2.18: Control of Beam Loss and Improving Efficiency in High-Repetition-Rate High- Power Klystrons

Transcription

1 .18 Accelerator Phsics.18: Control of Beam Loss and Improving Efficienc in High-Repetition-Rate High- Power Klstrons (Progress Report) Accelerator Phsics Contact person Chiping Chen Institution(s) Massachusetts Institute of Technolog

2 Progress Report Research supported under a supplemental to DOE Grant No. DE-FG-5ER4919 Control of Beam Loss and Improving Efficienc in High-Repetition-Rate High- Power Klstrons Chiping Chen Plasma Science and Fusion Center Massachusetts Institute of Technolog Cambridge, Massachusetts 139 Project Overview A major thrust in the International Linear Collider (ILC) program is the development of high-power klstrons to power a TeV-class LC. The high construction and operating costs of the ILC rf power sstem drive the need for research and development on Alternative Configuration Design (ACD). For ILC, the choice of Base Configuration Design (BCD) is the L-Band Multi-Beam Klstron (MBK). The required specifications for the L-Band power source are: 1 MW power output, ms pulse length, 1 Hz repetition rate, 65% efficienc, and several ears of lifetime. Much progress has been made in Europe, US and Japan on L-Band MBK (Adolphsen, 6). At Thales, 4 tubes were produced, and gun arcing problem occurred and seemed to be corrected in last two tubes after fies applied. However, Thales tubes recentl developed other arcing problems above 8 MW. Thales is to build two more without changes and two with changes after problem is better diagnosed. At CPI, one tube was built and factor-tested to 1 MW at short pulse. During full pulse testing at DESY, it developed vacuum leak after 8.3 MW was achieved. It has been repaired and will be tested again. At Toshiba, one tube was built, and after a vacuum problem was fied, ran at full spec for one da has been shipped to DESY for further evaluation. Despite these efforts, the ILC communit and industr still needs to develop an L-Band klstron that meets the full specifications for ILC. A leading choice of Alternative Configuration Design (ACD) is a ribbon-beam (or sheet-beam) klstron (RBK) powered b an electron beam with a large-aspect-ratio elliptic cross section. The ribbon-beam klstron (RBK) has the following advantages over the conventional multiple clindrical-beam klstrons: a) Higher efficienc (75% vs. 65%), b) Single beam (1 vs. 6 or 7), c) Lower magnetic field (1.4 kg rms vs. 5 kg rms), d) Energ-free permanent magnet vs. energ-consuming pulsed magnet. These advantages would reduce the construction and operating costs of ILC and improve the reliabilit of the rf power sstem. RBKs could provide the following savings: a) Klstron hardware: 66% (or $6M) saving. b) RF sstem electricit: % (or $M/ear) saving. These attractive features motivated MIT to pursue the R&D on a RBK. In FY6, SLAC began to establish a sheet-beam klstron R&D program with internal funding. SLAC hopes to receive support from GDE in FY7 to accelerate its sheet-beam klstron R&D. 1

3 The goal of MIT R&D program is to continue our innovative research on ribbonbeam klstrons, building upon the eperience we gained in the past several ears in the theor, design, fabrication and testing of ribbon beams and ribbon-beam devices. Progress Report Our recent accomplishments in our Ribbon-Beam Klstron research were: (a) Developed an award-winning design for an elliptic electron gun in 4-5 (Bhatt and Chen, 5; Bhatt, Bemis, and Chen, 5 and 6); (b) First eperimental demonstration of a 6:1 elliptic electron gun in 5 (Chen, 6); (c) Developed a cold-fluid equilibrium theor of a periodicall twisting elliptic beam in 5 (Zhou, Bhatt and Chen 6); (d) Published a kinetic equilibrium theor of a periodicall twisting elliptic beam, which showed that thermal effects are negligibl small (Zhou and Chen, 6); (e) Developed a kinetic equilibrium theor of nearl non-twisting elliptic beams (Bhatt, 6); (f) Developed a cold-fluid equilibrium theor of nearl non-twisting elliptic beams (Zhou and Chen, 7); (g) Designed a ribbon-beam transport sstem for the development of ILC ribbonbeam klstron (Zhou and Chen, 7). A brief description is given below for each of the 6-7 accomplishments (d)-(g), while our accomplishments (a)-(c) prior to 6 were reported earlier. Accomplishment (d): A Vlasov equilibrium of the Kapchinskij-Vladimirskij form was obtained for a periodicall twisted ellipse-shaped charged-particle beam in a nonaismmetric periodic magnetic focusing field. The single-particle Hamiltonian dnamics was analzed self-consistentl. A constant of motion analogous to the Courant- Snder invariant was found. The equilibrium distribution function was constructed. The statistical properties of the beam equilibrium were studied. In the zero-temperature limit, the generalized envelope equations derived from the kinetic equilibrium theor recover the generalized envelope equations obtained in the cold-fluid equilibrium theor. Eamples of periodicall twisted elliptic beam equilibria were considered, and potential applications were discussed. For ribbon-beam klstron applications, the kinetic equilibrium theor predicted that the effect of beam temperature on the beam envelopes is negligibl small. Detailed results were published (Zhou and Chen, 6). Accomplishments (e) and (f): Both cold-fluid and kinetic equilibrium theories of nearl non-twisting elliptic beams were developed (Bhatt, 6; Zhou and Chen, 7). In paraial approimation, the focusing field is epressed as with q () s k k e + e + B [ e e ] dbz B = Bz () s e z q +. (1) ds k k B B = B. The paraial cold-fluid equations, which consist q q ( s,,) ( s,,) of the continuit equation, the Poisson equation, and the force balance equation, are solved with the densit and transverse velocit of the form

