Paolo Michelato INFN Milano - LASA

Transcription

1 ACCELERATORS FOR ADS Paolo Michelato INFN Milano - LASA 1

2 TRASCO - ADS The Italian TRASCO program aims to study the physics and to develop the technologies needed to design an Accelerator Driven System (ADS) for nuclear waste transmutation. The main objectives of the research program are: Conceptual design of a 1 GeV - 30 ma proton LINAC. Design and construction of main accelerator components (proton source, RFQ, SC cavities). Development of methods and criteria for neutronics, thermal-hydraulics and plant design for an EA-like sub-critical system. Materials technologies and development of components to be used with lead or LBE. 80 kev 5 MeV 100 MV MeV 1 GV GeV Proton injector Low energy and superconducting preaccelerator Linac (ISCL) High energy superconducting linac (3 sections) with elliptical cavities 2

3 The subcritical core Accelerator Drive (Subcriticality) enables versatile and effective Nuclear Waste Destruction ATW Burner Typical Power: 2000 MWt Pump Liquid Lead Coolant Proton Beam Heat Exchanger: to steam-driven power production (up to 40% eff.) Beam Guide Window Spallation Neutron Source Transmutation Region (solid actinide fuel) why subcriticality has advantage for waste destruction: Power control is not linked to reactivity feedbacks, delayed neutrons or to control rods, but only to the accelerator drive ATW has no need for fertile materials. ATW uses pure transuranic cores Subcritical systems work independently of the fuel composition EOL inventory is not limited by criticality. Possible to have EOL burndown of inventory Neutronics and thermohydraulics are effectively decoupled Figure 4 3

4 ADS proton beam requirements Very high duty cycle, possibly CW Energy of the order of 1 GeV, determined by neutron production rate per GeV and per proton» (optimum value reached at ~1 GeV) energy dissipated in the input window 20» (rapidly decreasing with energy, when E<few GeV) 10 Beam current of the order of 10 ma 0 Beam power from several MW up to tens of MW few MW for a demo plant of ~100 MWth ~30 MW for an industrial burner of ~1500 MWth Very few beam trips per year accepted if longer then 1 second No limitation for very short beam trips: << 1 second new challenges in the overall design of the accelerator /GeV) (neutrons/ yield / E p proton energy, E_p (GeV) 4

5 Most powerful proton accelerators Linacs LAMPF/LANSCE (~1970)» 800 MeV» 1 ma H + average current» Peak H + current Hz and 625 μs pulse length» NC accelerator Cyclotrons PSI separated sector (1974)» Original design was for 100 μa» From 72 to 590 MeV» 1.8 ma average current» Beam losses at extraction < 1 μa» Plans for further upgrade (new cavities) Both linac and cyclotrons were considered as possible ADS drivers No fundamental obstacles have been found so far for a linac to deliver ~100 ma at 1 GeV or more 1 GeV and few ma are considered as limiting values for a cyclotron (multistage): possible for the demonstrator, not for the burner 5

6 The Cyclotron Circular accelerators: the cyclotron It has a limit in the maximum energy: if m is the relativistic mass, higher is the particle velocity, higher becames the relativistic mass and the particle looses its syncronism with the accelerating field. Synchrocyclotron (field frequency is changed during acceleration to compensate the increase of the mass of the particle) The magnet is huge for high energies mv R 2 = qbv v qb ω = = R m B The PSI Cyclotron PSI Ring Cyclotron Commissioning year 1974 Proton Beam current 1.8 ma CW Beam emittance 2 π mm mrad Injection Energy 72 MeV Extraction Energy 590 MeV Energy spread % Extraction Losses 0.03 % Number of turns ~ 220 6

7 Cyclotron option/1 Serious linac competitor when the needed d beam power is up to 4-5 MW ( ma) Reasonable extrapolation of the PSI cyclotron Some consensus that it should be possible to design and build a 10 MW cyclotron (1 10 ma) Few conceptual studies Three stages machines:» Source + Preinjector (Cockroft-Walton/Cyclotron/RFQ): few MeV» Intermediate cyclotron (at least 4 sectors): 100 MeV»Separated sector cyclotron (~ 10 sectors): 1 GeV 7

