Advanced basic layout of the HElmholtz LInear Accelerator for cw heavy ion beams at GSI

The design and construction of continuous wave (cw) high intensity linacs is a crucial goal of worldwide accelerator technology development. The standalone sc heavy ion linac HELIAC (HElmholtz LInear ACcelerator) is a common project of GSI Helmholtz Centre for Heavy Ion Research and Helmholtz Institute Mainz (HIM) under key support of Goethe University Frankfurt (IAP). In 2017 the first section of the linac has been successfully commissioned and extensively tested with beam at GSI, featuring the capability of 216.816 MHz multi-gap Crossbar H-mode (CH) DTL-structures. At present, the first fully equipped cryomodule of the HELIAC is under construction. In addition, six further superconducting CH cavities are being procured. The HELIAC beam dynamics concept foresees a total of twelve CH-cavities in order to accelerate ions with a mass-to-charge ratio of 6 up to an energy of 7.5 MeV/u. In this paper, an advanced very compact and less complex layout is presented, where the same number of accelerating cavities can be accommodated in three instead of four cryomodules, thus also reducing the number of solenoids and rebunchers. In addition, the integration and linking of the HELIAC to the GSI accelerator UNILAC will be outlined.


Introduction
The design and construction of cw high intensity linacs is a vital goal of worldwide accelerator technology development [1].Particularly compactness of a particle accelerator is a beneficial demand for the development of high intensity cw proton and ion linacs [2].In the low-and medium-energy range cw-linacs can be used for several applications, as boron-neutron capture therapy, high productivity isotope generation and material science.Besides, a high-energy linac is essential as a component of large-scale research facilities, such as spallation neutron sources or accelerator-driven systems.Thus the study and investigation of the design, operation and optimization of a cw-Linac, as well as progress in elaboration of the superconducting technology [3,4] is of high relevance.The design of the HIM/GSI cw-Linac HELIAC with its normal conducting injector linac comprising ECR-ion source, LEBT, RFQ and IH-DTL with an adjacent compact matching line is depicted in Fig. 1.The superconducting main linac consists of four cryomodules, each equipped with three CHcavities a rebuncher and two 10T solenoids.The superconducting CHcavities, operated at 217MHz should provide for ion acceleration to beam energies between 3.5MeV/u and 7.3MeV/u, while the energy spread should be kept smaller than ±3keV/u.The cw-Linac allows the acceleration of highly charged ions with a mass to charge ratio of up to 6.For proper beam focusing superconducting solenoids are located between the CH cavities.The general parameters are listed in Table 1 [5].Sc solenoids # 7 2. Demonstrator R&D-project R&D and prototyping -demonstrator and advanced demonstrator project [6] in preparation of the proposed HELIAC -are assigned to a collaboration of GSI, HIM and GUF.The demonstrator setup was located in straightforward direction of the GSI-High Charge State Injector (HLI).The demonstrator cryostat comprises a 15 gap sc CH-cavity (CH0) embedded by two superconducting solenoids; all three components are mounted on a common support frame (see Fig. 1) [7].The beam focusing solenoids provide maximum fields of 9.3T.The solenoids are connected to lHe ports inside the cryostat by copper tapes allowing dry cooling.The sc CHstructure CH0 is the key component and provide a variety of research and development [8].The sc 15 gap CH-cavity is directly cooled with liquid helium, supported by a helium jacket made by titanium.After high pressure rinsing (HPR) a performance test in a vertical cryostat at low rf power was performed at IAP, reaching gradients up to 7MV/m.After the final assembly of the helium vessel and further HPR preparation at RI, the cavity was tested again, but in a horizontal cryostat.As shown in Fig. 2, the cavity showed improved performance due to an additional HPR treatment, the initial design quality factor Q0 has been exceeded by a factor of four, a maximum accelerating gradient of Eacc=9.6MV/mat Q0=8.1410 8 has been achieved [8][9][10][11][12].
At June 2017, after a short commissioning and ramp up time of some days, the CH0-cavity first time accelerated heavy ion beams (Ar 11+ ) with full transmission up to the design beam energy of 1.866MeV/u (Wkin=0.5MeV/u)[13].For the first beam test the sc cavity was powered with 10 Watt of net rf power, providing an accelerating voltage of more than 1.6MV over a length of 69cm.Further on the design acceleration gain of 3.5MV has been verified and even exceeded by acceleration of beam with high rigidity (A/q=6.7).The beam quality has been characterized by measuring the phase space distribution.The measured emittance of the argon beam, showed an adequate beam quality: the total 90% horizontal beam emittance (normalized) is measured for 0.74µm, while in the vertical plane the total 90% emittance is 0.47µm only.All measurements have been performed without solenoidal field, therewith any additional emittance degradation effects by different beam focusing could be avoided.The measured (normalized) beam emittance growth at full beam transmission is sufficiently low: 15% (horizontal plane) and 10% (vertical plane).Selective measurements at other rf-amplitudes and -phases, as well as for other beam rigidities confirmed the high (transversal) beam performance in a wide range of different parameters.Besides beam energy measurements the bunch shape was measured after successful matching with the Feschenko monitor [14,15].As shown, an impressive small minimum bunch length of about 300ps (FWHM) could be detected, sufficient for further matching to and acceleration in future rf-cavities.Following the successful beam test of the first cavity (CH0) within the Demonstrator research project, the next step towards realization of the HELIAC is the manufacturing and operation of the first cryogenic module (CM1) within the "Advanced Demonstrator" project [16].As shown in Fig. 3 it contains the demonstrator cavity CH0 [17], two identical CH1 & CH2 cavities [18], a two-gap re-buncher cavity (B) [19,20] and two identical sc solenoids S1 & S2).For stable 4K operation of the entire HELIAC a cryo plant with 240 W total cooling power@4K is required.The cryo plant of the GSI-Series Test Facility (STF) has a cooling capacity of 700W and is already in operation for testing of superconducting SIS100 dipole magnets for FAIR project.After the magnet testing will be finished, the cryo plant is foreseen to supply mainly the HELIAC.Fig. 4 shows a 3d model of Helium transfer lines and the distribution boxes in the testing area.The lHe distribution box supplies the test area with liquid Helium and has been accomplished in 2020.In preparation for further beam test activities, the beamline, which connects the "GSI-HochLadungs-Injektor" (HLI) with the testing area, was installed within the radiation protection shelter.Recent findings have indicated that longer cryomodules than previously envisioned could possibly be used (Fig. 5).This approach with a CM lengths > 5m was initially rejected for handling reasons, among others.However, it turns out that the use of longer cryomodules is possible.For this, cryomodules 3 and 4 could be merged and combined into one cryomodule.At the same time, the significantly reduced drift distances could also save a solenoid and a rebuncher cavity.While the removal of these elements could result in a slight degradation of beam quality, it would also result in noticeable cost savings as well as valuable shortening of the overall length of the linac by 2.49m.Transversal and longitudinal beam envelopes of an advanced beam dynamics approach for this case are shown in Fig. 6.

