods. An example study is provided in Ref. [6] and typical modeling results available in Ref. [7]. None of these results suggest that undue concern is warranted.
We therefore focus our attention on the final issue: the impact on beam brightness of transport through a recirculation arc. In addition to traditional lattice issues (chromatic and geometric aberrations), we must in this case be concerned with synchrotron radiation (both incoherent and coherent) driven degradation of beam quality. Though there exists no consensus on the feasibility of beam quality preservation during recirculation, the potentially significant cost impact encourages serious investigation of this technique, at least in order to establish limits beyond which the use of this technique breaks down. There have, moreover, been initial studies [8, 9] suggesting that this approach can be successfully applied, at least in somewhat different regions of parameter space than those envisioned for JLAMP. In the following, we outline a notional approach to the design of an isochronous (but tunable) recirculation transport that would preserve beam quality while allowing us to double machine energy in the existing vault.
A JLAMP-Class Recirculator
The following discussion constitutes a design exercise intended to provide an existence proof of a recirculation arc that fits in the existing JLab FEL vault and preserves beam quality well enough to drive a short-wavelength FEL. We proceed with this exercise by first stating the top level design requirements, enumerating the major issues, and addressing them each in turn.
Design Requirements – JLAMP [10] will be a two-pass 600 MeV ERL driver for a short wavelength FEL in the JLab FEL vault. It will comprise a high-brightness 10 MeV injector, a 300 MeV linac based on three high-gradient (100 MeV) cryomodules and a two-pass recirculator (300 MeV and 600 MeV beam transport lines), with an FEL embedded in the second pass. The system will fit in the existing JLab FEL vault (within a ~12 m x ~65 m footprint).
The JLAMP FEL is intended to reach the 10 nm wavelength scale, and thus the electron beam must present a geometric transverse emittance of (10 nm/4p) at 600 MeV, corresponding to a normalized emittance of 1 mm-mrad. In order to lie within the FEL momentum acceptance and produce sufficient peak current using the 200 pC design bunch charge, we require a 50 keV-psec longitudinal emittance out of the injector. This would, for example, allow delivery of a 10-3 rms relative momentum spread with 0.5 psec FWHM bunch length to the wiggler (corresponding to 120 keV-psec at 600 MeV) while providing allowance for modest degradation of beam quality during acceleration, transport, and compression.
The FEL itself is assumed to have an extraction efficiency of ~0.3%, with a corresponding full-energy full exhaust momentum spread of 2% (~6 times the extraction efficiency). During energy recovery, this could (depending on the choice of longitudinal match) double in the final recirculation pass (at half energy), requiring ~4% momentum acceptance in the 300 MeV recirculator.
Physics Issues – The technical issues associated with the above requirements are apparent. Most obvious is the challenge of transporting a 600 MeV beam in a vault with footprint originally designed for the 210 MeV IR Upgrade. The beam transport system electron-optical design is therefore of primary concern; particular attention must be provided for the management of longitudinal and transverse matches and the control of aberrations (which can lead to significant degradation of beam quality). In addition, the move to high energy by way of multiple passes and high gradient will require thorough analysis of the BBU instability, although (as noted above) this is not expected to impose serious limitations. The use of relatively high energy in a small footprint may result in the generation of significant levels of incoherent synchrotron radiation (ISR), with attendant and potentially unacceptable levels of emittance excitation. Finally, considerable care must be taken to insure that CSR does not degrade beam quality.
Longitudinal Matching Scenario – Various longitudinal matching scenarios can in principle provide an appropriately compressed bunch to an FEL. However, the need to utilize energy recovery while avoiding parasitic compressions during the acceleration cycle and the necessity for adequate momentum acceptance for recovery of the FEL exhaust beam provides considerable guidance in choice of acceleration/deceleration phases and selection of momentum compactions. A preliminary study [11] indicates that the following longitudinal match is acceptable.
1. Inject a long, low momentum spread bunch (to avoid LSC effects).
2. Accelerate the first pass beam through the linac ahead of crest (on the rising portion of the RF waveform).
3. Use an isochronous first recirculation transport
a. Provide 4-5% momentum acceptance to support energy recovery
b. This will retain (future) option of lasing on both passes [12].
