Accelerator Design for the CHESS-U Upgrade

During the summer and fall of 2018 the Cornell High Energy Synchrotron Source (CHESS) is undergoing an upgrade to increase high-energy flux for x-ray users. The upgrade requires replacing one-sixth of the Cornell Electron Storage Ring (CESR), inverting the polarity of half of the CHESS beam lines, and switching to single-beam on-axis operation. The new sextant is comprised of six double-bend achromats (DBAs) with combined-function dipole-quadrupoles. Although the DBA design is widely utilized and well understood, the constraints for the CESR modifications make the CHESS-U lattice unique. This paper describes the design objectives, constraints, and implementation for the CESR accelerator upgrade for CHESS-U.


I. INTRODUCTION
The Cornell Electron/positron Storage Ring (CESR) was designed as an electron-positron collider, with two diametrically opposed interaction regions. The layout of the storage ring is constrained by the very nearly circular geometry of the tunnel that it shares with the existing synchrotron. The CLEO detector required a long straight section to accommodate strong IR quads for a mini-beta insert and anti-solenoids to compensate the 1.5 T CLEO solenoid, in addition to the detector itself. Hard-bend dipoles were required to bend the trajectory into the long straight, with gentler bends immediately adjacent to the straight to mitigate synchrotron radiation background in CLEO. The collider layout and characteristic optics are shown in Fig. 1.
The Cornell High Energy Synchrotron Source (CHESS), founded contemporaneously with CESR, was initially comprised of three x-ray extraction lines off the hard-bend dipoles from the counter-clockwise electron beam, expanding in 1988-89 with the addition of four beam lines (including one permanent-magnet wiggler) off the clockwise positron beam in CHESS East, and again in 1999 with the construction of G-line, adding three end stations fed by an insertion device from the positron beam.
Following the conclusion of the CLEOc HEP program in 2008, CESR was reconfigured for the CESR Test Accelerator program (CesrTA) [1] damping ring R&D program. The CLEO particle detector drift chamber was removed and the final focus replaced with a conventional FODO arrangement. The long straight was outfitted with six superconducting damping wigglers, relocated from two straights in the arcs.
The CesrTA layout also enabled the operation of CHESS in the arc-pretzel configuration with counterrotating beams of electrons and positrons [2]. Layout and optics are shown in the middle plots in Fig. 1. * shanks@cornell.edu CHESS was designed to run parasitically with HEP operations, with beam lines fed by both electron and positron beams. Running with two electrostaticallyseparated beams greatly complicates many facets of operation, including restricting bunch patterns and maximum per-bunch current, increasing the tune plane footprint, and impairing the dynamic and momentum apertures.
With the conclusion of HEP operations, CESR is being reconfigured as a dedicated high-energy, high-performance single-beam synchrotron light source, CHESS-U. The conversion to operation with a single clockwise beam requires rebuilding the beam lines fed by the counter-clockwise beam. The new layout eliminates the long IR straight and hard bends in favor of evenly distributed achromats and insertion straights, as shown in Fig. 1.

A. Design Objectives and Constraints
The accelerator requirements for the CHESS-U upgrade are: 1) Minimize the natural emittance for a single on-axis beam at 6.0 GeV; 2) Introduce straights for independent insertion devices for most end stations; 3) preserve compatibility with existing injector system (linac and synchrotron booster ring); and 4) Increase highenergy x-ray flux for all end stations.
Most of the CHESS beam lines are presently located in the L0 main experimental hall, fed by both electron and positron beams, see Fig. 2. Eight end stations are located in L0, four from each beam; a further three end stations were added in 1999 as an excavation out of the west CESR tunnel, fed by the clockwise-oriented positron beam. For single-beam operation, a clockwise orientation was chosen in order to preserve the existing six end stations from the F-line and G-line facilities.
The existing storage ring layout in L0 includes one long straight section for the former CLEO detector, which is not optimal for low-emittance x-ray light source operation. The present optics do not have enough degrees of freedom to constrain the dispersion in the hard bend dipoles which bring CESR into the long straight, and as a result, the hard bends dominate the natural emittance (see Fig. 3). Additionally, the long straight section cannot accommodate multiple source points for x-ray beam lines, which require angular separation between sources. The L0 experimental hall was also home to the threestory CLEO particle detector, which ran for HEP operation from 1979-2008. For single-beam CHESS operation, several x-ray extraction lines would pass through the CLEO iron; to make room for new source points, the CLEO detector was disassembled and removed in 2016.

