Installation and integrated testing of magnets for the ESS Linac

The European Spallation Source (ESS) linear accelerator is designed to accelerate a 62.5 mA, 2.86 ms, 14 Hz proton beam up to 2 GeV at the entrance of a rotating tungsten (W) target. Normal Conducting Linac (NCL) and Superconducting Linac (SCL) parts of the accelerator contain over 200 quadrupole, dipole and corrector magnets of different types and parameters for beam envelope and trajectory control. All magnets have been provided to ESS by in-kind collaborators and research institutes across Europe. Following the delivery of these components and their respective power converters to ESS, this proceeding presents the status of the installation together with the methodology and first obtained results from system local and integrated testing phase.


Status and System Design
ESS accelerator is composed of a Normal Conducting Linac (NCL), currently under commissioning [1], [2], and a cryomodule based Superconducting Linac (SCL) along with a High Energy Beam Transport (HEBT) and two (2) beamlines leading to the tungsten target and to a beam dump.ESS SCL magnets are installed in sections between the different types of cryomodules named Linac Warm Units (LWUs).Each LWU contains a horizontal and vertical quadrupole and a steering dipole magnet.In total, there are four (4) different types of quadrupole magnets (Q5, Q6, Q7, Q8) and three (3) types of combined dual-plane corrector magnets (C5,C6, C8) depending on proton beam energy.LWUs in Spoke area contain quadrupoles and correctors of types Q5 and C5 whereas, types Q6 and C6 are installed in Medium-β, High-β, HEBT and Dump Line.DogLeg contains quadrupole magnets of type Q7 and the larger types Q8, C8 are installed in the Accelerator to Target (A2T) area.Finally, two (2) dipoles at the beginning and the end of the DogLeg are used in order to provide a four-degree (4 o ) vertical bending of the beam towards the target.As of March 2023, the LWUs at Spoke, Medium-β, High-β and HEBT are installed whereas the installation is ongoing in A2T, DogLeg and Dump Line.Furthermore, the completion of cooling and electrical interface systems enabled the integrated testing of the first four (4) magnets in HEBT area.Different types of magnets present different apertures, maximum integrated field and magnetic length depending on the area of installation.The different magnet types, quantities, respective areas of installation and main design parameters are summarised in Figure 1 and Table 1 whereas a detailed description of the design parameters is given in [3].It is evident that the large number of magnets and the different types dictate the creation of a robust and versatile test procedure consolidating the different interfaces, permitting the delivery of a system that fulfills the ESS engineering requirements [4], [5] according to schedule.ESS SCL magnets are an in-kind contribution from Elettra Syncrhrotrone Trieste [6], [7] and are normal conducting operating in DC mode.Quadrupoles are equipped with water-cooled coils on a laminated yoke whereas correctors are air cooled window-frame magnets with yokes made of laminated steel sheets.One of the main goals during magnet system design was the standardisation of power converters which was achieved by specifying the same conductor cross section and current density.For quadrupoles and dipoles, an off-the-shelf commercial power converter solution was adopted (CAENels NGPS 200-50-10000) in contrary to corrector power  Interlock signals connected to thermal switches on the magnets and flow meters are connected to the power supplies through an interface board and an output signal representing overall power supply status is sent to Machine Protection System (MPS).In the case of the two (2) bending dipole magnets, MPS interfaces directly with the magnets employing a set of DCCTs that ensures the correct range of the set-point current for the selected beam energy.Magnets and power converters were thoroughly tested in different project phases during Factory Acceptance Tests (FATs) from manufacturing contractor companies (Danfysik, SigmaPhi) and Elettra prior to their delivery at ESS premises.Subsequently, each system follows a local and integrated testing phase, presented in Figure 2, following the installation of magnets and interface systems into their final position.Next sections provide a detailed description of system testing flowchart content.

