Barth, Winfried

Brodhage, R.

Busch, M.

Clemente, Gianluigi

Dziuba, F. D.

Groening, Lars

Mickat, Sascha

Podlech, H.

Ratzinger, U.

Schlitt, Bernhard

Vinzenz, Wolfgang

GSI Repository

∗G. Clemente† , W. Barth, L. Groening, S. Mickat, B. Schlitt, W. Vinzenz, GSI, Darmstadt, GermanyU.Ratzinger, H.Podlech, R.Brodhage, M.Busch, F.Dziuba,Goethe University, Frankfurt am Main, GermanyAbstractH-mode cavities offer outstanding shunt impedances atlow beam energies and enable the acceleration of intenseprotons and ion beams. Crossed-bar H-cavities extendthese properties to energies even beyond 100 MeV. Thus,the designs of the new injector linacs for FAIR, i.e. a 70MeV, 70 mA proton driver for pbar-production and a cwintermediate mass, superconducting ion linac are based onthese novel cavities. Several prototypes (normal and super-conducting) have been built and successfully tested. More-over, designs for a replacement of the 90 MV Alvarez sec-tion of the GSI - Unilac will be discussed to improve thecapabilities as the future FAIR heavy ion injector.INTRODUCTIONThe scientific program of FAIR requires the construc-tion of a new 70 MeV, 325 MHz proton injector [1] for theSIS18. This machine will provide at least 35 mA of protonsat a repetition rate of 4 Hz.Beside the new proton linac, FAIR also requires a mas-sive upgrade of the GSI UNILAC linear accelerator [2].The UNILAC started operation in 1975 and it is one ofthe eldest DTL still active. Almost 40 years of high dutyfactor operation (up to 25 %) led to the increasing num-ber of failures occurred in the last years. That represents astrong reliability issue for the new FAIR facility.Additionally, the Alvarez-DTL is equipped with DCquadrupoles, which is a disadvantage for short pulse op-eration and limits the machine flexibility in case of themultibeam operation (different ion beam with specific mag-netic rigidities and beam currents). That constraint, to-gether with the challenging requirements of 15 mA ofU28+, favours the replacement of the existing DTL sectionby a new high energy linac with short beam pulses at lowrepetition rate (HE-Linac).The linac upgrade program will be completed by a newsuperconducting cw heavy-ion linac [3] with an adjustableenergy from 3.5 to 7.3 AMeV for the super heavy elementsprogram (SHE-Linac).The choice of the RF cavity type represents the most crit-ical key for such a massive upgrade. The experience withhigh current operation at the HSI and HLI led to the naturaldecision to base the UNILAC replacement on IH-DTL cav-ities. On the other hand, the successful R&D performed atthe Frankfurt University on room temperature [4] and su-perconducting CH-DTL [5] justifies the use of that kindof structures for the new proton linac and for the cw SHElinac.∗Work supported by HicforFAIR and BMBF† g.clemente@gsi.deH-MODE CAVITIESH-Mode cavities are characterized by a slim tube ge-ometry resulting in a high shunt impedance and a uniformpower loss distribution (see Fig.1). Beam dynamics is de-signed according to an asynchronous injection energy lat-tice like the KONUS [6] or the EQUUS [3]. Alternativelyan APF [7] lattice can be used.Figure 1: Effective shunt impedance of several ion and pro-ton accelerators as function of the velocity profile.The IH-DTL, exited in the TE11(0) mode is by far themost efficient RF structure in the β range from 0.01 to 0.2.For β ≤ 0.1 it is competitive with superconducting struc-tures, concerning the plug power demand, even at cw oper-ation. Several IH-DTL are routinely in operation and thisstructure has established as standard solution for heavy ionin the frequency range between 30 and 200 MHz.The CH-structure, exited in the H21(0) mode has a largerdiameter for a given frequency compared with the IH-structure. Cavities with frequencies from 150 MHz to 3GHz can be realized, making the CH-structure feasible forproton and ion acceleration up to 100 AMeV. Additionally,the crossbar geometry is so robust to ensure the mechan-ical stability required for superconducting operation. Atpresent, the s.c CH-DTL is the only multi gap supercon-ducting structure available at low energy.THE CH-DTL FOR FAIRThe FAIR requirement of a dedicated proton injectorpushed an intensive R&D activity in order to