Longitudinal Beam Physics Experiments at the University of Maryland - PowerPoint PPT Presentation

Longitudinal Beam Physics Experiments at the University of Maryland Electron Ring John Richardson Harris Institute for Research in Electronics and Applied Physics University of Maryland August 23, 2004

Outline • Motivation: “Intense” Beams • University of Maryland Electron Ring (UMER) • Longitudinal Effects • Evolution of Modulated Beams • Longitudinal Focusing • Future Work • Conclusions

Motivation • New Accelerators and Applications – High Quality, High Current Needed “High quality” = “low emittance” • Limit of: Low Emittance Coulomb Forces Dominate Low Energy “Space Charge Dominated” High Current “Intense”

Motivation • All Beams: SCD at birth (low γ ) γ → • SC-driven effects “frozen in” as big RF Accelerating RF Gun : Terahertz Sections: 5 MeV Diagnostics 75 MeV Time Diagnostics E-Beam Modulation UV Laser Input 4 3.5 0 Terahertz Output 0 3 0 2.5 0 2 0 1.5 0 1 0 0 0.5 0 0 -1.5 -1 -0.5 0 0.5 1 1.5 -11 x 10 (J. Neumann, U. Maryland; Experiment performed at Brookhaven Source Development Lab) Effects: Good or Bad

Motivation • Some Beams: Always SCD Ex: Heavy Ion Fusion 1000 1000000 1000000000 1E+12 1 keV 1 MeV 1 GeV 1 TeV e - electrons p protons HI heavy ions UMER

Longitudinal Expansion • Space Charge – Beam tends to expand b Beam a Beam Pipe • Transverse SC Force – Contain using transverse focusing (Quads) • Longitudinal SC Force – Beam will expand unless contained • Longitudinal E-Field (long wavelength): ∂ λ ⎛ ⎞ g b = − ≈ α + ⎜ ⎟ E sz 2 ln g ∂ πε γ ⎝ ⎠ 2 a 4 z 0 Local Line Charge Density λ [C/m]; Geometry Factor g ; 0 < α < 1

University of Maryland Electron Ring (UMER) 3.7 m

University of Maryland Electron Ring (UMER) Beam Energy: 10keV ( β = 0.2) Beam Current: 0.6 – 100 mA Pulse length: 30 ns – 150 ns (1/2 ring filled at 100ns) Bunch charge: ~5 nC Compact: 12m Circumference Complex: 36 Dipoles > 78 Quadrupoles > 36 Steering Dipoles 17 Diagnostics Ports ε ≈ µ 2 m n 1 Diagnostic End Station

UMER Diagnostics End Station: Every 64 cm: Faraday Cup Beam Position Monitors Pepperpot and Slit-Wire System Energy Spread Analyzer (Under development) Phosphor Screens 0.2 eV resolution At Injection and Extraction: Fast Current Monitors (Bergoz)

64 cm UMER Today

Longitudinal Effects and Experiments

Longitudinal Effects (1) • Beam Expansion/End Erosion λ � c 0 ( z ) � 2c 0 z • Generating Perturbations/Wave Propagation

Longitudinal Effects (2) • Modulation/Wave Interference • Combinations

Longitudinal Effects (3) Common Theme: These effects all evolve at the Sound Speed λ Zqg c = 0 πε γ 0 5 4 m 0 Z Charge State λ Line Charge Density [C/m] 0 ⎛ ⎞ b ≈ α + ⎜ ⎟ 2 ln g Geometry Factor ⎝ ⎠ a typ 6 c ~ 10 m For UMER s 0 One example…

Modulation in UMER Modulation observed when Bias Voltage ≈ 60 BV = 0 BV = 40 BV = 5 BV = 50 BV = 10 BV = 55 BV = 20 BV = 60 BV = 30 BV = 66 Simple Argument – density mod. should become energy mod., vice versa

Modulation in UMER Bergoz (62.6 cm) Modulation observed to disappear, BPM 0 (82.6 cm) return, then start to disappear again BPM 1 (194 cm) as beam travels through UMER BPM 2 (258 cm) BPM 3 (323 cm) BPM 4 (386 cm) BPM 5 (450 cm) BPM 6 (514 cm) BPM 7 (578 cm) BPM 8 (642 cm) BPM 9 (706 cm) BPM 10 (770 cm) BPM 11 (834 cm) BPM 12 (898 cm)

Modulation in UMER Two Questions: 1. Where does it come from? 2. Why does it disappear, then come back?