Energy savings
The highly efficient superconducting technology used allows accelerator operation with up to 90% reduced primary energy input compared to the existing accelerator facility UNILAC: Power measurements showed that for a GSI-typical beam experiment operation at medium heavy ion energy (i.e. 7 mega volts/mass unit, which corresponds to about 10% of the speed of light) the primary power requirement is in the order of about 4.9 megawatts.The HELIAC, on the other hand, will require only IOP Publishing doi:10.1088/1742-6596/2687/5/0520096 about 1.1 megawatts under the same conditions, although it will deliver four times as many ions per unit time in the process because of the continuous beam operation.For a typical six-month UNILAC beam period, HELIAC operation could save about 20 gigawatt hours of electrical energy due to the higher efficiency and four times the number of particles on over time, thus making a significant contribution to climate neutrality.In addition, the novel ECR ion source will make significantly higher ion intensities available, so that a larger number of ions per time unit will be available for the user experiments and thus the energy savings can be significantly increased again.

Basic approach and link to UNILAC
Recently it was decided to realize HELIAC in a stepwise approach.After finalizing the R&D-phase with installation and commissioning of CM1, it is planned to setup ECR-ion source and the normal conducting injector part 23, 24] in the new HELIAC radiation protection shelter.The to be built basic version comprises CM1 -CM3, while the CM3-cold string is equipped with a basic set up of 3 CHcavities and a rebuncher and two solenoids only.As the injector linac will be supplied with rf amplifiers capable for pulse operation (30% rf-duty cycle), first of all HELIAC operation is restricted to 25% beam duty factor.

Outlook
The GSI UNILAC will be upgraded to supply the FAIR facility currently under construction with short high-intensity heavy ion pulses [25][26][27][28][29][30][31].In future, the HELIAC will then meet the requirements of users, including those from SHE research, for long pulses with the highest possible power density.For this purpose, an optimized HELIAC beam dynamics layout was developed that meets all the required beam parameters [32,34] and was designed to be very compact and efficient at the same time.In addition, the HELIAC will save considerable amounts of primary energy (approx.90%) in cw heavy ion user operation compared to the existing UNILAC system.Together with the results already achieved in the performance of the prototype, the promising beam dynamic design and the significant energy savings due to the use of superconducting RF technologies, the HELIAC is of general interest to the accelerator community.The upcoming extended beam test with the first fully equipped cryomodule CM1 [35][36][37][38][39] will take place in 2023 and will pave the way for the realization of the entire HELIAC.

Figure1.
Figure1.Design of the HELIAC with normal conducting injector linac comprising ECR-ion source, LEBT, RFQ and IH-DTL -the superconducting main linac consists of four cryomodules each equipped with three CH-cavities a rebuncher and two 10T solenoids.

Figure 3 .
Figure 3. Photo of CM1 cold string, equipped with three CH cavities, a rebuncher and two solenoids

Figure 4 .
Figure 4. 3d Model of the testing area at GSI

Table 1 .
Design parameters of the cw-Linac