explore thecapabilities of the room temperature CH-DTL. In 2006 afirst test model was successfully built at the University ofFrankfurt and later copper plated at the galvanic workshopof GSI. This 340 MHz cavity comprises eight equidistantcells of length βλ/2=45 mm, and was used to test all fabri-cation steps including welding, copper plating and design-ing of an efficient cooling system. Moreover, a tuning strat-DEVELOPMENT OF H-MODE LINACS FOR THE FAIR PROJECTMO3A01 Proceedings of LINAC2012, Tel-Aviv, IsraelISBN 978-3-95450-122-9120Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Proton and Ion Accelerators and Applications2G Other Proton/Ionegy based on exchangeable drift tubes was adopted, and anew geometry based on larger half drift tube was developedto tune the end cells (see Fig.2). This cavity was success-fully tested with a 2 kW, cw amplifier.Figure 2: The geometry of the first room temperature CH-DTL.Later on, the 3 MW 324 MHz klystron developed forJPARC opened new perspectives for the optimization ofthe CH-DTL. At low energy, the KONUS beam dynam-ics requires short sections in contrast with the high shuntimpedance which favours longer cavities . To overcomethis problem a coupling scheme to match one klystron totwo CH sections have been developed [8].Figure 3: The electric field on the axis of a coupled CH-DTL.Figure 4: The magnetic field orientation inside a coupledCH. Field is shown in the plane A-A’ of Fig.3.The coupled structure consists of two CH-DTL con-nected through a single cell resonator. This intertank sec-tion oscillates in the Alvarez mode and the large drift tubehouses an electromagnetic quadrupole triplet. The radiusof the single cell resonator has to be adjusted so that theresonance frequency of this unit matches the adjacent CHcavities. RF simulations have proven that an effective cou-pling is achieved with this geometry. Fig. 3 and 4 show thedistribution of the electric and magnetic field along the cou-pled structure. In particular, Fig.5 shows how the couplingcell oscillates in a TM mode like the standard DTL.Figure 5: The magnetic field orientation inside the couplingcell. Field is shown along plane B-B’ of Fig.3Successful RF measurements on the scaled model of thesecond cavity of the FAIR proton injector was the mile-stone for the construction of the full scale prototype [9].The main parameter of this structure are listed in Tab.1Table 1: Main Properties of the second resonator of theFAIR Proton Linac. * indicates that calculation have beenperformed with a 15% reduced σ.Energy Range (MeV) 11.7-24.4RF frequency (MHz) 325.2Effective Voltage (MV) 13.5Beam Aperture (mm) 20Magnet Aperture (mm) 30Length (m) 2.7RF Losses∗ 1.34The cavity was delivered at the university of Frankfurt inlate 2011. In a first step dummy stems and exchangeabledrift tube made of Aluminum were mounted inside the cav-ity. Seventeen provisional tuners were installed along thestructure, six in the fist CH, one in the intertank section andten in the second CH, respectively.First frequency measurements were in good agreementwith simulations. The experimental value of 323.7 MHz isvery close to the expected one of 324.4 MHz. The cavitywas designed at a lower value with respect the operationalfrequency of 325.2 MHz to take into account the inductiveeffect of the tuners.A measured field distribution along the beam axis isshown in Fig.8 while, Figs.9 and 10 compare the measuredvoltage with the reference one calculated with LORASR.At present, stainless steel stems are ready for beingwelded inside the cavity. Copper plating will be performedin early 2013. The prototype will be then tested with a 3MW klystron in the dedicated test bench recently built atGSI.Proceedings of LINAC2012, Tel-Aviv, Israel MO3A0102 Proton and Ion Accelerators and Applications2G Other Proton/IonISBN 978-3-95450-122-9121 Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)Figure 6: The second resonator of the FAIR proton linac.Figure 7: The dummy drift tubes.Figure 8: The measured field distribution on the axis.The Proton LinacThe design of the FAIR proton injector is sketched inFig.12: up to 100 mA of proton will be extracted from anECR ion source at 95 keV. The beam is first accelerated byan RFQ to the energy