MOTIVATION

The Isotope mass Separator On-Line facility (ISOLDE) at CERN occupies a leading position in the field of radioactive ion beams research, as it can produce the largest range of isotopes worldwide —over 1300 isotopes of more than 70 elements. HIE-ISOLDE (High Energy and Intensity – ISOLDE) is an upgrade that aims to increase the facility’s energy and intensity reach, opening the way to new opportunities in multiple fields of physics: nuclear and atomic physics, astrophysics and fundamental interactions. The energy upgrade of the facility entails the construction of a superconducting linear accelerator (HIE-linac) to increase the energy of radioactive ion beams, a high energy beam transfer line to bring the beam to the experiments, as well as new beam diagnostic tools. The intensity upgrade aims to improve the target and ion source, the mass separators and charge breeder.

ENERGY UPGRADE

HIE-linac

The installation of the energy upgrade is divided in three phases. This schedule allows for the best handling of the available resources and staff whilst delivering increased beam energies without delay to the ISOLDE users. In the first phase, two cryomodules were added to the existing REX machine to provide energies of up to at least 5.5 MeV/u and allow Coulomb excitation studies to be carried out across the entire range of radionuclides available at ISOLDE. The second phase of the upgrade continues in this manner until all four high-β cryomodules have been added to provide energies of up to at least 10 MeV/u, which will make available a wider range of reactions aside from Coulomb excitation. In the final phase, all the existing REX accelerating structures after the IHS will be replaced with two low-β cryomodules to improve beam quality and ensure that energy is continuously variable between 0.45 and 10 MeV/u.

Figure 1 Layout of the HIE-linac with four high-beta cryomodules installed. The REX-ISOLDE post-accelerator can be seen at the right. The beam lines are connected to Miniball (right), ISS (middle) and the SEC, Scattering Experiments Chamber (left).

Superconducting cavities

Superconducting cavities were identified early on in the design process as a reliable and robust structure, able to provide the stable high gradients and the velocity acceptance demanded by the beam specification at HIE-ISOLDE. Superconducting quarter-wave cavities are used in many facilities around the world for applications in heavy ion beam acceleration, e.g. ANL, LNL-INFN and TRIUMF.

In order to boost the beam to over 10 MeV/u, a total of 32 cavities are required with two different families of cavity, with geometric velocities (β_g ) of 6.3 and 10.3%, of which there are 12 and 20 cavities respectively. The total effective accelerating potential of 39.6 MV can be achieved by assuming a gradient of 6 MV/m and an average synchronous phase of −20^◦ . The geometric velocities were chosen such that the total number of cavities was minimised and the transition energy between the two types of cavity made as quickly as practically possible to benefit from the increased accelerating potential per cavity in the high-β cavity, corresponding to 1.8 MV as opposed to 1.17 MV in the low-β cavity. The flexibility of the output beam energy, which can be varied by switching off cavities for the upper and lower bounds of the A/q acceptance.

Two different materials were considered for the production of the quarter wave resonators of HIE-linac: bulk niobium and copper sputter-coated with niobium. Both options result in similar radiofrequency performance and high gradients. Copper, however, was the material of choice, as it can be used to manufacture stiffer, more massive cavities, reduces microphonics effects, can eliminate electron beam welds and is more cost-effective than niobium for the operating temperature (4.5 K) and RF (100 MHz) chosen. The use of copper also  eliminates the need for magnetic shields in the cryomodules, because the material is insensitive to magnetic flux trapping, and results in higher thermal stability against quenches.

Two designs were developed for the high-β RF cavities. Initial prototypes (Figure 2, left), which were manufactured from two rolled copper sheets and contained a helium reservoir spread out over the shorting plate, were used to optimise manufacture and coating procedures. An improved cavity design was later produced (Figure 2, right), comprising inner and outer conductors that were shrink fitted and electron beam welded with a single weld on the RF side. The new design improved shape accuracy and repeatability and minimised sensitivity to fluctuations of helium pressure.

Diode sputtering was considered as an efficient option for the niobium coating of the cavities; it has the advantage of minimising impurities and promoting the mobility of niobium atoms during film growth, thanks to high substrate temperatures and negative bias.