4. Dechirp (accelerate on the falling part of the waveform) during the second pass so as to energy compress the beam to get to small momentum spread.
5. Compress the bunch length in the full energy linac-to-wiggler transport.
6. Decompress the bunch length (to set up energy compression during energy recovery) using the full energy wiggler-to-linac transport.
a. Select linac reinjection phase and wiggler-to-linac transport compaction to keep the momentum spread of the recovered beam within the first recirculator acceptance during energy recovery.
A simple simulation of this process [13] is presented in Figure 1, which shows the evolution of a longitudinal phase region space as it is accelerated and transported through the system. The model includes nonlinear RF and compaction terms, but no collective effects. It gives an existence proof for a longitudinal matching solution taking a 50 keV-psec injected phase space to a wiggler to deliver ~0.25% rms momentum spread in ~0.120 psec rms (about 150 keV-psec, including nonlinear distortions from the acceleration and bunching process). As indicated above, the 300 MeV recirculator is isochronous, and the full-energy reinjection phase and wiggler-to-linac transport compactions are selected to provide energy compression during energy recovery.
The FEL exhaust energy spread at 0.3% extraction efficiency of ~17 MeV at 600 MeV – or about 2.8%=1% (core beam) +1.8% (lasing induced) – is compressed to ~9 MeV (or 3%) during the second transit of the first arc, and then to ~1.5 MeV at the dump energy (here, ~12 MeV). Accelerating phases and compaction values are given in Table 1. We note that the injected longitudinal phase space is rather similar to that already produced (albeit at 135 pC) in the JLab IR Upgrade, and the choice of first-pass accelerating phase was made in part based on Upgrade operational experience (these parameteric choices alleviate LSC [14]) and in the desire to present to the first arc a beam of moderately large momentum spread (sdp/p ~ ½%) so as to assist in CSR management during the recirculation transport.
The simulation locks the phase difference from third to fourth pass to match that from the first to second (as the common transport has identical time of flight). As the solution straddles crest from pass to pass on acceleration (to dechirp the phase space on the second pass), the beam also jumps from one side to the other of trough during recovery. This fundamentally limited the recovered bunch length (and hence the recovered energy spread) because one or the other working points will – as the momentum spread (and hence recovered bunch length) increases – eventually have electrons tailing off into trough and forming a high energy tail. It will therefore be useful to devise a very large acceptance dump line so as to recover as large a final energy spread as possible. The solution also tends to be a bit “twitchy”, inasmuch as small phase changes can result in large swings of energy spread in the recovered beam. This is not at all surprising, given the rather large linac energy gain. Note also the system engages in “incomplete” energy recovery. Care should be taken to define RF drive requirements appropriately.
Table 1: Longitudinal Matching Parameters
Einj (MeV) / 10DElinac (MeV) / 304.6
Pass 1 / Pass 2 / Pass 3 / Pass 4
Eafter pass (MeV) / 310 / 596.3 / 304.63* / 11.99*
Phase during pass / -10o / 20 o / 156.8 o / 186.8 o
Compactions (m) / M56 / T566 / W5666
1st arc / 0 / 0 / 0
linac-to-wiggler / 0.38 / 36 / 3300
wiggler-to-linac / 0.24 / 7 / 200
*lasing at 0.3% extraction efficiency
Figure 1: Longitudinal matching scenario for two-pass 600 MeV JLAMP ERL driver while lasing at 0.3% extraction efficiency. Vertical axis: energy in MeV; horizontal axis: time in 1497 MHz RF degrees. Phase space should be viewed, notionally, as subtending ±2s in each variable.
Transport System Constraints – The multipass recirculation transport must satisfy numerous constraints:
1. It must separate each pass for recirculation (implying the need for spreaders and recombiners)
2. It must provide for betatron matching into/out of the linac on each pass and into/out of the FEL at full energy
3. It must support the longitudinal matching process by providing path length and compaction control (through appropriate nonlinear order). The low energy pass must nominally be isochronous. Tuning range on each longitudinal parameter must be available.