LowEmittance Beam Acceleration in Induction Linac Vyacheslav Yakovlev

Low-Emittance Beam Acceleration in Induction Linac Vyacheslav Yakovlev, Fermi National Accelerator Laboratory EIC Hadron Cooling Workshop October 7 -8, 2019

Induction Linac for Electron Cooling On behalf of the LOI team: • S. Nagaitsev, • V. Lebedev, • Yu. Tereshkin, • I. Gonin, • A. Saini, • N. Solyak. 2 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Outline: 1. Introduction; 2. Concept of the Induction linac and parameters; 3. Emittance preservation – issues and possible mitigation; 4. High-brightness electron gun; 5. The beam matching to the linac; 6. The linac cell concept; 7. Summary 3 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling • Induction Linac Parameters: - Energy - Current 54. 5 Me. V 100 A - Pulse duration 380 ns - Repetition rate 200 s-1 - Beam transverse emittance at the injection: ≲ 6 μm • The beam transverse emittance at injection into the ring should be determined by the beam emittance at the cathode. • The corresponding rms normalized emittance is: where me is the electron mass, c is the light speed, rc is the cathode radius, and Tc its temperature. 4 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling 100 m Block chart of the accelerating system. Strict requirement for the emittance of the electron beam constitutes the most challenging part of the injector and the transport line design. 5 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Beam emittance preservation: • The gun: - Thermal emittance; - The cathode roughness; - Aberrations (cathode, anode, cathode edge) ; - Misalignments; - Pulse flatness; -. . . • The gun matching to the acceleration system: - Aberrations; - Space charge transverse force non-linearities; - Longitudinal space charge effects; - Misalignments; -. . . • The linac (emittance preservation): - Aberrations; - Misalignments; - Space charge effects; -. . . 6 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling The Gun. q The gun concept: • Low – aberration electron gun; Anode • Magnetic yoke for matching with the solenoid. q Example: Electron gun for 34 GHz magnicon* • Beam voltage: 500 k. V; • Beam current: 200 A; • Beam transverse area compression: 3000: 1 (low emittance is essential) • Built, tested and operated. Solenoid HV insulator * V. P. Yakovlev, O. N. Nezhevenko, et al, PAC 2001 7 Magnet yoke 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling Cathode Focusing electrode

Induction Linac for Electron Cooling • Cathode type: - The Gun. Lower temperature → lower thermal emittance. Low temperature dispenser M-type cathode (Tc =1050°C) (Ir. Ce, Ir. La: Tc >1400° C ; La. B 6: Tc >1500° C)* • Cathode radius: - Thermal emittance ~ rc; Cathode longevity decreases when the current density increases. ~6 A/cm 2 allows ≲ 50, 000 hours. Semi-empirical curve showing M-type Compromise between thermal emittance and the cathode longevity. rc =23 -25 mm looks OK to get εn<6 μm *G. Kuznetsov, NIM A 340 (1994); J. L. Cronin, IEE Proc, v. 128, N 1 (1981). 8 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling dispenser cathode life prediction versus cathode loading, space charge limitation. (E. Wright, A. Balkcum, H. Bohlen, et al. , “Development of 10 -MW, L-Band Multiple-Beam Klystron For TESLA, ” Proceeding of 2003 Particle Accelerator Conference, Portland, May 11 -16, 2003, pp. 1144 -1146. )

Induction Linac for Electron Cooling Cathode aberrations 9 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling The Gun. • Current density distribution must be uniform! Aberration compensation*: r’(r) + = Cathode Anode • Thermal gap aberrations: j(r) Gun output Thermal gap between the cathode and the focusing electrode • • Mitigation: Mo ring attached to the cathode having a Pierce angle with respect to the cathode. It provides the “perfect” optics. Strict control of “hot” dimensions, precise perveance and the beam envelope measurements. * V. P. Yakovlev, O. N. Nezhevenko, et al, PAC 2001 10 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling The cathode edge

Induction Linac for Electron Cooling The Gun. • Misalignment analysis (longitudinal, transverse, tilt); • HV pulse flatness; • Tangential component of magnetic field on the cathode; Still should be done in details. Filament magnetic field is not an issue. q The gun matching to the focusing system*: Beam trajectories should coincide to the magnetic force lines in the gun and in the magnetic system; The beam radius in the magnetic system should be close to the Brillouin limit rb. The beam envelope scalloping in the magnetic system should be minimized; - The cathode magnetic field Bc is to be 12 Gs (cooling requirement); 11 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling Magnet yoke *Y. V. Baryshev, et al, NIM A 340 (1994)

