A Superconducting Undulator Research and

Development Plan for Free-Electron LasersD. Arbelaez (LBNL), P. Emma (SLAC), E. Gluskin (ANL/APS), Y. Ivanyushenkov (ANL/APS),

H.-D. Nuhn (SLAC), S. Prestemon (LBNL)

February 6, 2014

Introduction and Motivation

Undulators serve as the primary source of radiation for modern storage rings (SR) and Free-

Electron Laser (FEL) light sources. The advent of permanent magnet (PM) undulators in the early 1980’s

ushered in an era of 3rd generation light sources [i], and since then improvements in undulator

technology have served to continually enhance the flux and brightness provided to experiments by SR

and FEL beamlines. A comparison of different undulator technologies clearly shows superconducting

undulator (SCU) technologies, with much higher magnetic fields, as the leading candidates for future

FEL-based x-ray sources (see Figure 1).

Figure 1: Peak magnetic fields vs. undulator period length for four different technologies (blue-solid: PM-NdFeB, cyan-dotted: PM-NdFeB-in-vacuum, green-dashed: SCU-NbTi, red-dash-dotted: SCU-Nb3Sn). The vacuum chamber full vertical (internal) gap is 5.7 mm in all cases, with the three “out-of-vacuum” magnets using a 7.5-mm magnetic gap, and the “in-vacuum” magnetic gap at 5.7 mm (i.e., all have the same chamber gap for comparison purposes). Note the existing LCLS-I PM-NdFeB undulator has a 30-mm period and a peak field of 1.25 T. Fields for the two SCUs are evaluated at 80% of the short-sample limit, and a 60-m thick insulator is assumed for Nb3Sn.

These strong magnetic fields, with respect to PM undulators, can produce much better FEL

efficiency, higher photon power, a wider spectral tuning range, shorter undulator lines, and/or relaxed

electron beam brightness requirements. This can be shown by plotting (see Figure 2) the required LCLS-

II undulator beamline length (including 1.15-m long break sections, 3.4-m undulator segments, 50%

additional undulator length for self-seeding, and a reasonable saturation length safety margin of 20%) as

a function of several key FEL parameters, such as electron energy, undulator magnetic gap, electron

emittance, peak current, lower-limit photon energy, and upper-limit photon energy, all with a 5.7-mm

beam stay-clear (vacuum chamber aperture). Note the “in-vacuum” device has a 5.7-mm magnetic gap

while the other 3 technologies have a 7.5-mm magnetic gap ( i.e., the beam stay-clear is the same for all

cases at 5.7 mm to allow a fair comparison). The lower-limit and upper-limit photon energies represent

the FEL spectral tuning range possible with a constant 4-GeV electron energy. These plots show that

SCU technologies using commercially-available Nb3Sn or NbTi superconductors offer superior FEL

performance and can reduce the LCLS-II undulator beamline length by 50-70 m with respect to a PM

undulator.

Figure 2: LCLS-II undulator beamline length, adding 50% for self-seeding and 20% for a safety margin, as a function of electron energy (a), magnetic full gap (b), electron emittance (c), peak current (d), lower-limit photon energy (e), and upper-limit photon energy (f). Clearly the SCU technologies of Nb3Sn or NbTi offer superior performance and can reduce the final LCLS-II undulator beamline length by more than 50-70 m with respect to PM undulators.

Note that the undulator period, u, is not fixed in these plots, but is reset at each point (for all but plots c

and d) in order to maintain the lower-limit of the photon tuning range (1.2 keV nominal here). Electron

beam and undulator parameters for the plots in Figure 2 are summarized in Table 1. The undulator

periods for each curve, evaluated at the nominal working point (red dots on each curve), are 27.5 mm

(blue-solid), 24.8 mm (cyan-dotted), 20.3 mm (green-dashed), and 18.7 mm (red-dash-dot).

Table 1: Electron beam and undulator parameters for the plots in Figure 1.

Parameter Symbol Value Unit

Electron energy E 4.0 GeV

Lower-limit photon tuning range u1 1.2 keV

Upper-limit photon tuning range u2 4.0 keV

Magnetic gap gm 7.5 mm

Vacuum chamber aperture (stay-clear) gvc 5.7 mm

Transverse norm. rms emittance x,y 0.45 mm

Peak current Ipk 1.0 kA

Slice energy spread (rms) E 500 keV

Average beta function in undulator 15 m

Additional undulator length to allow self-seeding 50 %

Additional undulator length to allow design safety margin 20 %

Number of gain lengths required for saturation NG 18 -

Length of undulator magnet segments Lseg 3.4 m

Length of undulator breaks (between segments) Lbrk 1.15 m

Additional length to allow self-seeding monochromator L 4 m

In addition, SCUs offer the possibility of lower electron energy (for reduced linac costs), a larger

beam stay-clear (important for high bunch repetition rate FELs such as LCLS-II), less sensitivity to beam

emittance (relaxed injector requirements), and/or a broader photon tuning range. Clearly these SCU

technologies need to be developed for future high-performance FELs. A list of SCU advantages with

respect to the HXR undulator in LCLS-II is tabulated below.