B. Linear Optics
In order to maximize the number of end stations, a layout was chosen with six 13.8 m double-bend achromats with 3.5 m insertion device (ID) straights. It is worth noting the cell length is approximately half that of many third-generation storage ring light sources. DBAs were chosen over a multi-bend achromat design for a number of reasons: 1) The space available is extremely limited; 2) The dynamic aperture requirements would be incompatible with the existing accumulation injection scheme; and 3) the cost and complexity would be beyond the scope of this project.
The new DBAs utilize 2.35 m defocusing combinedfunction sector dipole-quadrupoles (DQs). Though the bend radius of the new CHESS-U dipoles is nearly identical to the old hard bends (31.4 m vs. 31.7 m), the dispersion is well suppressed in the new achromats, lead-ing to a factor of four reduction in the global emittance while only replacing one-sixth of the storage ring.
A single DBA shown in Fig. 4. The natural emittance for a single CHESS-U cell is 2.56 nm·rad at 6.0 GeV. Matched into the remainder of the CESR, the full ring optics are shown in Fig. 5. Linear and nonlinear optics were optimized using Tao [3], which is built on the Bmad accelerator library [4].
The six new achromats span the 100-meter region between the two superconducting rf straights, and begins and ends with straights for insertion devices. Five of the seven straights will be used for CHESS IDs; there is not presently space for end stations for the remaining two straights. The remaining two straights will have one CESRc superconducting damping wiggler each [5], to be used only in low-energy machine studies. The new CHESS-U lattice will enforce zero dispersion through the rf cavities, a condition which is not met in present twobeam CHESS optics. The new achromats are matched into the existing FODO structure with four additional quadrupoles in each of the rf straights.
The storage ring energy will be increased to 6.0 GeV. Aside from CLEOc HEP operation in 2003-2008 and occasional CesrTA machine studies from 2008-2016, when the energy was lowered to 2.085 GeV/beam, CESR has operated at 5.3 GeV/beam. CHESS operation has been exclusively at 5.3 GeV. CESR was originally designed for an energy range of up to 8.0 GeV, though the highest operating energy to-date is 5.6 GeV. Machine studies conducted in 2017 in the present CESR configuration demonstrated injection and accumulation at 6.0 GeV. Every quadrupole and sextupole in CESR is indepen-  dently powered, allowing significant flexibility in operating conditions. Therefore the remaining 5/6 of the storage ring requires no modification.
CHESS-U lattice parameters are summarized in Table I.

C. Nonlinear Optics
Due to the complete lack of periodicity in CESR (the nearest symmetry in the ring is a mirror symmetry about the North-South axis), nonlinear optics are numerically optimized for CHESS-U using 76 independentlycontrolled sextupoles with an implementation of Bengtsson's resonance driving term (RDT) formalism [6] in Bmad. As there is no symmetry, there are no sextupole families, and all sextupoles are allowed to vary separately.
An unusual design feature of the CHESS-U achromat is a lack of sextupoles in the new sextant. The peak dispersion in the DBAs is O(20 cm), whereas the average dispersion in the remaining 5/6 of CESR is O(1 m). As such, any sextupoles in the new sextant would have little leverage on the chromaticity. Optimizations showed the inclusion of harmonic sextupoles in the new DBA cells did not significantly improve the nonlinear optics, and were therefore omitted.
The new DQ magnets have a significant b 2 multipole, contributing roughly ∓0.5 to the horizontal and vertical chromaticity, respectively. This is compensated during the RDT reduction optimization.   FIG. 6: SPECTRA modeling of flux through a 1 mm 2 pinhole at 20 meters, before and after CHESS-U upgrade [12]. "F-line wiggler" refers to the 24-pole wiggler for Sector 1.