Figure 2 Old and new design for the superconducting cavities. Left is the original prototype from rolled copper sheets with the helium reservoir on top. Right is the machined copper cavity design.

Some cavities produced in industry were subject to certain defects, mostly located close to the EB weld (projections, inclusions, etc.). A new cavity design was developed with the aim of making it  possible to machine them from a single copper billet, thus avoiding completely the EB weld. The main modification required was to reduce the protuberances of the RF surface at the beam ports (cavity noses). Removing the weld from the high magnetic field region would eliminate the main source of potential imperfections in the location where the sensitivity to RF losses is highest.

Figure 3 3D model of the seamless cavity

Cryomodule

The high-beta HIE-ISOLDE cryomodule consists of about 10,000 parts, from the smallest nut to the main vacuum vessel. The design is built around the active components; five quarter wave superconducting cavities and one superconducting solenoid assembled in an environment offering the best performance of these components with respect to the specifications; temperature, pressure, vacuum level and alignment. The active components are aligned and fixed to a common support frame, which is suspended from the underside of the top plate of the vacuum vessel. All services to the cryomodule enter through this top plate. A thermal shield cooled between 55 K and 70 K surrounds and insulates the cold mass made up of the components operating at 4.5 K; the active components, the frame and the bi-phase helium reservoir located above the components.

The cryomodule comprises three distinct volumes; the common beam and insulation vacuum named "vacuum volume", the 55−70 K gaseous helium circuit named "thermal shield circuit" and the bi-phase helium volume contained inside the reservoir the RF-cavities and  solenoid and the piping supplying and welded onto the frame, named "helium bi-phase volume". The design takes into account the combination of the different possible relative pressures between the volumes.

Figure 4 Layout of a high-beta cryomodule. 1 Vacuum vessel lower box, 2 Vacuum vessel top plate assembly, 3 Thermal shield lower box, 4 Support frame, 5 Suspension end plate, 6 Tie-rod, 7 Inboard cavities, 8 Outboard cavities, 9 Down tube to solenoid, 10 Helium vessel, 11 Chimney assembly, 12 Support frame cooling supply, 13 Support frame cooling return.

The cryogenic design of the cryomodule together with its interfaces to the supply of cryogens ensures that it can be cooled down and warmed up in reasonable time and can be maintained at sufficiently constant temperature and pressure during steady state operation. The cryostat design aims at ensuring at reasonable cost a stable temperature of the superconducting cavities of 4.5 K ±0.5 K. In nominal operation the static bath pressure is stabilised within a ±10 mbar range in order not to substantially change the tuned frequency of the RF cavities by deforming their structures. In operation, the solenoids must remain completely immersed in liquid helium at all times and must allow their stored energy to be locally transferred to the liquid helium during quenches (19 kJ of stored energy equivalent to nine evaporated litres of liquid helium at 1.3 bara)

HIGH ENERGY BEAM TRANSFER LINE (HEBT)

The High Energy Beam Transfer Line (HEBT) was designed to transport the post-accelerated beams from HIE-linac to the experimental stations. It consists of three identical beam lines (XT01, XT02, XT03), shown in Figure 5, that can transfer beams with various energies (from 0.3 MeV/u to 10 MeV/u) and mass-to-charge states. A uniform design was chosen for all three to facilitate their assembly and operation. The HEBT was built over two years and the first two beam lines were operational in early 2015. Installation works for the third one started in 2017. It is possible to extend the third beam line in the future to bring the beam to either a storage ring or a larger experimental station at the back of the ISOLDE hall.

Figure 5 A 3D visual of the HIE-ISOLDE linac, composed of three cryomodules, and the three experimental stations in the HEBT. At the end of the first beam line, XT01, Miniball is located, at the second (XT02) the ISOL Solenoidal Spectrometer, ISS, and the third, XT03, is reserved for movable setups. In the drawing, XT03 is occupied by the scattering chamber, SEC, which presently connected to XT02.