SCU allows full LCLS-II performance to 5 keV (SASE & self-seeded at 4 GeV) and with 50-70 m

less undulator length (or preserve the length and taper the fields to extract more FEL power).

SCU has orders of magnitude less sensitivity to radiation dose, which is a serious risk for the

PMU in LCLS-II with a 120-kW electron beam power.

SCU is much less sensitive to e emittance, a parameter which is not yet demonstrated in the

new high-rate LCLS-II electron gun.

SCU with a cold bore produces very low pressure in a small vacuum chamber, reducing gas

scattering without adding the complexity of anti-chambers for enhanced pumping in a PMU.

SCU system can likely be built with a much smaller, more contained transverse tunnel profile

with respect to the adjustable-gap PMU, which needs a very large support structure for

mechanical rigidity.

SCU enhances the LCLS-I FEL power (when using the existing Cu linac), whereas a new PMU

degrades the power (as shown in Figure 3).

Two or more SCUs can likely be packed in one cryostat with only modest increase in its length.

Figure 3: LCLS-I performance (plotted from 2.5 to 15 GeV) with the existing 30-mm PMU (blue-solid) and with a

20-mm period Nb3Sn SCU (red-dashed). The x-ray pulse energy is increased (10%) by using an SCU here, whereas it

is reduced (25%) by using the baseline 26-mm PMU (not shown); an undesirable result.

General Description of the Proposed Project

The main goal of this project is to demonstrate the viability of superconducting undulator (SCU)

technology for the FEL-type undulator by building, measuring, testing, and correcting prototype SC

undulators. The FEL-specific undulator parameters, such as the magnetic field strength for the chosen

period and magnetic gap, and the magnetic field quality over the chosen length of the undulator, must

be demonstrated. The parameter selection is dictated by the desired spectrum range of X-rays and the

electron beam energy of the LCLS-II. As a result, the undulator period lies between 18 and 21 mm

depending on the superconducting material. The full magnetic gap is chosen at 7.5 mm with the beam

stay-clear diameter of 5.7 mm. The choice of the undulator segment length is somewhat arbitrary, but it

cannot be much smaller than the gain length and should not be too long in order to avoid potential

technological pitfalls. Therefore the prototype undulator length is chosen to be 1.5 m which would

represent a reasonably long building block for a 100-m long undulator line, possibly in the LCLS-II project

at SLAC.

The goal of the project is to build two 1.5-m long superconducting undulator magnetic

structures, one from NbTi and another from Nb3Sn, conducting independent cryogenic tests on each of

them using the same cryostat, and verifying the magnetic performance of each undulator using the

same magnet measurement bench. In parallel, a magnetic tuning technique will be developed and

applied to each undulator as needed. The project design and execution will rely heavily on already well-

established designs of the undulator cryostat, undulator magnet core, and magnetic measurement

systems developed and implemented for shorter undulators at the APS. It will also employ a unique

magnetic tuning approach developed at LBNL. The APS SCU magnetic measurement system

incorporates Hall-probe and rotating coils sensors modified for the smaller vacuum pipe diameter; the

system will furthermore incorporate a pulsed-wire technique developed at LBNL. The project

deliverable will be two fully functional superconducting undulators that meet LCLS-II undulator

specifications. Prototype undulator parameters are listed in Table 2, showing the slightly different

period for each.

We note that the project is focused on the demonstration of the applicability of

superconducting technology for FEL undulators. It does not address directly the viability of such

technology with respect to other important elements of the FEL undulator line: focusing quadrupoles,

phase shifters, electron beam positions monitors. It should be mentioned, however, that several of

these types of elements have been already developed and used at superconducting magnet-based

accelerators, and their adaptation to FELs appears to be straightforward. It is the superconducting

undulators that must overcome the barrier of the “newcomer” and become the mainstream of light

sources technology.

Table 2: SCU prototype parameters for each technology based on parameters of Table 1, allowing the FEL to reach as low as 1.2 keV photon energy with a 4-GeV electron beam.