B. Error Tolerance
Error tolerance for CHESS-U was assessed using the tools and low-emittance tuning methodology developed for the CesrTA program [13]. The optics correction procedure used here is identical to that in CesrTA: 1. Measure closed orbit; correct using horizontal and vertical steering magnets.
2. Measure betatron phase advance and local coupling [14] and horizontal dispersion; correct using quadrupoles and skew quadrupoles.
3. Measure orbit, local coupling and vertical dispersion; correct using vertical steerings and skew quadrupoles.
Though the new DQs do have trim windings, the correction simulation did not allow for the DQs to vary. Error amplitudes are listed in Tables V and VI, and represent the best estimate of anticipated alignment and BPM errors.
A summary of the misalignment study is shown in Fig.  7. The statistical impact of misalignments and corrections on the Twiss parameters is shown in Fig. 8. The 95 th -percentile vertical emittance due to optics errors after correction is 4.1 pm.

C. Dynamic and Momentum Aperture
Dynamic and momentum aperture are evaluated via frequency map analysis [15]. Particles are tracked for a total of 2048 turns at each initial amplitude; dynamic aperture plots are normalized to beam sigmas, assuming 1% emittance coupling.
Systematic multipoles for dipoles, quadrupoles, and sextupoles are determined via field modeling or measure-  Table VII. Due to the compact nature of the CCU magnet arrays, the field roll-off is significant: vertical field is reduced 10% at a 10 mm horizontal displacement. Systematic multipoles and the field rolloff and measured field integrals for CCUs are included in tracking simulations where stated [2]. Dynamic aperture and its footprint in the tune plane are shown in Fig. 9 for combinations of systematic multipoles, insertion devices, and misalignment/correction; corresponding momentum aperture with systematic multipoles, IDs, and misalignment/correction are shown in Fig. 10.

D. Injection Simulation
The injection scheme for CESR relies on accumulation from a 60 Hz synchrotron booster ring. Injection efficiency is simulated via multi-particle tracking. 200 macroparticles are launched with a phase space matching the beam at the end of the synchrotron booster extraction line. Particles which survive for 400 turns are considered captured. Details are found in [2].
Results of the injection tracking simulations are shown in Fig. 11. It should be noted that the injection efficiency is not expected to reach 100% as there are two 10 mm   full-aperture vertical collimators used to mask against particle losses on the small-gap permanent-magnet IDs.

IV. MAGNET DESIGN
Most constraints on magnet design are more or less universal. The specific impact on the CHESS-U magnets is discussed here. Details are available in [16].  Color scale indicates capture efficiency in percent.

B. Quadrupoles
All achromat quadrupoles are horizontally-focusing, as the vertical focusing is done by the DQs. There are two families of quadrupoles, with magnetic lengths 0.400 m and 0.362 m, both with the same pole profile; the primary distinctions are length and the extension of the iron to allow for x-ray extraction on one of the two families. Both quadrupole families have maximum gradient 39 T/m, with a bore radius of 23 mm. Fig. 13 shows the magnetic field in one quarter of a non-extraction quadrupole and the excitation curve; an extraction quad model is shown in Fig. 14.

C. CHESS Compact Undulators
Four of the five new sectors will be equipped with canted pairs of CHESS Compact Undulators (CCUs). The design for the CCUs is detailed elsewhere [7][8][9]. An end-view of the CCU profile is shown in Fig. 15a; perspective CAD drawing of CCU assembly is shown in  Table  II.
The CCUs utilize a variable-phase design rather than variable-gap in order to minimize the supporting infrastructure. As implemented in CESR the CCUs have a 7.0 mm out-of-vacuum vertical aperture between poles, mandating the vacuum chamber to fit inside this aperture. The associated vacuum chamber for the CCUs is    CHESS has operated with two 1.5 m CCUs in a canted straight since 2014, with two counter-rotating beams. Total beam current has routinely exceeded the CHESS-U target 200 mA in single-beam positron tests at the present CHESS beam energy of 5.3 GeV.

