Rapid Cycling Magnets - HTS-based Design Concepts Henryk Piekarz with J. Blowers, S. Hays and V. Shiltsev Fermilab MAP Winter Meeting, SLAC, 3-7 December, 2014

1.Conductor placement within magnetic core and core options for simultaneous 2-beam acceleration 2. Current waveforms for magnet excitation 3. Technology limitations of current sources 4.Single versus multiple-turn power cable 5. Why HTS power cable for rapid-cycling magnet 6. Progress on rapid-cycling HTS magnets at FNAL Outline Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)2

Placement of Drive & Return Conductors Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)3 For rapid-cycling operations minimization of conductor exposure to magnetic field is a fundamentally important requirement. As shown in figures, conductor must be narrow and properly placed within somewhat enlarged cable cavity to minimize the field crossing its space. For C-shape core the return conductor is still exposed to magnetic field, even if it is placed at a far (impractical)distance. Magnetic core design: - C-shape (1/2 shown) - Window-frame (1/4 shown)

Magnetic Core Options for Simultaneous 2-Beam Acceleration Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)4 Option C is the most desirable one, but the flight direction for one of the electron or muon species must be reversed before entering collider ring. Single window fram e Dual C-shape horizontal & vertica l NOTE: Advantage of conductors within core: Sharing magnet & RF power systems in simultaneous 2 beam acceleration

Current Wave-Forms for Magnet Excitation (I) The dB/dt rate must be constant over the acceleration period to allow multiple beam passes match RF system cycle. A standard sine-wave current induces limited linear B - field response (< + / T) through acceleration period. B-field is always ramping faster than the current with the exception of the field saturation regions. Example of hypothetical current form for a perfectly linear B-field response in acceleration period of -2 T to +2 T. Together with the slow rising & falling current before and after the acceleration the power loss would be ~ factor 4 reduced relative to a standard current sine-wave. 5 Due to core magnetic saturation current and magnetic wave-forms do not replicate each other. Below is example of response of core constructed of Surahammar Steel: Non-oriented Fe3%Si, 0.005” (104 µm) thick laminations. Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) Dec 5, 2014

Sine-wave current form can be generated using 2 modes of power supply operation: (1) White Circuit (continual) and (2) Ringing Circuit (periodical). Current Wave-Forms for Magnet Excitation (II) Beam bunch is accelerated using the first full leading edge of the current sine-wave. In both White and Ringing Circuit modes the power loss is not limited to acceleration period. The Ringing Circuit mode, however, operates at the same frequency as beam, e.g. 15 Hz, and rapidly decaying current and B-field within the acceleration cycle considerably reduce power losses. Contrary to the Ringing Circuit the White Circuit mode is well developed and it has been applied in many fast-cycling accelerators (e.g. Booster at Fermilab). 6 Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)

Technology Limitation of Rapid-Cycling Magnet Current Supply 7 The IGBT semiconductor technology is typically used to construct fast ramping & fast switching high-current power supplies. A commercial IGBT (e.g. POWEREX CM600HX- 24A) allows for a pulse current of 1200 A in a transient time of 10 µs. Muon acceleration magnet of 25 mm gap requires A current in a 500 µs transient time, or 1440 A/10 µs ramp rate. The IGBT unit should sustain higher current than 1200 A for the transient time 50 times longer (0.5 ms). However, without further improvements there will be 60 IGBT units per power supply of magnet string. Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) Significant R&D is required to develop power supply for a large scale rapid –cycling accelerator magnet application. One very important limitation is lack of large capacitor banks operating at cryogenic temperature. The industry is pursuing such an R&D effort but an involvement of HEP community would probably be very beneficial.

Single & Multiple-turn SC Cable for Rapid-Cycling Magnet String Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)8  Single-turn SC cable constitutes the best option for the cable and magnet constructions, but a high-current source is required. - This is proper application for DC-type magnets, e.g. in RLA’s it will allow use a single power source for all rings. - For rapid-cycling magnets a significant high-current source R&D is needed.  Multiple-turn conductor allows use of lower current source, but it complicates cable design, and increased magnetic inductance shortens magnet string to match available current source power.  Multiple parallel single-turn conductors require multiple but lower current sources, and lower inductance allows construction of longer magnet strings. This may be the most acceptable option.