Parameter Symbo

l

NbTi Nb3Sn Unit

Undulator magnetic full gap gm 7.5

5.7

1.5

mm

Vacuum chamber full gap (internal) gvc mm

Prototype undulator segment length Lseg m

Undulator period u 20.0 18.5 mm

Peak magnetic field Bmax 1.7 2.0 T

Maximum K value Kmax 3.2 3.4 -

Minimum K value Kmin 1.0 1.1 -

The Cryostat

The SCU prototype cryostat will be a longer version of the 2-m long SCU0 cryostat at APS, shown

in Figure 4. The design of the SCU0 cryostat has been proven by the dedicated cold tests before

installation of the device into the storage ring. Since January 2013, the SCU0 operates very successfully

in the Sector 6 of the APS, demonstrating a cryogen loss-free behavior.

Figure 4: SCU0 installed in the APS storage ring.

The cryostat will include a vacuum vessel that incorporates 60-K and 20-K thermal shields, and a

cold mass as shown in Figure 5. The cold mass includes a magnet assembly and a LHe tank with the

piping. The cold mass is mounted on a stainless steel frame that is held inside the vacuum vessel by

Kevlar strings.

LHe vessel

SC magnet He fill/vent turret

20 K radiation shield

60 K radiation shield

Beam chamber

Beam chamberthermal link to cryocooler

LHe piping

Figure 5: Cutaway view of the inside of the cryostat.

The experience of SCU0 shows that such a support system minimizes cold mass displacement

during the cryostat cool down to a level of 300 µm. In addition, the cryostat will be equipped with

optical windows to measure the actual position of the cold mass that can be adjusted by vertical

displacement of the cryostat. The superconducting magnet is indirectly cooled by liquid helium (LHe)

which is moved by a thermosyphon effect through the channels in the magnet cores. The LHe is

contained in a 100-liter tank with the cores and the piping makes a closed loop system. In such a

system, the evaporated helium is condensed back into liquid inside the LHe tank, as shown in Figure 6.

LHe

Current lead assemblies

1HTS leads

Heater

Cryostat vacuum vesselCold mass support

2

3 4

He recondenser

Cryocoolers 4K/60K

Cryocoolers 20K/60K

20K radiation shield

60K radiation shield

RF fingers

LHe vessel

SC coils

He fill pipe

Beam chamber @ 20K

Figure 6: Superconducting undulator cooling scheme concept.

Four cryocoolers provide cooling power of 3W at 4K, 40 W at 20 K, and about 220 W at 60 K. The

expected static heat load at 4 K is about 1 W. The SCU0 experience demonstrates that the observed

temperatures were equal to or less than the calculated temperatures for given heat loads, thus proving

the thermal design of the cryostat. It is important to note that the beam chamber is thermally isolated

from the superconducting magnet. It is cooled by the two bottom cryocoolers that also provide cooling

to 20-K and 60-K thermal shields. The two upper cryocoolers cool the LHe circuit, current leads and 60-K

shield. The current leads, one pair for the main coil and two pairs for correction coils, are of a hybrid

type with resistive top parts and commercial HTS leads at the bottom parts.

NbTi Magnet (ANL)

The design of a 1.5-m long NbTi prototype magnet is based on the design of the 0.33-m, and 1.2-

m long magnets for the first two superconducting undulators at the APS – SCU0 and SCU1. The SCU0

magnet, shown in Figure 7, has achieved performance characteristics that meet the APS requirements

and that are consistent with LCLS-II specified requirements.

Figure 7: Assembled SCU0 magnet at APS (33 cm long).

The 1.5-m magnet assembly will be comprised of two identical magnets separated by the

magnetic gap where a beam vacuum chamber is accommodated. Superconducting coils are going to be

wound with 0.6-mm NbTi on the low carbon steel formers, or cores. Each magnet will include a main coil

and a pair of correction coils, one at each end of the magnet. An existing computer controlled winding

machine will be used for coil winding. After winding, the magnets will be impregnated with epoxy resin.

The magnet assembly will be tested in 4-m deep vertical LHe bath cryostat that exists in the APS SCU

facility.

The quality of the magnet field in terms of the phase errors will be achieved by the precise

winding onto the former machined and assembled with a high precision. For example, the former of the

SCU0 magnet was machined with the precision better than 10 µm. As a result, the achieved phase error

was less than 2 deg rms for that magnet. The manufacture of the SCU prototype magnet will follow the

same approach. In addition, the magnet will be equipped with a tuning circuit developed by the

Berkeley team.

The SCU magnets are cooled by liquid helium passing through channels inside the cores. The

magnet assembly is thus interfaced to the cryostat cooling system through standard flanges (see Figure

6). This simple interface will permit both NbTi and Nb3Sn magnets to be installed and tested in the same

2-m long test cryostat at APS, which was described in the previous section.