8 Cyclotron option/2 Beam extraction ti is critical Minimise beam losses for a 10 MW beam Single turn extraction is needed» Well separated orbits High energy gain per turn (high accelerating voltage) Narrow beams (small energy spread) Requirements for the RF cavities at high voltages H 2+ acceleration and extraction by stripping has been proposed too RF, injection and extraction systems for a high current, high energy cyclotron still need intense study and R&D programs. So far only conceptual studies have been performed. 8

9 The linac: a resonant accelerator An RF source is used to generate an electric field in a region of a resonant metallic structure ( cavity ). The particles of the beam need to be localized li in bunches and properly phased with respect to the field so that the beam is accelerated. d ( 2 γmc ) = qe ( s t) ds In order to keep acceleration along the linac this synchronism condition needs to be maintained. z, bunches Electric field 9

10 Linear accelerator /1 10

11 General block diagram of an RF Linac The linac delivers energy to the beam by the application of an electric field Δ W kin = qδv The use of electrostatic fields is limited by the electric breakdown (practically, ΔV can reach a few tens of MV) Particle Source Linac structure: Acceleration (cavities) Transverse focusing (magnets) Output beam (experiments, users, applications...) Subsystems Electric power Vacuum Cooling RF power and control The use of time-varying harmonic (RF) electric fields allows to increase the energy gain, partially overcoming the breakdown effects) 11

12 The ADS Linac Linac benefits of impressive progresses in the field of SC elliptical RF cavities (CEBAF, LEP2, TRISTAN and KEK2, TTF-TESLA and now also SNS) R&D going on from several years demonstrated t d that t this technology can be extended d to proton linac down to β ~ 0.5 (β = v/c) Intrinsic modularity simplify reliability issues Redundant design strategy based on the spare-on-line concept Strong focusing and large beam aperture produce negligible losses The scheme generally considered consists of four different sections The proton source: (proton energy kev) The Radio Frequency Quadrupole (RFQ): (up to 5 MeV ) A medium energy section, either NC or SC (up to 100 MeV ) A high energy section made of SC elliptical rf cavities (up to final energy 1 GeV) most of the linac is here! ( 12

13 TRASCO Reference Linac Design 80 kev 5 MeV ~100 MeV 200 MeV 500 MeV >1000 MeV Proton Source RFQ Medium energy ISCL linac 3 sections high energy SC linac Source RFQ ISCL High Energy SC Linac 80 ke ev High current (3 5mA) Micro owave RF Source High transmissi on 90% 30 ma A, 5 MeV ( 352 MHz) 5-85/100 MeV SC linac Spoke cavities (352 MHz) Lambda/4 cavities (176 MHz) Reentrant cavities (352 MHz) or NC Drift Tube Linac (DTL) 8βλ focusing 3 section linac: 85/ MeV, β= MeV, β= /2000 MeV, β=0.85 Five(six) cell elliptical cavities Quadrupole doublet focussing: multi-cavity cryostats between doublets MHz 13

14 Linac: injector Consists essentially of a source (microwave RF source, operating voltage <100 kv) and an RFQ (copper-bulk structures, best tool for initial beam acceleration and bunching when disruptive space-charge effects dominate) LEDA at Los Alamos has demonstrated the feasibility of injectors able to provide CW proton beams of 100 ma and 6.7 MeV Similar injectors in construction in France (IPHI)» ECR source (SILHI): 100 ma, 95 kev (operating)» RFQ: 352 MHz, CW, 5 MeV, 100 ma in Italy (TRASCO injector)» ECR source (TRIPS): 35 ma, 80 kev (operating)» RFQ: 352 MHz, CW, 5 MeV, 30 ma 14

15 Source & RFQ fully operational since 1999 LEDA Source: Proton Beam current 110 ma Total Beam current 130 ma Beam emittance 0.2 π mm mrad Operating voltage 75 kv LEDA (LANL) LEDA RFQ: Beam current 100 ma Beam emittance 0.22 π mm mrad 0.17 π deg MeV Final Energy 6.7 MeV Length 8 m (4 sections) RF Power 670 kw (beam) 1.2 MW (structure) Peak Field 1.8 Kilpatrick Beam halo tests have been performed on the LEDA HEBT to compare simulation codes with experimental results 15