The HEBT consists of doublet cell units, achromatic 90-degree bends and focusing triplets positioned before the experimental stations. The doublet units are composed of quadrupoles, dual-plane trajectory corrector magnets (dipoles) and beam instrumentation devices. As upgradability was a key consideration, the length of the doublet unit period was designed to be the same as the length of a cryomodule. As a result, it is relatively easy to replace a doublet with a cryomodule to increase the energy reach of the linac in the future.

Experimental instrumentation

Experimental instrumentation at HIE-ISOLDE includes a variety of set-ups. The first beam line hosts the high-resolution germanium detector array Miniball, which is used for Coulomb excitation and transfer reactions. It has been in operation for more than ten years. Three complementary set-ups are often employed alongside Miniball: T-REX, a set of double sided silicon strip detectors, the CD-type Double sided Silicon Detector and SPEDE (Spectrometer for Electron Detection), which performs electron conversion studies. In the second beam line, a general purpose scattering chamber was installed at first. In 2017, it was replaced by the ISOLDE Solenoidal Spectrometer (ISS), intended for transfer reaction studies. ISS will be available for physics exploitation after CERN’s second long  shutdown (LS2). The active target SpecMAT will be used in separate experiments inside the ISS magnet. The third beam line is reserved for smaller, movable set-ups, e.g. the scattering chamber, with different charged particle detector arrays such as CORSET  (CORrelation SETup) for quasi-fission measurements. The scattering chamber will have to be removed eventually, when ACTAR (Active Target Detector) or the optical time projection chamber are installed in its place.

PHYSICS OPPORTUNITIES

The HIE-ISOLDE upgrade substantially enhances research opportunities in most aspects of nuclear structure and nuclear astrophysics. The wide variety of exotic nuclei produced, their availability at different energies, and the new instrumentation that has been developed paves the way for a robust physics programme in the coming years. The higher energies made available by HIE-linac allow the exploration of interesting regions in the nuclear chart, increase the cross section (in most cases), and improve accessibility to detailed nuclear structure information. Over thirty experiments have already been approved; two of them finished data-taking in autumn 2016 and 13 are scheduled for the 2017 physics run. Experiments at HIE-ISOLDE cover a wide range of topics, such as isospin symmetry, collectivity vs. single particle structure, magic numbers far from stability, shape coexistence, as well as quadrupole and octupole degrees of freedom. Most planned experiments plan to use Coulomb excitation of the beam for their studies, but also there is a number of transfer reaction experiment proposals, as well as a few experiments to use elastic scattering reactions. In the heavy mass region, Coulomb excitation experiments will investigate quadrupole and octupole collectivity in Te, Xe, and Ba isotopes, unravel the structure around the doubly magic nucleus ¹³²Sn, as well as measure octupole collectivity in Rn and Ra nuclei. In the superheavies, Coulomb excitation of the single magic  two-proton-hole  nucleus ²⁰⁶Hg will be used to obtain information of quadrupole and octupole collectivity, (improving the predictive power of the shell model for more exotic nuclei). Some proposals for transfer reaction experiments focus on a few regions of the nuclear chart, mainly neutron rich Ni isotopes, while others are motivated by unsolved questions in nuclear astrophysics, and specifically stellar nucleosynthesis, for example the long-standing ⁷Li abundance problem.

COMMISSIONING

Phase 1 of the ISOLDE high-energy upgrade was completed in 2015 with the installation of the first high-beta cryomodule, and phase 2 finished one year later, when the second high-beta cryomodule was added to HIE-linac, increasing the energy reach to 5.5 MeV/u for A/q = 4.5. Before the first experiments took place in the new facility, a wide range of tests and measurements were performed as part of the hardware and beam commissioning.

Hardware commissioning

In preparation for the 2016 physics run, the first cryomodule was removed, vented and equipped with new couplers in the ISO5 clean room. It was installed again in the beam line with the second cryomodule in May 2016. The cool down of the two cryomodules took longer than expected, and highlighted limitations in the management of transients from the cryogenics plant. Active cooling of the two cryomodules was performed in several stages with floating periods in-between and, as a result, passive cooling of cavities and solenoids through radiation on the thermal shields lasted a few weeks. As customary, multipacting levels at a low field were conditioned before reaching the superconducting transition. A spontaneous increase in vacuum pressure was observed in the second cryomodule some days after its temperature reached 4.5 K. Pressure became steady at 10^-9 mbar.