D. Power Distribution
For the new CESR south arc, the magnet power distribution was chosen to be be compatible with existing infrastructure. There are two main pieces to this: the CESR dipole circuit and the CESR quadrupole bus system. The controls for both are derived from a proprietary (and perhaps unique) computer bus known as the Xbus [17,18]. The Xbus handles all digital and analog controls, read back signals, and interlocks associated with the operation of these magnet power supplies. All magnets can be controlled individually or collectively as needed from a central control room.
The CESR Dipole circuit is essentially a series circuit of approximately 120 dipole magnets driven by two 0.5 MW Transrex power supplies at 500 V and 740 A to obtain 6 GeV. The series chain is made up of normal bend, soft bend, and hard bend dipole magnets. The new DQ magnets will be part of this existing circuit.
The operation of CHESS-U will require 5 quadrupole buses: East, South East, South Center, South West, and West. The SE, SC, and SW are new buses installed expressly for the south arc installation. The West and East quad buses are preexisting. Each quad bus is driven by an 80 V 1125 A EMHP power supply. These buses supply a fixed 70 V primary voltage to a large number of switching power supplies known as choppers (DC-to-DC converters). These power supplies excite all quadrupole, sextupole, steering, skew quadrupole, skew sextupole, and octupole magnets. Through the Xbus, each switching power supply can be controlled independently of all others, giving total flexibility to tuning the optics. The current draw from these buses is entirely dependent on the optics requirements, typically scaling with beam energy.

E. Support and Alignment
Each new CHESS-U cell is split into three sections: an upstream girder, a downstream girder, and an insertion device straight. The two girders support all achromat magnets, each holding two quadrupoles and one DQ, plus any corrector magnets. The insertion device straight is suspended from above in order to allow access to the upstream sector's front end. One sector is illustrated in Fig. 16.
Magnetic alignment of the DQ and quadrupoles is achieved using a combination of vibrating wire [19] and Hall probe. The procedure is as follows: 6. Align DQ using Hall probe The setup used for this alignment procedure is diagrammed in Fig. 17. Using this method, the magnetic centers of quadrupoles and the DQ on one girder are aligned to within 32 µm.