16 SILHI Goals: Achievements Beam current 110 ma 157 ma Beam emittance π mm mrad 0.11 π mm mrad Operating voltage 95 kv 95 kv Beam noise (rms) 2 % 1.2 % Source operational, RFQ under fabrication, First RFQ beam expected in 2004 IPHI Several reliability tests were performed on the source 3 before extraction system changes: 99.96% availability (1 stop in 104 hours of operation) 2 with new extraction system: 99.8% availability (8 stops in 162 hours, automatic restart procedures in 2.5 min, MTBF=23.1 hours) IPHI RFQ: Beam current 100 ma Beam emittance 0.2 π mm mrad T 0.2 π deg MeV L Final Energy 5 MeV Length 8 m (3 sections) RF Power 500 kw (beam) 1.2 MW (structure) Peak field 1.7 Kilpatrick 16

17 Source operational, RFQ under fabrication, First RFQ beam expected in 2004 TRASCO injector TRASCO RFQ: Beam current 30 ma Beam emittance 0.2 π mm mrad T 0.18 π deg MeV L Final Energy 5 MeV Length 7.13 m (3 sections) RF Power 150 kw (beam) 600 kw (structure) TRIPS Goals: Achievements Peak Field 1.8 Kilpatrick Beam current 35 ma 55 ma Beam emittance 0.2 π mm mrad To be measured Operating voltage 80 kv 80 kv Peliminary reliability test at 65 kv/15 ma 24 h with no beam interruptions Reliability at 80 kv is not yet achieved, need a few more month work 17

18 Proton cavities What is the difference between an electron and a proton SC cavity? me=0.511 MeV 1 β = 1 γ γ 1 2 = mp= MeV (~2000 me) 2 A proton varies its velocity on a much higher kinetic energy range with respect to an electron Synchronous condition i for a multicell ll cavity (π mode): λ RF β 2 L = Time needed d to traverse one spatial period is equal to half an RF period The cell length depends on the particle velocity! Synchronism is exact only for a given velocity value and the cavity can be operated in a velocity range Technologically impossible to vary the cell length continuously with β + W k mc 18

19 Linac: low energy section In this low energy sections, two basic solutions are possible: Well proven nc structures (DTL or similar)» It is difficult to implement spare-on-line strategy t» Huge power dissipation in CW operation (technological challenge for average currents of 10s ma)» Efficient structures require a rather small beam bore (losses!) Indipendently phased sc cavities» Structures with wider energy acceptance (easy to implement spare-on-line )» Moderate energy gain at each cavity, the linac can be designed to tolerate the loss of a few cavities without losing the beam» Wider beam holes SNS: nc section to 200 MeV (DTL 90 MeV, CCL) The proposal for RIA is to use sc cavities right after 10 MV 19

20 R&D on very low β sc structures Several options are being considered for the low energy sc linac, in the range (or 200) MeV: Quarter wave resonators (or half-wave) Reentrant cavities Spoke cavities (could be the best choice for high current) Quarter Wave resonator (QWR) 2 gap structure of the ALPI linac in INFN-LNL Reentrant cavity single gap structure, under study for TRASCO. He Vessel integrated in the cavity β= MHz 2 gap spoke cavity for AAA, under construction by Zanon spoke Beam tube Coupler port stiffener 20

21 Linac: high energy part All designs are based on the technology of sc rf elliptical l cavities (developed for electron accelerators) very ygood rf efficiency relatively high field gradients (shorter length of the accel.) large bore radius at the frequency usually considered (reduced beam losses even for very intense beams) lower operating costs w.r.t. nc and possibility of CW operation The low velocity of protons, varying from β=0.43 at 100 MeV to β= at 1 GeV, imposes a variable length of cavities 3 sections matched at three β values may give an efficient coverage (even up to ~2 GeV)» lower # is bad for transit time effects (cavity efficiency vs. β)» higher # means more R&D and smaller production series 21

22 Reduced β cavities The electron cavity geometry can modified to accelerate protons with velocities as low as β ~0.5 (approximately 100 MeV proton energy) At lower velocities (energies) different geometries need to be chosen Frequency depends mainly on the cavity radius (see pillbox), so the geometry is that of a squeezed electron cavity Lowering β we have: Worse Ep/EaccE /E factor Worse Bp/Eacc factor And therefore operation is limited to lower accelerating fields β = v/c 22