of 3 MeV. After a matching section,the beam enters the first part of the drift tube linac, com-posed of three pairs of coupled CH-DTLs. At the energyof 37 MeV a 1.6 meter long dedicated diagnostic section isinstalled.At that energy, space charge effects are reduced and theKONUS offers the possibility to build long lens free sec-tions. For that reason, the high energy section is based onstandard lens free CH cavities with slightly larger beamFigure 9: The voltage distribution in the first CH section.Figure 10: The voltage distribution in the second CH sec-tion.aperture at each cavity ends. In this way the numberof quadrupole triplets is reduced and the mechanical andRF design is simplified. Electromagnetic 3D simulationsshowed that three CH-DTL made of 21 gaps each couldcover the energy range from 37 to 70 MeV within an rfpower demand around 1 MW per individual cavity.Figure 11: Beam envelope for the FAIR proton linac when70 mA are injected in the DTL section.The linac was designed assuming an RFQ output currentranging from 45 to 70 mA using the LORASR code. TheMO3A01 Proceedings of LINAC2012, Tel-Aviv, IsraelISBN 978-3-95450-122-9122Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Proton and Ion Accelerators and Applications2G Other Proton/IonFigure 12: The schematic view of the FAIR proton linac.Table 2: Beam Evolution of the FAIR proton injector at 70mARFQ Output 95% ǫ X-X’, norm. (mm mrad) 1.22RFQ Output 95% ǫ Y-Y’, norm. (mm mrad) 1.14RFQ Output 95% ǫ∆W-Φ (keV ns) 9.01DTL Output 95% ǫ, norm. X-X’(mm mrad) 2.35DTL Output 95% ǫ, norm. Y-Y’ (mm mrad) 2.68DTL Output 95% ǫ∆W-Φ (KeV ns) 17beam envelopes, shown in Fig.11, for the design current of70 mA shows a large safety margin against beam losses.Fig.13 presents the emittance including the current statedalong the abscissa at the injection point. The red curve isthe upper limit for the emittance that the injector, at givencurrent, must provide. This limit is obtained from the fore-seen multi-turn scenario and it depends mainly on severalsynchrotron parameters, such as circumference, injectionenergy, acceptance and filling factor. For a proper injec-tion the linac emittance must remain below the straight line.This brilliance analysis shows that the best window for aneffective injection would be around 55 mA.Figure 13: Brilliance analysis at the end of the proton linacin comparison with the synchrotron acceptance.Finally, Tab.2 shows the beam evolution to the final en-ergy of 70 MeV.THE CW SHE LINACAll chemical elements from 107 to 112 have been dis-covered at GSI.Table 3: Main parameter of the s.c. cw SHE LINACA/q 6Frequency (MHz) 217Max. Beam Current (mA) 1Injection Energy (AMeV) 1.4Output Energy (AMeV) 3.5-7.3Length (m) 13To keep the Super Heavy Element (SHE) program at acompetitive level an upgrade of the High Charge Injector(HLI) is ongoing, including a new 28 GHz ECR sourceand a new cw four-rod RFQ [10].At present, behind thenew RFQ the existing IH-DTL injects a 1.4 AMeV beaminside the Alvarez DTL section of the UNILAC.As the UNILAC should be operated as injector for FAIR,a strong limitation of the beam time for the SHE program isexpected. To offer and adequate beam time for the nuclearchemistry program a dedicated linac is request by manyscientific users. The design of a low energy cw linac wouldconsiderably profit from a multigap superconducting struc-ture. At present, the CH-DTL is the only multigap structurewhich can be operated as superconducting DTL.For that reason, a collaboration between GSI, IAP Frank-furt and HIM Mainz worked out a conceptual design basedon s.c CH-DTL.That new linac consists of nine superconducting CH-cavities operated at 217 MHz. The first three cavities ac-cepts the beam from the HLI and perform the first stage ofacceleration to 3.5 AMeV. At this point, the energy variablepart begins and the final energy can be adjusted up to 7.3AMeV within an energy spread of ± 3 AkeV. Beam focus-ing is provided by seven superconducting solenoids. Thegeneral parameters are listed in Table 3.The demonstratorThe so called Demonstrator project includes the con-struction of the first s.c CH cavity with the s.c. solenoisat the cavity ends, and the assembly of the components inthe cryostat. A full performance beam