Nb3Sn Magnet (LBNL)

Nb3Sn SCUs could lead to higher fields and shorter FEL lines when compared to NbTi SCUs due to

the higher performance of the Nb3Sn conductor. Alternatively, the risk of a quench can be reduced by

operating with a larger margin while still obtaining the magnetic performance of a NbTi SCU. From an

SCU performance standpoint, Nb3Sn is therefore preferable to NbTi. However, due to the brittle nature

of the Nb3Sn conductor, a wind-and-react process must be used to produce the coils. This introduces

one additional step in the SCU fabrication process with respect to NbTi. LBNL has many years of

experience working with Nb3Sn conductors and it is believed that the additional reaction step, with a

proper tooling design, will not lead to distortions of the coils. The goal of this prototype is to

demonstrate that Nb3Sn SCU technology attains the expected peak field performance without

degradation of the field quality due to the reaction process.

A significant amount of work has already been done at LBNL in order to develop Nb 3Sn SCU

technology. In 2002, with Lab-Directed R&D (LDRD) funds, two 10-period prototypes were fabricated.

The two devices demonstrated a) that extremely high current density undulators can be safely

protected, and b) that simple “trim-coils” situated on poles can provide sufficient field perturbations to

allow for trajectory tuning [2]. This effort lead to a collaboration with the APS SCU team in 2005-2006: a

λu = 14.5 mm, 10-period undulator-half was fabricated at LBNL and reached 97% of its theoretical limit

on the fourth quench [3]. The combination of results described above demonstrated the general

viability of Nb3Sn for SCU application.

More recently an R&D effort was initiated at LBNL to bring SCU technology to a state of

readiness for FEL facility application. The focus includes a) developing optimized end designs to control

beam entrance and exit from the device within LCLS-II tolerance levels; b) measuring trajectory wander

through the device with sufficient accuracy to meet LCLS-II tolerance levels; c) developing and

demonstrating a tuning mechanism and paradigm compatible with FEL requirements [4], and d)

developing tolerance requirements for fabrication scale-up [5]. These areas are currently being

addressed on a λu = 20 mm, 25-period Nb3Sn device funded with LBNL FY13-FY14 LDRD funds. We note

that all of these recent developments (items a-d above) are applicable to both NbTi and Nb3Sn SCUs.

Undulator Tuning

For short devices, such as the SCU0 at the APS, tight fabrication tolerances and precise assembly

can yield sufficient field quality so that only end tuning is needed. However, the phase and trajectory

wander through the device, due to both random and systematic errors, can grow rapidly with increased

device length. While the goal in the fabrication of the long devices is to maintain the same machining

and winding accuracy that was achieved for the short ones, distributed tuning, as is common on PM

devices, may nevertheless be necessary. A variety of tuning concepts for SCU’s have been considered in

the community and a concept is under development at LBNL that is expected to be scalable to the full-

length device, and can be industrialized [4]. This concept uses patterned high temperature

superconductor (HTS) tapes in order to locally correct magnetic field errors under specific poles of the

device. For the commissioning process, an in-situ tuning method is desirable where the tuning locations

can be determined in an efficient manner. A concept has been developed which accomplishes this goal

by using heater networks in order to direct the current to the appropriate pole location. With current

LDRD funds, proof-of-concept tests have demonstrated the functionality of the switching scheme.

Within this proposed project, the scale-up of the concept and test on a full-scale prototype will take

place.

Magnet Measurements

The magnetic performance of the SCU prototype will be measured with an existing horizontal

measurement system that was successfully used to measure the SCU0 at APS. The system is based on a

‘warm tube’ concept shown in Figure 8. In this concept, a titanium guiding tube is located in the cold

beam chamber of the cryostat. It is thermally isolated from the beam chamber and is self-heated by

passing electrical current through it. The guiding tube bore is open to air and accommodates a carbon

fiber stick with Hall probes or wire measurement coils. In this design, the temperature of the Hall

sensors is kept close to room temperature. The wire coils are of rectangular or ‘figure-8’ types to

measure the first and the second field integrals.

Figure 8: Cold SCU beam chamber with warm guide tube. Dimensions here are for the APS SCU0 beam chamber.