Induction Linac for Electron Cooling The gun matching to the focusing system analysis: • Space charge transverse force non-linearities; • Longitudinal space charge effects; • Misalignments. Still should be considered in details. 12 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling • Three low-emittance high power guns 1, 2, 3 based on this concept have been designed, built, tested and operated successfully: Application Beam current, A Beam voltage, k. V Beam area compression Year 7 GHz magnicon 236 436 2500: 1 1992 11 GHz magnicon 200 500 1500: 1 1997 34 GHz magnicon 200 500 3000: 1 2001 • Careful beam envelope measurements 1, 4 give assurance that the beam emittance is close to thermal one. 1. 2. 3. 4. 13 Y. Baryshev, et al, NIM A 340, (1994) V. Yakovlev, O. N. Nezhevenko, et al, PAC 97 (1997) V. Yakovlev, O. N. Nezhevenko, et al, PAC 2001 (2001) O. Nezhevenko, et al, IEEE Trans. Plasma Sci, v. 30 (2002) 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Electron gun : a. Generate the beam with uniform transverse current distribution b. Use micro-perveance ~1. 2 micro-perv*. • The gun voltage is 300 k. V, which at the modest compression ratio will provide reasonable surface electric fields, which is necessary at high repetition rate. • To obtain reasonable cathode lifetime (up to 50, 000 hrs), the dispenser cathode diameter is chosen to be 50 mm, limiting loading to 12 A/cm 2 • Modest transverse beam are compression ~10 -20: 1. • This gun is to be optimized for low emittance and contains a special near-cathode molybdenum electrode (ring following the cathode edge), having a Pierce angle with respect to the cathode surface. It prevents emittance dilution caused by the gap between the electrode and the cathode (required for thermal insulation). • The beam must have uniform current distribution at the gun output to avoid aberrations in the transport line. Scheme of low-emittance diode gun with enlarged electric strength*. *V. P. Yakovlev, O. A. Nezhevenko, R. True, "Electron gun for a high-power X-band magnicon amplifier", PAC 97, Vancouver, 1997, p. 3186. 14 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling The electron gun for the LIA – an initial 200 A version: Cathode max. surface field ~ 19. 6 MV/m H = 35 mm Anode max. surface field ~ 39. 4 MV/m Emitter max. surface field ~ 3. 1 MV/m Surface fields on the focusing electrode and on the anode nose look OK 15 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling • Current density at the output close to homogeneous; • Aberrations are still not compensated completely, εeff ~20μ 16 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling • Matching system: - Solenoid; Magnet yoke; Trimming coil. • Filed distribution in the matching system: - Bsol =650 Gs → rb = 7. 5 mm • Beam envelope: scalloping amplitude 9% • First considerations give assurance that it is feasible to bulid the gun having parameters acceptable for Electron cooling. 17 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Next steps: • Consider 100 A gun according to the latest requirement; • Optimize the beam radius and the gun voltage to minimize emittance dilution in the magnetic system; • Determine the gun geometry for the optimal voltage; • Optimize the beam matching for the optimal voltage; • Tolerance analysis. 18 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling q The linac lattice: • Schematic of the induction accelerator showing accelerating cells. • Each cell contains two cores, two focusing solenoids and an acceleration gap. 19 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling • The linac contains 727 accelerating cells with lengths of 141 mm each. Each cell provides the energy gain of 75 ke. V. • The total length of the linac is ~100 m (without injector). • The power dissipation in the cores of each cell is ~6 k. W. The total power dissipation in the LIA is ~6. 3 MW. • Concept scheme of a pulser to feed a cell of the LIA is suggested that meets the requirements of the voltage and pulse length. • Work on minimization of the emittance dilution in the linac. 20 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Next steps: • Optimize the beam dynamics in the linac (Trace. Win); • Tolerance analysis. 21 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling

Induction Linac for Electron Cooling Summary: • Strict requirement for the emittance of the electron beam constitutes the most challenging part of the injector and the transport line design. • Previous extensive experience with low-emittance 100 MW magnicon guns including careful beam envelope measurements gives assurance that the beam emittance is close to thermal one. • First gun optics considerations give hope in the linac concept. • Further work is in process to complete the linac concept design. 22 10/8/2019 V. Yakovlev , Induction Linac for Electron Cooling