A. Design Considerations
As a part of particle beam transport system, the CHESS-U vacuum chambers need to provide adequate physical beam aperture, while maintaining sufficient clearance to all CHESS-U magnets. The vacuum pumping system is designed to achieve and to maintain ultrahigh vacuum (UHV) condition, with average vacuum pressure < 1 nTorr with stored 200 mA beam at 6.0 GeV. Additionally, all vacuum chambers must be designed to manage heating from synchrotron radiation (SR) generated by the bending magnets, with a minimum factor of safety (FOS) >2. The vacuum chambers will also house a suite of BPMs for beam instrumentation. extremely limited longitudinal spaces between the magnets, each achromat cell vacuum string is made into three flanged chambers, as shown in Fig. 18. More specifically, each achromat is comprised of two long dipole chambers, a 4.91 m long downstream "Dipole C" chamber and a 5.03 m long upstream "Dipole A" chamber, which are integrated with magnet girders, and a 3.69 m long undulator vacuum chamber.
The majority of the chambers are made from 3 styles of 6063 aluminum extrusions (Fig. 19): dipole extrusion, quad extrusion and undulator extrusion. The minimum clearances between magnet pole tips and the extrusions are slightly less than 2 mm, as illustrated in Fig. 20. The dipole and quad extrusions have a beam aperture of 52 mm (H) × 22 mm (V), while the vertical aperture in the undulator extrusion is 5 mm. The dipole and undulator extrusions have ante-chambers for distributed pumping.
The bending section of a dipole C chamber, as shown in Fig. 21a, is made from the dipole-style extrusion (Fig.  19b). Precision bending (31.17 m radius, 4.26 degree bending angle) of the extrusion was achieved via the stretch-forming technique, in which an aluminum extrusion was formed to a precision die while being stretched to its yield stress. The stretch-formed section was then machined to its design form, and the quad-style (Fig.  19a) extrusions welded to both ends. Four sets of BPMs were welded directly onto the quad extrusions near locations of quadrupole magnets. Other functional vacuum components, including an RF-shielded sliding joint and pumping ports, were also welded to the quad extrusions. The chamber completes with a pair of stainless flanges with aluminum-to-stainless steel bi-metal transitions.
The dipole A chamber design is much more complicated than the dipole C chamber, as it needs to provide egress for the X-ray from the canted undulators. The dipole A design is illustrated in Fig. 21b. To incorporate the compact geometry needed for the passage of both stored electron beam and egress of X-ray from the undulators, the chamber main body is constructed by welding of two precision machined "clam-shells" of 6061-T6 aluminum alloy. Cooling channels and NEG strip mounting features are also machined in the main body. Similar to the dipole C chamber, quad extrusions are welded to both ends of the main body. Two sets of BPMs, one RF-shielded sliding joint and multiple pump ports are also directly welded to the chamber. At separation junc- tion of the electron beam and the X-ray, an insertable crotch absorber is mounted onto the dipole A chamber, to intercept up to 5.4 kW of SR power from the dipole magnet, while allowing required passages of both electron beam and the X-ray. The crotch absorber is one of the most challenging CHESS-U vacuum components, due to very limited available space. Its design is based on the existing CHESS crotches, consisting of a pure beryllium ring (5 mm thick) and a water-cooled copper core. The beryllium ring, vacuum brazed to the copper core, dilutes the SR power density by bulk absorption and scattering along its depth. The detailed thermal stress analysis of the crotch is beyond the scope of this paper. However, the analysis indicates that the crotch absorber has sufficient margin of safety to handle SR power at designed CHESS-U beam parameters. Design and construction of the CHESS-U undulator chambers are similar to an ex- isting CCU chamber in operation for the last 2+ years. As shown in Fig. 21c, the undulator beampipe is made from the CHESS-U undulator extrusion (Fig. 19c). Two sections (1.51 m each) of the extrusion are machined to a thickness of 0.61 mm to accommodate the 7.0 mm CCU pole gap. The machining of the undulator extrusion is done completely dry to avoid the difficulty in cleaning the long beampipe with 5 mm vertical aperture with thin walls. The wall thickness at the thin section was constantly checked using ultrasonic thickness gauges during the machining. Two end assemblies are welded to the undulator beampipe, that gently transition vertically from 5 mm to 22 mm, the nominal CHESS-U vertical aperture. Six pumping ports are also welded to the ante-chamber of the undulator extrusion.

C. Vacuum Pumping and Performance Simulation
All CHESS-U vacuum chambers were baked to 150 • C and then back-filled with ultra-high purity nitrogen through a MATHESON NANOCHEM R purifier that reduces H2O/HC level below ppb level. A post-installation 95 • C hot water bakeout of all CHESS-U chambers is also planned. With these measures, SR-induced desorption (SRID) is expected to be the dominant source of gas load. To achieve the required level of vacuum, adequate vacuum pumping systems must be implemented.
Owing to distributed nature of the SRID gas load and very restricted gas conductance of the CHESS-U  beampipe, it is essential to have distributed vacuum pumping. Non-evaporable getter (NEG) strips (st-707 SAES Getters) are chosen for the distributed pumps in the dipole chambers over distributed ion pump in CESR, due to the limited space available. The structure of the NEG strip assembly is shown in Fig. 22. A 30 mm wide NEG strip is supported on to a stainless steel ribbon with periodic stainless steel clips. The clips are pinched to the NEG strip, and are riveted to the stainless steel ribbon through sets of alumina spacers. The NEG strip is to be activated by resistive heating to 500 • C with a DC power supply. Flexible connections at both ends of the NEG strip to the electric vacuum feedthroughs allow thermal expansion during activation. Very high SRID gas load is expected in the initial beam conditioning stage. Simulations showed that the NEG strips may not have sufficient pumping capacity during the CHESS-U commissioning phase, and it can be disruptive to the initial operation with frequent activation cycles. Therefore, compact high capacity NEG modular pumps, CapaciTorr Z200 and NexTorr Z100 (SAES Getters), are installed to aid the initial beam conditioning. To handle potential noble (or non-gettable) gases, a large sputtering ion pump (approx. 110 l/s) is also installed on each achromat cell.
The CHESS-U vacuum pumping performance was evaluated using MolFlow+, a Test-Particle Monte-Carlo simulator developed at CERN. The simulations showed that the installed vacuum pumping system is capable of achieving required level of vacuum for CHESS-U operations. Figure 23 displays an example of simulated pressure profile in one CHESS-U achromat cell, after 100 Amp-hr beam conditioning, achieving an average pressure of ≈ 1.8 × 10 −9 torr. Large pressure "bumps" are due to lack of pumping in the straight sections populated by the magnets.