test is foreseen atthe HLI within 2014 at earliest [11].The s.c. 217 MHz CH-structure [12] consists of 15equidistant gaps for a gradient of 5.1 MV/m. The structure,shown in Fig.14, has inner total length is 0.69 m. Opti-mized field distribution is shown in Fig.15Nine static tuners, one slow and two fast bellow tuners,will be mounted to ensure the proper frequency matching.Further rf, mechanical and multipacting simulations areProceedings of LINAC2012, Tel-Aviv, Israel MO3A0102 Proton and Ion Accelerators and Applications2G Other Proton/IonISBN 978-3-95450-122-9123 Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)Figure 14: The demonstrator project.Figure 15: Field distribution of the demonstrator CH-DTL.in progress at the moment to determine the final geometryof the dynamic bellow tuners.THE HE LINACA new High Energy Linac (HE-LINAC) is presently un-der discussion at GSI. This new machine has to replace the90 MV DTL section of the UNILAC.A first proposal [13] was to increase the prestripper en-ergy to 3 AMeV with 3 IH-DTL. The higher energy wouldallow generation of U37+ which would be further acceler-ated to 11 AMeV by another 4 IH-DTL.Recently a less ambitious solution, consisting in the re-placement of the Alvarez DTL, is under investigation. SixIH cavities would cover the energy up to 11.4 AMeV inapproximatly 25 meter including a new matching sectionbehind the charge separator.This solution is more cost effective compared to the pre-vious one, and will leave plenty of space available for afuture linac upgrade. At the energy of 11.4 the frequencycan be raised to 325 MHz and CH cavities could be usedto increase the energy. Additionally, alternative solutionsto the present gas stripper, like a C-foil or a plasma stripperare under investigation in order to generate higher chargeFigure 16: The proposed upgrade of the UNILAC insludigthe new HE and SHE linac.states.ACKNOWLEDGMENTThe authors would like to express their gratidute tothe collegues of the mechanical workshop of IAP Frank-furt and of the galvanic laboratory of GSI. This work hasbeen supported by the Federal German Ministry for Educa-tion and Research BMBF-06FY9089I and by HICforFAIRwithin the LOEWE funding program of the State of Hesse.REFERENCES[1] L. Groening et al., “Status of the FAIR Proton Linac” ThisProccedings, THPB034.[2] W. Barth et al, ”The injector system of the FAIR project”,LINAC08, Victoria, BC, Canada, MO204, p.35 (2008)[3] S.Minaev et al., “Superconducting, energy variable heavyion linac with constant beta, multicell cavities of CH-type”Phys. Rev. ST Accel. Beams 12, 120101 (2009).[4] G.Clemente t al., “Development of room temperaturecrossbar-H-mode cavities for proton and ion acceleration inthe low to medium beta range” Phys. Rev. ST Accel. Beams14, 110101 (2011).”[5] H.Podlech et Al, ”Superconducting CH structure” Phys.Rev. ST Accel. Beams 10, 080101 (2007).[6] U. Ratzinger and R. Tiede, ”Status of the HUF RF linacstudy based on H-mode cavities”, Nucl. Inst. and Meth. inPhysics Research A, 415, 1998.[7] Y.Ywata at al, ”ALTERNATING-PHASE-FOCUSED IH-DTL FOR HEAVY-ION MEDICAL ACCELERATORS”,EPAC06, Edimburgh, UK, WEPCH169, 2328[8] U.Ratzinger at al, ”A 70-MeV proton linac for the FAIR fa-cility based on CH cavities”, PAC05, Knoxville, TN, USA,TH1004, 526[9] R.Brodhage at al, ”First Coupled CH Power Cavity forthe FAIR Proton Injector”, IPAC11, San Sebastian, Spain,WEPS036, 2565[10] P. Gerhard et al, this proceedings THPB035[11] W. Barth et al., “Superconducting CW Heavy Ion Linac atGSI” This Proccedings, TUPB074.[12] F.Dziuba et al., “Development of superconducting crossbar-H-mode cavities for proton and ion accelerators” Phys. Rev.ST Accel. Beams 13, 041302, 2010[13] B.Schlitt et al, “Design studies for a new heavy ion injectorlinac for FAIR” HIAT’11, Chicago,IL, WEC05MO3A01 Proceedings of LINAC2012, Tel-Aviv, IsraelISBN 978-3-95450-122-9124Copyrightc ○2012bytherespectiveauthors—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Proton and Ion Accelerators and Applications2G Other Proton/Ion

Development of H-mode linacs for the FAIR project

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Development of H-mode linacs for the FAIR project

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