Why HTS Conductor for Rapid-Cycling Magnet? 9 Attributes of rapid cycling accelerator magnet powered with HTS cable:  High current density leads to small cross-section of power cable allowing packing multiple sub-cables into a small space.  Small cable space leads to a smaller core size reducing in this way power losses in the core.  Dynamic power losses of HTS cable can be much reduced with a combined optimization of core cable cavity and cable designs [1,2,3].  Allowable wide temperature margin (e.g. 20 K) secures stability.  Resilience of HTS strands to radiation secures magnet longevity. Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) [1] H. Piekarz, S. Hays, J. Blowers, V. Shiltsev, “Design Study and Test Arrangement of HTS Transmission Line Power Cable for Fast-Cycling Accelerator Magnets”, IEEE, Vol. 20, No 3,(2010) [2] H. Piekarz, S. Hays, J. Blowers and V. Shiltsev, “A Measurement of HTS Cable Power Loss in a Sweeping Magnetic Field”, IEEE, Vol. 22, No 3, (2012) [3] H. Piekarz, J. Blowers, S. Hays and V. Shiltsev, “Design, Construction and Test Arrangement of a Fast Cycling HTS Accelerator Magnet”, IEEE, Vol. 24, No 3 (2014)

Example of Power Loss Expectations with HTS-Based Rapid-Cycling Magnet Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)10 Magnet parameter Magnet length [m] 7.5 Beam gap (v. x h.) [mm x mm] 25 x 100 Ceramic beam pipe 4 mm wall 20 x 60 Maximum current [kA-turns] 72 Number of sub-cables 6 Current per sub-cable [kA] 12 HTS strandSup-Power HTS strands per sub-cable 16 HTS mass (with Ni5%W) [kg] 10.7 Cryogenic pipe mass [kg] 15.1 dB/dt in cable space [T/s] 20 Cable power loss at DF [W] 4.4 Core size (v. x h.) [mm x mm] 180 x 350 Lamination: Fe3%Si, NO [mm] 0.1 Core mass [kg] 3095 Core power DF (exp.) [W] 740 Magnetic inductance [µH] 390 Ramping DF (exp.) [MVA] 0.65 Synchrotron Parameter Circumference (Tevatron tunnel) [m] 6283 Injection energy [GeV] 375 Extraction energy [GeV] 750 Muon bunch polarity + & - Acceleration per orbit *) [GeV] 7.2 Number of orbits 52 Orbit circulation time [µs] 21 Nominal acceleration period [ms] 1.09 Beam gap : vert. x hor. [mm x mm] 25 x 100 B injection / B extraction [T / T] +1.8 / -1.8 B-field wave-form sin-wave Nominal dB/dt (beam gap) [T/s] 2000 Nominal dI/dt [A/µs] 3000 Bunch frequency [Hz] 15 Magnet power loss projections based on tests of HTS cable exposed to cycling magnetic field [2] Expected cryogenic power loss of ~2 kW /accelerator ring is in acceptable range. For comparison; Tevatron => 24 kW and LHC => 300 kW at 4.2 K.

Current HTS Test Magnet at FNAL Window-Frame, 20 kA, 0.5T, 40 mm gap Expected: 30 T/s in White Circuit Mode 11 HTS sub-cable with 20 strands. Splicing arrangement of HTS strands to power lead. HTS strands connected to power lead. View of HTS cable with lead and cryogenic support. Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)

HTS Test Magnet Construction (I) 12 Inserting HTS cable into magnet core HTS cable installed in magnet core 6 HTS coils stacked into a cable HTS cable spliced to lead-connecting copper blocks Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014)

Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) 13 Expected assembly finished by March, followed by tests through summer HTS Test Magnet Construction (II)

Multiple-Turn Rapid-Cycling HTS Magnet (Concept I) FNAL LDRD 3-year program: L Vertical dual C-magnet, e + e -, µ + µ - & p-p acceleration Featured 3-turn conductor, each with2 sub-cables Triple cryostat pipe gives room for radiation absorber Connection to power leads and cable return are placed at the ends of magnet string (e.g m apart ), so the difficult cable return ends are not too frequent. Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) 14 Most of e/µ decay radiation exits into outer space, and absorbers protect cables from the inward radiation. e + e -, µ + µ -, p-p

Multiple-Turn Rapid-Cycling HTS Magnet (Concept II) FNAL LDRD 3-year program: L Horizontal dual C-magnet, e.g. 16 GeV Dual Proton Booster. 3-turn conductor, each with 2 sub-cables. Swapping beam orbits is required for use of common RF system. 2 beam simultaneous acceleration allows doubling intensity of proton source. Front viewRear view Connection to power leads and cable return are placed at beginning and end of a magnet string (e.g.100 m). The difficult cable return ends are not too frequent. Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) 15

Summary Dec 5, 2014 Henryk Piekarz | MAP Winter Meeting (SLAC, Dec 3-7, 2014) 16  HTS- based rapid-cycling accelerator magnets are suitable for 2-beam simultaneous acceleration.  Vertical dual C-shape core secures minimal cable exposure to µ + µ - decays and to e + e - synchrotron radiation in the 2-beam acceleration mode.  Up-to-date designs and tests indicate that cryogenic power losses of HTS power cable in rapid-cycling operation should be in an acceptable range.  HTS-based rapid-cycling magnet R&D continues through LDRD program at FNAL with focusing on multiple-turn power conductor to facilitate use of lower current sources (at expense of more complicated cable structure).