VI. INSTRUMENTATION
CESR is well-instrumented for a multitude of beambased measurement techniques, thanks to the CesrTA program. Details are documented elsewhere [20]. A few highlights are noted here.

A. CESR Beam Position Monitors
CESR is presently equipped with 110 peak-detection BPMs of an in-house design (CBPM-II), capable of measuring bunch-by-bunch turn-by-turn positions for bunch  spacings ≥ 4 ns. 100 CBPMs are used for routine optics correction and orbit monitoring. Each achromat in CHESS-U will be instrumented with four CBPM-II modules, one adjacent to every quadrupole. CBPM-II specifications are listed in Table III; further details are available in [21].
With the installation of canted CCU undulators in 2014 [22], three Libera Brilliance Plus processors have been used for continuous turn-averaged position monitoring at the existing 3.5 m-long 4.6 mm-aperture CCU chamber.

B. X-Ray Video Beam Position Monitors (vBPMs)
Each x-ray front end will be equipped with two vBPMs, one for each extracted beam, to provide horizontal and vertical position. The vBPMs will use an edge-on diamond blade monitor which was prototyped on A-line in 2015 [23], rendered in Fig. 24. The diamondblade style vBPM allows for a compact design close to the source (roughly 10 m), where the canted undulator beams are only separated by 10 mm.
The vBPMs are presently used in conjunction with Libera BPMs around the small-gap undulator chamber to monitor x-ray trajectories in real-time and correct positions after every topoff, typically every five minutes. This strategy will be expanded in CHESS-U, where each sector will have two Libera Brilliance Plus monitors dedicated to real-time position monitoring for position feedback.

C. Turn-by-Turn Feedback
Bunch-by-bunch turn-by-turn horizontal, vertical, and longitudinal feedback is accomplished using three Dimtel feedback front-end and processing modules [24]. The low level outputs are connected to 200 W, 150 MHz ENI amplifiers which drive horizontal and vertical stripline kickers. The longitudinal processor drives a 200 W solidstate RF amplifier that in turn drives a 1 GHz low Q cavity kicker. While the system can handle bunch spacings as short as 2 ns, gain falls off because of the short interval required to provide feedback. A bunch spacing of 14 ns provides for the maximum kick as well as matching better with the injector RF frequency.

VII. BUNCH PATTERNS
Historically, bunch patterns were constrained by the beam-beam interaction from electrostatically-separated counter-rotating beams in the same vacuum chamber. With the simplification of single-beam operation, there are many more possibilities.
There is increasing interest in time-resolved experiments (see, for example, [25,26]), and a number of possible bunch patterns are under consideration. The constraints on bunch patterns are now discussed, and several bunch patterns proposed.

A. Mode Coupling Instability Threshold
Many light source upgrades are favoring mult-bend achromat designs with strong focusing and small vacuum apertures (see, for example, [27,28]). However, the resistive wall impedance scales as 1/r 3 , quickly limiting the single-bunch current due to mode coupling instability [29]. The vacuum chamber aperture for CHESS-U has been kept relatively large (52 mm × 22.5 mm fullaperture) in an effort to keep the maximum single bunch current as high as possible.
The transverse mode coupling instability (TMCI) threshold has been estimated for CHESS-U based on analytic resistive wall calculations [30] and detailed T3P modeling for discontinuities [31,32]. The limit is estimated to be around 25 mA (64 nC) [33]; conservatively, only bunch patterns up to 20 mA are considered.
The present single-bunch current limit is 10 mA (26 nC) per bunch, to protect instrumentation (BPM modules and feedback amplifiers). Options for raising the limit are under discussion, opening the path to high bunch-current operating modes. These options include preventing damage to the feedback amplifiers by limiting beam-induced power transmitted back to the amplifier.