Local Tests
Magnet power converters are installed in racks dispersed inside ESS Klystron gallery area and the power/interlock cables are guided to the tunnel through wall openings called stubs.Local tests are the first types of tests performed in each rack once the interfaces are ready using ESS developed air-cooled resistive loads (Figure 3 -(a)).Local tests are divided in two (2) parts: a. power and interlock tests and b. power quality and EMI/EMC compliance tests.The first part is performed for all magnet power converters and includes the verification of correct functionality of multiple power converter interlocks at rack level ensuring the protection of the system from unwanted operational conditions.Power tests include the ramping up of current setpoint at 25 %, 50 %, 75 % of power supply maximum (200 A) for ten (10) minutes followed by a four (4) hour stability test at 100 % current setpoint.During this procedure the current and voltage to the Ohmic load are constantly monitored from the power supply GUI and with DCCTs (Danisense DS600ILSA) connected on the output cables.In parallel, the tests of EPICS based magnet control system [11] ensures the correct functionality of network interfaces and communication with the power supplies.Power Quality and EMI/EMC tests procedures are developed according to the international and European Standards, [12], [13], [14], modified according to ESS requirements and conducted only at different rack types in order to verify power converter performance.Power quality tests are performed using the Zimmer LMG670 power analyzer, in combination with AC and DC current transformers.Acquired current and voltage waveforms in AC input and DC output of power converters followed by an FFT analysis of the obtained spectrum are used to evaluate setpoint stability and harmonic content.EMI immunity tests are conducted using an EFT/burst generator (Emtest-NX5 b-1-300-16) while monitoring system response after introducing fast transients in AC main line, DC output and interlock signal circuits.Power converters are subject to dual-polarity 2 kVp, 5 kHz pulse, 15 ms duration bursts employing a 33 nF coupling capacitor or capacitive clamps for one (1) minute.Last step in the local testing phase is the EMI emission tests at AC main power line and DC output cables in order to evaluate the disturbance levels to nearby equipment.For AC line disturbance measurements a linear impedance stabilization network (LISN) is connected to rack power distribution unit and to an EMI receiver (Narda PMM9010F) whereas DC output disturbance assessment is performed using a passive probe (Narda SHC-1-1000) directly connected to the EMI receiver via 50 Ω RF cable.Extracted waveforms via Narda PMM Emission Suite in cases of no/full output load are used to verify that the conducted disturbance levels are withing the limits stipulated in [14].Zimmer LMG67A power analyzer is used also in this testing phase for waveform acquisition and FFT/ harmonics content calculation using DCCTs attached to the magnet power cables (Figure 3 (b)).Figure 5 presents an example of obtained waveforms for current, voltage and power to a quadrupole magnet with a current setpoint of 170 A and an acquisition time of one (1) minute after the four ( 4) hour stability test.The results are within the specified stability range defined from the power supply manufacturer.A DC current waveform FFT analysis is presented in Figure 6.Except from the main DC component the higher harmonics related to switching frequency of the power supplies are present in the spectrum.However, the connection of the magnet inductive load reduced harmonic distortion levels compared to the tests on the resistive load.A further study will be conducted in order to assess any effect of these harmonics on system losses and magnetic field.A Hall probe (Gaussmeter Bell 5180) is used to measure the magnetic field on the quadrupole pole tips and polarity in order to deduct preliminary excitation curves.Figure 7 presents the excitation curves for the first 4 (four) tested HEBT magnets and calculated fitting coefficients.The results were compared and are well in agreement with the pole-tip field measurements from Danfysik during FAT tests [15].In addition, the Hall probe was used to measure possible remnant fringe fields and compare these results with fringe field Opera 3D simulations [16] during system design phase [15].Measurements along the beam axis indicated  full magnetic field attenuation 40 cm away from the magnet in accordance with simulation results, therefore, the impact on the nearby instrumentation equipment and cryomodules needs to be further assessed.During integrated system testing the control of the magnets is performed from the control room using the EPICS control interface.In order to mitigate transient voltage fluctuations due to the present inductive loads current setpoint rates limiters are introduced in power supply and EPICS interfaces.

Conclusions
ESS magnets integrated testing phase is the final step before system delivery for beam commissioning validating and verifying the performance with respect to design requirements.The presented preliminary testing procedure and results will be further updated as the different system interfaces become available.In the next months, magnet testing will be mainly focused in HEBT, DogLeg, A2T and DupmpLine areas in parallel with cryomodule installation at the other areas of ESS Superconducting Linac.

Figure 1 :
Figure 1: Different types of ESS magnets with respect to area of installation.

Figure 2 :
Figure 2: Local and integrated tests flowchart for ESS magnets.

Figure 3 :
Figure 3: (a) An ESS developed Ohmic resistive load used during magnet system local testing phase.(b) LWU tunnel testing configuration using Zimmer LMG670 Power Analyzer.

Figure 4 :
Figure 4: Snapshot of quadrupole temperature distribution using thermal camera after four (4) hours of operation at full nominal load (173 A) and water flow of 2.0 l/m.Quadrupole coils reached a surface temperature value of approximately 38 o C.

Figure 5 :
Figure 5: Quadrupole current, voltage and power stability waveforms as measured with power analyzer Zimmer LMG670 and DCCTs for 170 A current setpoint at the end of the long-term stability test.

Figure 6 :
Figure 6: Magnet current FFT analysis measured with power analyzer Zimmer LMG670 showing the DC component and higher harmonics on magnet output cables during full load operation.High harmonics are associated with power converter switching frequencies spanning to the kHz range.

Figure 7 :
Figure 7: Quadrupole excitation curves and coefficients measured at magnet pole tips with Hall probes.

Table 1 :
Different types of ESS magnets, quantities and main design requirements.