23 High energy sections with different β values β=0.50 section β=0.65 section β=0.85 section 23

24 Efficient use of the cavities In order to efficiently design a linac it is necessary divide it in sections, each using a different cavity geometry in an energy range If N is big, too many sections are needed (low velocity acceptance) 12 Transition energies at 190 MeV and 430 MeV: ty [MeV] Energy Gain/Cavi S3 S2 S S1: MeV (β=0.5, i.e. 145 MeV) S2: MeV (β=0.65, i.e. 296 MeV) S3: MeV (β=0.85, i.e. 843 MeV) Increasing β: Higher accelerating field Longer cavities Greater energy gain! Beam Energy [MeV] 24

25 Why Superconductivity in RF linacs? In normal conducting linac a huge amount of power is deposited in the copper structure, in the form of heat, that needs to be removed by water cooling (in order not to melt the structures) Dissipated power can be much higher than the power transferred into the beam for acceleration Superconductivity, at the expenses of higher complexity, drastically reduces the dissipated power and the cavities transfer more efficiently the RF power to the beam In short: NC linac: lower capital cost, but high operational cost SC linac: slightly higher capital cost, but low operational cost 25

26 The surface resistance For a good but not perfect conductor, the fields and currents penetrate into the conductor in a small layer at the cavity surface (the skin depth) Therefore it is possible to define a surface resistance (inverse of the conductivity divided by the skin depth) and evaluate the average power dissipated on the cavity surface S P diss = Rs 2 2 S H ds The dependency d of the surface resistance from the frequency and the operating temperature of superconducting materials allows to understand the benefits of the use of superconductivity 26

27 But... Take into account Carnot The R s predicts a factor of reduction in losses, s but we need to keep in mind that in the SC case, this power is deposited in the cold bath: this means a power in the refrigerator that, at least, has to compensate for the Carnot cycle efficiency: i η Carnot = T 1/ / 70 for T = = 4 = 1 300K, T2 4.2K 25 30%at T 4.2K 2 = ηideal/real = T1 T2 1/150 for T1 = 300K, T2 = 2K 15 20%atT = 2K η tot = η η C th 250W at 300K for 1W at T = 4.2K 800W at 300K for 1W at T = 2K Of course, life is a bit worser than that, since here we neglected (at least): Static power losses in the He bath (power flowing directly in the He) Material impurities which lower R s (higher dissipation) Still, a wide frequency range favors superconductivity 27

28 Realistic SC cavity geometries The pillbox geometry has the drawback of the presence of resonant electron trajectories at the cavity flat-top and between the vertical walls, which are seeded by electron emission (multipacting) A smoother cavity geometry is used to mitigate the multipacting phenomenon 2 Ez along beam axis 1 z [MV/m] E z z[cm] Cavità β=0.5 Cavità ideale Axisymmetric computational model 28

29 Quality factor: (as high h as possible) Figure of merits of a cavity Q = ωu Where U is the power in the cavity and P diss the power P diss dissipated along the surface, typical value (Normal Conductive ) This parameter gives the frequency bandwidth of the cavity, L R C ω 0 = Q = =ω 0 RC 1 LC 3 db, r Shunt Inpedance ( ΔV ) 2 = Where ΔV is the voltage seen by the beam P diss I ΔI Q = f/δf f I 29

30 R&D on sc cavities Special care is needed d in designing i low-β cavities as short cavity length leads to peculiar problems electromagnetic» smaller volume implies higher surface fields than electron cavities mechanical» Lorentz forces detuning (pulsed operation?) and microphonics R&D activities on low-β cavities on-going since several years (France, Japan, Italy, USA, ) Very successful activity on prototypes» In spite of some initial fears, no multipacting limitations have been detected in all tested cavities One accelerator based on these cavities, the Oak Ridge SNS, ready for the cavity production phase ESS is considering i a sc design 30

31 700 MHz SC cavities protons (France & Italy) β=0.47 Various low-β 700 MHz single cell cavities (bulk Nb) have been tested, yielding excellent performances, well above the design specifications: β=0.65 cavities from CEA/CERCA tested at CEA/Saclay β=0.47 cavities from INFN/ZANON, tested at TJNAF Multicell cavities under fabrication for both programs 1E+11 1,E+11 T = 2 K INFN/MI β=0.47 cavity Test #3 T = 2 K CEA/Saclay β=0.65 cavity T = 1.5 K Q 0 1E+10 Q 0 1E+10 1,E+10 Design goal ASH goal 1E E 1,E E acc [MV/m] E acc 31