The RF measurements at cold highlighted a degraded performance on the first and the last cavity of the first cryomodule, which had been transported back to the clean room and vented to exchange the RF couplers. All other cavities reached the nominal field of 6 MV/m, close to the required power dissipation of 10 W, in particular those of the second cryomodule, which was a confirmation of the soundness of the assembly procedure and slow pump-down. As in the first commissioning experiment of 2015, it was found that the RF performance of the cavities in the cryomodule exceeded that shown in the vertical tests.

Figure 6 Q vs E_acc curves of diode sputtered cavities, data from July 2016. Label XLL2 refers to cryomodule 1 and XLH1 to cryomodule 2, which are installed at positions XLL2 and XLH1 in HIE-linac respectively.

Beam commissioning

Beam commissioning activities at the REX normal-conducting injector were performed simultaneously with hardware commissioning at HIE-linac. A mixture of helium, carbon, oxygen, neon and argon beams with A/q = 4 was used for the setup beam. The beam was drifted through the cryomodules into the HEBT lines with REX energy and was used to commission the diagnostic boxes. Then, the superconducting cavities were phased, a process which also verified that the calibration of the gradients was accurate. The beam energy was measured using the first dipole of the first HEBT line, silicon detectors and a time-of-flight system.

Characterising the energy and energy spread of the beam is important for the users of the facility. The post-accelerator is equipped with several diagnostic devices that can be used to do these measurements that were commissioned during the 2016 campaign:

-              Silicon detectors. Four detectors located in different positions along the linac and HEBT lines were used to measure changes in the beam energy and the energy spread of the beam.

-              Dipole as an energy spectrometer. Slits before and after the first dipole of the first HEBT line were used to collimate the beam and select ions on the beam axis after the focusing and steering optical elements were turned off. The beam current past the dipole was measured for different magnetic fields. Both the energy spread and the absolute energy could be determined by this method using the previously-measured effective magnetic length of the dipole.

-              Time of flight. The time-of-flight information provided by two of the silicon detectors located after the second cryomodule and separated by 7.76 m was also used to measure the energy of the beam. The results of the measurements conducted during the beam commissioning showed that the uncertainty in the beam energy using this method was lower than ± 0.5 %.

-              Calibration of the RF systems. The energy of the beam can be calculated using the accelerating gradients, the Transit Time Factor (TTF) of the cavities and the synchronous phases of the beam. The typical uncertainty determining the energy of the beam using this method for a single cavity is ± 1 %.

The transverse profiles of the beam were measured using the scanning slits in several diagnostic boxes. The measurements conducted during the beam commissioning showed that beam profiles could be clearly determined for intensities as low as 10 epA.

Physics run

On 9 September 2016, the first exotic beam marked the start of operations for the upgraded facility. The first experiment investigated excitation states of tin isotopes, using transfer reactions and Coulomb excitation of an ¹¹⁰Sn²⁶⁺ beam, post‑accelerated to 4.5 MeV/u. See Table 1 for an overview of the six experiments conducted at HIE-ISOLDE and Figure 5 for a gamma ray spectrum of a 4.5 MeV/u ¹⁴²Xe beam. Besides demonstrating the experimental capabilities of the facility, this successful first run validated the technical choices of the HIE‑ISOLDE team and provided a fitting reward for eight years of rigorous R&D efforts.

Table 1 Overview of the six experiments that took place during the 2016 physics run.

Figure 7 Doppler-corrected gamma ray spectrum of 4.5 MeV/u ¹⁴²Xe beam Coulomb excited on ²⁰⁶Pb target (in red) and the equivalent gamma spectrum for 2.8 MeV/u ¹⁴²Xe beam on a ⁹⁶Mo target (in blue). The higher energies of HIE-ISOLDE markedly improve the population of higher-lying states (Courtesy of C. Henrich).