4 ~ b( s) a( s) ~ θ ( s) n Fig. 1 Coordinate sstems. ~ ~ N () b (, ) 1 ()() () (), s = Θ πa s b s a s b s ( ) = [ µ ( s) ~ α ( s) ~ ] β ceˆ + µ ( s) ~ + α ( s) b [ ~ ] β bc V, e, (3) In Eqs. () and (3), + is a transverse displacement in the twisted coordinate sstem illustrated in Fig. 1. () s Θ( ) = if <. The functions ( s) s b ~ ˆ ~ = ~ eˆ ~ ~ eˆ ~ θ is the twist angle of the ellipse. ( ) = 1 a, b ( s), µ ( s), µ ( s), α ( s), () s Θ if > and α and θ () s obe the generalized envelope equations, which will be presented elsewhere (Zhou and Chen, 7). Equations (1)-(3) together with the generalized envelope equations provide a theoretical framework for the design of elliptic electron beams in RBKs. Accomplishment (g): Using the cold-fluid equilibrium theor, we determined the parameters for the realization of an elliptic electron beam for an ILC ribbon beam klstron. Table 1 shows the progress we made in our consideration of beam design options for RBK. Figure shows the simulation results (Zhou and Chen, 7). Table 1. Sstem parameters for an elliptic beam design for ILC RBK Parameter Value Current (A) Voltage (kv) 1 S (cm). k k.158 B (kg). a/b a (cm) 1. θma (deg).75 3

5 .5.4 s/s=9. s/s=9.. d/ds (rad) (cm) (cm) d/ds (rad) (cm)..5 (cm) (cm) (cm) (cm) (cm) s/s=9.5 s/s=9.5. d/ds (rad).5 (cm) s/s= (cm) s/s=9.75 s/s=9.75. d/ds (rad).5 (cm) 1..4 s/s= (cm) s/s=1 s/s=1. d/ds (rad).5 (cm)..5 (cm) (cm) Fig. Plots of 5, particles (a sample of the particles in the PFBD simulation) in the (, ) plane and (, d ds ) plane for five snapshots within one period: s S = 9., 9.5, 9.5, 9.75 and 1. for the parameters listed in Table 1 (Zhou and Chen, 7). 4

6 Plans for 7 and Beond We plan to stud how to form elliptic electron beams, appl the knowledge we gain to forming the elliptic electron beam, and participate in the engineering design, fabrication, and testing of ILC RBK in collaboration with SLAC and industr. Deliverables include: a) Design of the formation of a relativistic elliptic beam from an elliptic diode (7); b) Design of the transformation of a relativistic round beam from a round diode into a relativistic elliptic beam (7); c) Design the focusing magnets (7-8); d) Design the rf sstem including beam tunnel and rf cavities (7-8); e) Participate in the fabrication and testing of ILC RBK (8-9). We plan to submit a supplemental request to Department of Energ to support these actives in FY7. Beond FY7, we plan to submit a research proposal directl to Global Design Effort (GDE) to participate in the engineering design, fabrication, and testing of ILC RBK. The estimate cost for FY7-FY9 is given in Table, which is modest for the proposed research efforts. Table. Approimate breakdown of estimate costs for 7-9 Item FY7 FY8 FY9 Total Chiping Chen (PI) $3, $3, $35, $97. Postdoctoral Research Associate $45, $47, $49, $141, Student $7, $7, $7, $1, Travel $3, $4, $4, $11, Total $85, $9, $95, $7, References Adolphsen, C., 6, ILC Main Linac and RF Sources, (SLAC DOE Review). Bhatt, R., and C. Chen, 5, Theor and Simulation of Non-Relativistic Elliptic Beam Formation with Child-Langmuir Flow Characteristics, Phs. Rev. ST-AB 8, 141. Bhatt, R., T. Bemis and C. Chen, 5, Three-Dimensional Theor and Simulation of Large-Aspect-Ratio Ellipse-Shaped Charged-Particle Beam Gun, Proc. 5 Part. Accel. Conf., p First Prize for Outstanding Technical Paper b Student. Bhatt, R., T. Bemis, and Chen, 6, Three-dimension theor and simulation of nonrelativistic elliptic electron and ion beam generation, (Invited Paper) IEEE Trans. Plasma Sci. 34, 187. Bhatt, R., 6, Inverse Problems in Elliptic Charged-Particle Beams, Ph.D. Thesis, MIT. Chen, C., 6, High Performance of Ribbon-Beam Amplifier for 3G and 4G Wireless Base Stations, Final Report for MIT Deshpande Center Grant. Zhou, J., R. Bhatt, and C. Chen, 6, Cold-Fluid Theor of Equilibrium and Stabilit of a High-Intensit Periodicall Twisted Ellipse-Shaped Charged-Particle Beam, Phs. Rev. ST-AB 9, Zhou, J., and C. Chen, 6, Kinetic Equilibrium of a Periodicall Twisted Ellipse- Shaped Charged-Particle Beam, Phs. Rev. ST-AB 9, 141. Zhou, J., and C. Chen, 7, Cold-Fluid Equilibrium of Elliptic Charged-Particle beams, manuscript in preparation. 5