The mechanical parts include a guiding tube inside the cryostat and two stages externally

mounted on the cryostat flanges. A horizontal 4-m long linear stage is used to drive the Hall probe

inside the cryostat under the test. All external components of the existing measurement system will be

employed. The modifications will concern internal parts including the Hall probe assembly with the

carbon fiber holder, and the guiding tube. These components are currently made to fit a 7.2-mm

aperture of the SCU0 beam chamber. Their diameters need to be smaller to fit the 5.7-mm aperture of

the SCU prototype beam chamber. This would possibly require a custom made Ti tube. The Hall probe

3-sensor assembly currently has an outside diameter of 3.8 mm and this will be reduced to about 3 mm.

Custom-made Hall sensors will be required. A possible vendor is Arepok Ltd in Slovakia. The existing

LabVIEW-based data acquisition system will be employed for the SCU prototype measurement. This

system was successfully used to measure SCU0.

The LBNL group has focused on the development of a complementary approach to magnetic

measurements of the first and second field integrals, using the pulsed wire technique. A thin,

conductive wire is stretched in the undulator and pulsed with a current; the interaction of the magnetic

field with the current produces a force that deflects the wire, resulting in a mechanical wave that

propagates along the wire. A simple but accurate laser/diode/scope setup allows for the accurate

temporal measurement of the wave at the end of the wire, which can then be related to the local value

of the field integral. Dispersion in the wire can be very accurately compensated for using mathematical

manipulations of the resultant waveform [7]. The method has been experimentally validated on an LCLS

undulator and compared to accurate Hall-probe scans made by the SLAC team. The method is fully

compatible with SCUs, and will allow for fast trajectory tuning of devices. The pulsed wire can be easily

added to the Hall-probe and stretched wire systems already implemented on the APS horizontal

measurement system, resulting in further enhancement of the system. The combination of stretched

wire, rotating coil, Hall-probe, and pulsed-wire capabilities provide the requisite toolbox of

measurement capabilities needed for SCU tuning and commissioning.

It should be noted that the existing power supplies might not be accurate enough to deliver the

required precision of 10-4 in setting the SCU prototype magnet current. Therefore a high precision

custom-made power supply might be required.

Cost and Schedule

The experience and resources of the Argonne and LBNL teams are coordinated using detailed

resource-loaded schedules. These have been developed by the Argonne and LBNL teams and are based

on experience of previous prototypes designed, fabricated, and tested by the teams. Milestones for the

efforts of each team are clearly identified, as are coordination points. The schedules are designed to

yield results as quickly and efficiently as possible, while minimizing risk. Table 1 shows the total cost

estimate, and Table 2 shows the integrated schedule and WBS.

The LBNL schedule is focused on four main areas:

1. Modifications of an existing cryo-cooler-based, cryogen-free tuning cryostat to allow

development of a tuning system commensurate with the 1.5-m SCU prototypes. The

modifications are straightforward – extending the cryostat ends to allow for an increase of test

components from 1 m to 1.5 m – and will not introduce risk to the cryogenics of the system.

2. Development of a 1.5-m scale tuning system, based on concepts already tested and proven in

previous work at LBNL. The tuning system can be largely tested in the tuning-cryostat,

described above, in advance of implementation with an undulator in the ANL test cryostat.

3. Nb3Sn SCU design and fabrication. The Nb3Sn SCU prototype will have a period λu = 18.5 mm.

LNBL has recently completed construction of a λu = 20 mm device; all of the tools and end

optimization developed for this device can be rapidly applied to the new, longer prototype.

4. Contributions to prototype testing and tuning at ANL. The baseline plan is that both the

Argonne-produced NbTi SCU prototype and the LBNL-produced Nb3Sn SCU will be tested in the

new Argonne 2-m long test cryostat.

Table 3: Combined cost estimate of Argonne and LBNL contributions, using FY14 burdened rates for resources and procurements. M&S estimates are based on recent purchases, previous experience or industry quotes. The total listed for both laboratories, without contingency, is 3.1 M$. Individual laboratory contributions are also shown.

Table 4: Rolled-up schedule showing ANL and LBNL components of the SCU R&D project.

Conclusions

The full development of superconducting undulator technologies will open up a revolutionary

breakthrough in FEL design, providing more efficient light sources, shorter and more compact

undulators, higher FEL power, and a broader tuning range. This advanced technology has many

advantages but needs to be demonstrated so that new FEL designs can incorporate the great leverage

available from these high-field devices. The availability of this technology, if established by

approximately May of 2015, might also allow a baseline design change to the LCLS-II Hard X-Ray FEL

undulator, allowing self-seeding at photon energies up to 5 keV, which is otherwise not possible using

the presently planned PMU device.

References

i K. Halbach, J. Chin, E. Hoyer, and H. Winnick, “A Permanent Magnet Undulator for SPEAR”, IEEE Trans. Nucl., 1981.