32 TRASCO 700 MHz β=0.5 Single-cell Prototypes LASA is responsible for the design and realization of prototypes t for the high energy section SC cavities fabrication and testing Beam dynamics activities 32

33 Cavity test infrastructure in Milano Clean Room, HPR station ti & vertical cryostat t HPR station in a class 100 clean room RF test bunker with a vertical cryostat Z101 Clean room and HPR Ultrapure water HPR mechanics and filter before mounting in the clean room RF Test Bunker 33

34 Prototypes of sputtered (Nb on Cu) cavities with the CERN MHz technology for the LEP cavities 350 MHZ SC cavities protons (CERN INFN) Single cell sputtered β = cell sputtered β = Cavity integration in a LEP type cryostat Q [10 9 ] B peak [mt] T = 4.2 K Test 1 Test 2 He Processing 1 He Processing 2 The cavities performed as the best LEP cavities, but difficult to fabricate lower β moderate gradients w.r.t. 700 MHz longer linac TRASCO Specifications 0.8 Q=2 x MV/m E acc [MV/m] 34

35 Multicell structures have been built for the SNS project, for both the β=0.61 and the β=0.81 cavities All tests t reached the design goals with good margins The cavity fabrication for all the cavities of the SNS linac will start soon The actual RIA linac proposed design will use the SNS cavities and extend to lower energies with a β=0.47 structure 805 MHz SC cavities for protons (SNS + RIA) 1,E+11 β=061 6 cell cavity SNS004 Q 0 vs. E acc 1,E+11 6 cells β=0.81 cavity 6SNS81-1 stiffening ring at 80mm Q 0 vs. E acc Test #1 Test #1 1,E+10 1,E+10 Q 0 Design Goal Q 0 Design goal 1,E+09 1,E+09 T = 2 K T = 2 K 1E+08 1,E E acc [MV/m] 1,E E acc [MV/m] 35

36 The TESLA experience TESLA aim is the design of a 250 GeV GeV collider For these energy high gradients are needed to limit the linac length Present goal is set for a 20 km + 20 km SC linac: 25 MV/m The TESLA Test Facility cavities, originally planned for 15 MV/m reached the 25 MV/m goal The Q design value needs to be chosen from the tolerable power losses in the He bath of the accelerator It goes directly in the cryogenic plant dimensioning (1W-800W) 36

37 Material defects Limiting performances of SC cavities Impurities in the superconductor increase the surface resistance (residual surface resistance) or can lead to hot n.c. spots that induce the cavity quench. In order to limit losses (and going towards high fields), it is necessary to use very high quality materials and strict procedures for reduction, refinement, casting, furnace treatments, rolling, welding, polishing, rinsing, etc... Treatments to improve the thermal conductivity are needed to avoid thermal breakdown. Magnetic fields To avoid trapping of magnetic flux, the ambient magnetic fields needs to be reduced to a few mt! Also, the B field associated to the RF needs to be lower than the critical field B~170 mt to avoid magnetic field penetration in the superconductor. Electron Emission At high fields electron can be emitted from the surface, accelerated by the fields and release their kinetic energy heating the surface. Surface treatment is necessary:» aggressive chemical etching,» rinsing with ultrapure water in clean room conditions. 37

38 Practical limits & state of the art The peak electric and magnetic fields limit the cavity performances, in terms of achievable accelerating gradients For an electron linac SC cavity geometry, where Ep/Eacc ~ 2 Bp/Eacc ~ 4 mt/(mv/m) the state of the art in a multicell cavity under operating conditions has been reached by the TESLA collaboration: E acc > 25 MV/m E p > 50 MV/m with no field emission B p > 100 mt with an acceptable Q value (low losses) Proton cavities are limited to lower accelerating fields due to higher values of peak fields 38

39 Reliability and availability/1 Studies of ADS dynamics seem to suggest that short beam trips (< 1 s) can be tolerated due to the rather big thermal inertia the allowable number of longer (> 1 s) beam trips depends on tech details of target and reactor estimated m of the order of hundreds per year unexpected shutdown (long term beam interruption) should not exceed 10 per year (in a conventional power plant is 1-2/year) above numbers (normally) far exceeded d by accelerators rs operating for nuclear and subnuclear physics new challenges in the overall design of the accelerator but a considerable potential for improving should be there 39