B. Momentum Aperture and Touschek Lifetime
The momentum compaction factor α p for CHESS-U is rather large compared to other modern light sources, resulting in a limited momentum aperture (Fig. 25, top). The Touschek lifetime (Fig. 25, bottom) at the design emittance coupling of 1%, total RF voltage of 6 MV, and design bunch current of 2.22 mA (in 18 × 5 mode) is roughly 40 hrs.
Extrapolating to 25 mA, the anticipated Touschek lifetime at 0.1% coupling is approximately one hour; at 1% coupling, this increases to around 3 hours.

C. Electron Cloud
Initial commissioning will be done with positrons. As such, bunch patterns will be limited by tune shift and head-tail instability due to the electron cloud effect [34]. Each of the bunch patterns proposed here have been examined for electron cloud build-up and trapping [35]. The electron cloud tune shift was constrained to be less than or equal to the electron cloud tune shift in present two-beam bunch patterns (around 3.5 kHz, or    ∆Q ≈ 0.009 in both horizontal and vertical). It is possible that larger tune shifts from electron cloud would be tolerable.

D. Proposed Bunch Patterns
Bunch patterns proposed for CHESS-U are listed in Table IV. Commissioning and early operation will be in the 18 × 5 pattern, with the option of exploring other bunch patterns as user interest dictates.
In any of the proposed bunch patterns, CESR would run with topoff. In present CHESS two-beam conditions, the topoff interval for positrons is 5 min, with a Touschek lifetime of approximately 12 hrs.
CESR will preserve the ability to store electrons in the counter-clockwise direction for machine studies. Total current in the counter-clockwise direction will be limited to 5 mA (13 nC) at 6 GeV, limited by synchrotron radiation heating on the sliding joints in the new achromats. This current limit will scale as E −4 ; therefore, the restriction in the counter-clockwise direction at 2.1 GeV (frequently used for machine studies) will exceed the existing administrative limit of 200 mA.

VIII. INSTALLATION AND COMMISSIONING
Installation of the CHESS-U upgrade began in June 2018, and is scheduled to be completed in November 2018. Beam commissioning will follow. All insertion devices are out-of-vacuum, which allows for initial commissioning and vacuum processing without IDs. Undulators will be installed one canted pair at a time, with a brief recovery period after each installation to establish operating conditions. Beam will be delivered to all end stations by April 2019.
The positron beam presently circulates in the clockwise direction; as such, initial commissioning and operation of CHESS-U will be with positrons, with the option to invert the polarity of the injector and storage ring to run electrons in the clockwise direction at a future date.

IX. SUMMARY
CHESS-U will deliver high flux at high energy, enabling a diverse set of new experiments [12]. The upgrade will improve the performance of the CESR storage ring for x-ray production by almost an order of magnitude with CCUs and a factor of three for the 24-pole "F-line" wiggler. In turn, the transition to single-beam will drastically simplify operations. Bunch patterns compatible with timing mode are under consideration, pending evaluation during commissioning and machine studies.  Standard deviations for misalignments applied in error tolerance simulations are listed in Table V. The errors for dipoles, quadrupoles, and sextupoles are based on survey information. DQ and girder errors are determined by extrapolation from survey information and magnetic alignment characterization.
BPM misalignments and errors are listed in Table VI.

Appendix B: Systematic Multipoles
The multipole expansion is parameterized as a fractional field error dB/B with normal (b n ) and skew (a n ) components, where n = 1 corresponds to a quadrupolelike field. The reference radius for all multipoles listed in Table VII is 20 mm. All multipoles are normalized to the main field.