2017 Commissioning

During the 2016 operations, several issues were identified in the cryogenic system, mainly related to CP flow limitations, bad LHe quality supply and strong oscillations in the 4.5 K return line. However, once the operational conditions were found, the cryogenic availability exceeded 99% and no unplanned stops occured. Preventive and corrective maintanance was carried out during the Extended Year-End Technical Stop of 2016-17, including updates to the cryo plant logic to allow full performance and ease restarts and the development of a more robust refrigirator control system. and processes, with automatisms allowing reconnections of cryomodules during cool down. Endoscopic investigations revealed that the excessive heat load in the distribution system was caused by contact of the 4.5 K process pipes with the shield, as well as a hole in the shield that allowed the 300 K surface to radiate inside the JB. To remove major non conformities, a crash programme was launched, involving the insertion of spacer material around the process pipes to remove   direct contacts. Thanks to the successful intervention, the performance of the cryo plant is now nominal. Capacity tests confirmed that it will be able to cope with the fourth cryomodule, scheduled for installation in 2018.

The 2017 hardware commissioning began on March 20 with warm tests. Cool-down started one week later and all cavities reached 4.5 K on April 18. Multipacting conditioning and radiofrequency measurements were done on all 15 cavities. While the overall RF performance is very good, two cavities were found to suffer from field emission: the fifth cavity of CM1 (XLL2.5), which was known to be  vulnerable from the previous year, and the second of CM3 (XLH2.2). Their field emission onsets were at 4 MV/m and 3 MV/m respectively. Once again, a frequency perturbation was detected in the superconducting cavities, correlated to the liquid helium temperature and pressure oscillations, although it was less severe than the one observed in 2016. As the cavity frequency shift was slow (repeating every 10-20 minutes) and limited to only about 10 Hz, it could be easily corrected by the cavity tuning system. In the following weeks, the LLRF system was set up and the solenoids powered.

A busy physics run was in line for 2017 with 12 scheduled experiments in 136 days. Most of them used the Miniball spectrometer, mainly to conduct Coulomb excitation studies, while others take advantage of the scattering chamber and visiting setups. A wide range of isotopes was be studied, from ⁹Li to ²⁰⁶Hg.

Phase 2 of the HIE-ISOLDE energy upgrade will be completed in 2018, with the installation of the fourth high-beta cryomodule, which is currently being assembled. Cavity production for CM4 has almost been concluded. It will be ready for bunker tests in September. Its addition to the linac will allow beam energies up to 10-15 MeV/u, opening up new physics opportunities and making ISOLDE the only facility in the world capable of accelerating medium to heavy radioactive isotopes in this energy range.

INTENSITY UPGRADE

Aside from longer term plans to upgrade the Proton Synchrotron Booster (PSB), the beam intensity of the CERN accelerator complex is to be increased in accordance with the LHC luminosity upgrade and the replacement of Linac 2 with Linac 4. The upgrade will increase the injection energy of the PSB from 50 MeV to 160 MeV and reduce the space-charge bottleneck in the PSB so that the beam intensity can be increased by a factor of 2.

Along with H − charge-exchange injection and an increased cycling rate of the PSB, a factor of between 2 and 3 increase in the average driver beam power can be realistically expected at the ISOLDE target stations. Radiation protection studies have been launched to understand the effects of the increase in proton intensity at the target stations.

The radioactive beam intensities will benefit from the increase in the driver beam intensity and developments in target technology are being pursued. Options for new target materials are explored, with a focus on radiation resistance. Target materials that are presently used were extensively tested to examine their suitability to higher radioactivity levels. Research into target geometry modifications and improvements to the target change systems is also undertaken. Various upgrades to the REX charge breeder are being investigated including the test of a new ECR ion source and an RFQ cooler named ISCOOL is already on the HRS beam line and capable of injecting cooled beams directly into the EBIS, overcoming the present space-charge bottleneck in the Penning trap. To increase the intensity and improve the repetition rate of the ion beam, it is necessary to address the ultra‑high vacuum, the electron current and density, the high voltage and the magnetic field. Preliminary tests of a prototype electron gun with a pulsed electron beam of 1.5 A and 30 keV are ongoing at BNL and have already produced promising results.