40 Reliability and availability/2 Strategy to improve reliability and availability Very advanced control systems based on fast electronics a certain amount of over-design careful design in order to allow fast interchange-ability of components that may fail availability of ready-to-operate replacement units careful component tests and severe acceptance criteria Linac Lnac» the modular structure allow an easy implementation» a spare-on-line strategy for some components is also conceivable» very low losses even for very intense beams Cyclotron» allows only partially the above strategy, but a different approach may be based on several machines in parallel» beam losses may be a critical item 40

41 Accelerator Driven Transmutation of Nuclear Waste ATW Consists of Three Major Functional Blocks Accelerator APT Technology Pyrochemical Processes Proliferation resistant, low environmental impact MS U Spent Fuel Rare Earths Spent Fuel Bi Noble Metals Pu,... (Ac) Bi Subcritical Burner (multiple units) Liquid Lead Nuclear Technology Power Production Residual Waste to Repository Power to Grid: ~ 90% Power to Accelerator: ~10% Figure 1 41

42 The subcritical core Accelerator Drive (Subcriticality) enables versatile and effective Nuclear Waste Destruction ATW Burner Typical Power: 2000 MWt Pump Liquid Lead Coolant Proton Beam Heat Exchanger: to steam-driven power production (up to 40% eff.) Beam Guide Window Spallation Neutron Source Transmutation Region (solid actinide fuel) why subcriticality has advantage for waste destruction: Power control is not linked to reactivity feedbacks, delayed neutrons or to control rods, but only to the accelerator drive ATW has no need for fertile materials. ATW uses pure transuranic cores Subcritical systems work independently of the fuel composition EOL inventory is not limited by criticality. Possible to have EOL burndown of inventory Neutronics and thermohydraulics are effectively decoupled Figure 4 42

43 TRASCO - ADS The Italian TRASCO program (INFN, ENEA and italian industries) aims to study the physics and to develop the technologies needed d to design an Accelerator Driven System (ADS) for nuclear waste transmutation. t ti The main objectives of the research program are: Conceptual design of a 1 GeV - 30 ma proton Materials technologies and development elopment of LINAC. components to be used in a plant in which lead or LBE Design and construction of main accelerator (Pb 44.5 %, Bi 55.5 %) acts as a primary target and/or components (proton source, RFQ, SC cavities). as the coolant (e.g. the interface accelerator / reactor). Development of methods and criteria for neutronics, thermal-hydraulics and plant design for an EA (Energy Amplifier)-like sub-critical system. 80 kev 5 MeV 100 MeV 1 GeV Proton injector Low energy and superconducting preaccelerator Linac (ISCL) High energy superconducting linac (3 sections) with elliptical cavities 43

44 XADS proton beam lay-out 44

45 XADS reactor (Ansaldo, INFN, ENEA) CORE MIDPLANE FREE LEVEL PB-BI DIAGRID LEVEL

46 The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. TRASCO - WINDOWLESS Outgassing rate [g/year] Source PbBi LBE Evaporation (T = 450 C) Po Still unknown Nuclear reaction Hg 150 Nuclear reaction H 2 6 Nuclear reaction H 2 O 48 Oxidation control He 4 Nuclear reaction 46

47 Interface Vacuum/Reactor LBE (Pb Bi Eutectic) ti properties in UHV Weight loss measurement at SAES Getters 1.E+0 Experimental Data Pb Bi PbBi Raoult approximation 1.E-1 Evaporation Rate (mg/cm 2 s) 1.E-2 1.E-3 1.E-4 1.E-5 1E-6 1.E T ( C) Evaporation measurement at LASA 47

48 High Power Hadron Linacs: other application Very high beam power: 100 kw 100 MW High efficiency, up to 50%, from plug to beam power Suitable for secondary bem production Through Spallation» Neutron microscope» Waste Transmutation Radioactive beams for Nuclear Physics Neutrino beams Future muon colliders 48

49 Options and special requirements Both linac and cyclotrons are considered d as possible ADS drivers No fundamental obstacles have been found so far for a linac to deliver ~100 ma at 1 GeV or more 1 GeV and 10 ma may be considered as limiting values for a (multistage) cyclotron an accelerator for ADS must have an extremely high reliability an availability target, core and structures should not be too frequently exposed to the thermal shocks induced by a beam interruption to allow hands on servicing i and keep downtime as low as possible, beam particles losses have to be minimised design and construction have to be done introducing additional safety margins into all accelerator components 49