Source of Modulation • Gun acting like Triode • Increase BV – no longer space charge limited • Gun amplifies ripple, droop, etc., of pulser • Assume Triode/Diode behavior and pulser voltage shape: Triode 5 0 Anode (A) or Plate (P) Pulser Voltage (V) Grid (G) 20 + E B - E C K PV t ( ) 40 “Step” (short path 60 length Beam Pipe reflection?) Anode (A) + − 62.719 80 - 10kV 5 . 10 8 1 . 10 7 0 Grid (G) − − 10 9 t 150 10 9 Cathode (K) − ⋅ ⋅ 30 Droop (Common in pulse BV Time (ns) PV circuits) Ringing (Common in pulse UMER Gun circuits; frequency chirp assumed)

0.01 0.01 0.01 0.01 0 0 0 0 0.02 0.02 0.02 0.02 0.04 0.04 0.04 0.04 − − − − I out ( ) t I out t ( ) I out t ( ) I out ( ) t 0.06 0.06 0.06 0.06 0.08 0.08 0.08 BV ~ -10 V 0.08 0.1 0.1 0.1 0.1 − − 0.110 0.110 − 0.110 − 0.110 2 . 10 2 . 10 4 . 10 6 . 10 8 . 10 1 . 10 1.2 . 10 7 1.4 . 10 2 . 10 2 . 10 4 . 10 6 . 10 8 . 10 1 . 10 1.2 . 10 7 1.4 . 10 8 8 8 8 8 7 7 8 8 8 8 8 7 7 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 0 0 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 0 0 − 9 t − 9 − 9 t − 9 − − − ⋅ ⋅ − ⋅ ⋅ − ⋅ 9 t ⋅ 9 − − 30 10 150 10 30 10 150 10 30 10 150 10 9 t 9 − ⋅ ⋅ 30 10 150 10 BV ~ -70 V 0.01 0.01 0.01 0.01 0 0 0 0 0.02 0.02 0.02 0.02 0.04 0.04 0.04 0.04 − I out t ( ) − − I out t ( ) − I out t ( ) I out t ( ) 0.06 0.06 0.06 0.06 0.08 0.08 0.08 0.08 0.1 0.1 0.1 0.1 − − − − 0.110 0.110 0.110 0.110 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 2 . 10 8 2 . 10 8 4 . 10 8 6 . 10 8 8 . 10 8 1 . 10 7 1.2 . 10 7 1.4 . 10 7 0 0 0 0 − − − − − − − − − ⋅ 9 t ⋅ 9 − ⋅ 10 9 t 150 10 9 ⋅ − 10 9 ⋅ t 150 10 9 ⋅ − ⋅ 9 t ⋅ 9 30 10 150 10 30 30 30 10 150 10

Modulation Amplitude vs. Distance 1 1 Modulation Amplitude (arb) 0.8 〈 〉 0.6 data 1 1 2.5 ⋅ ⋅ 0.915 cos 0.44 x ( ( ) ) ~ cos( t ) 0.4 0.2 − 6.13 10 3 × 0 0 2 4 6 8 10 〈 〉 x 0 data 0 10 , Distance from Cathode (m) This would make sense for interfering cosine waves 2.5 2 − + cos t x ( ) 1 0 + − cos t x ( ) 1 2 − 2.5 10 5 0 5 10 − 10 x 10

Phase Velocity of Waves Calculate phase velocity from location of nulls in data: = ± × 6 v 1 . 80 10 m (85 mA settings) s p Compare with sound speed: λ qg c = 0 πε γ 0 5 4 m 0 = × 6 (85 mA settings) c 1 . 76 10 m s 0 2.3% Error Result: Modulation splits into forward, backward traveling space charge waves

Longitudinal Focusing

Longitudinal Focusing • Prevent beam expansion to enable extraction • Study compression for HIF • Allow direct manipulation of beam • Concept: c + β 2 c v(z) v(z) 0 v(z) z z z c − β 2 c 0 Initial Condition Beam Expanding Focusing Applied Beam Contracting Direction of Travel

Longitudinal Focusing Voltage Higher Voltage Needed v(z) Lower Voltage Needed E(z) Focusing Voltage – Triangular Pulses

Spiral Generator Disadvantages: Advantages: • “Swingback” Voltage • Triangular Pulse • Spark Gap switching usual • Simple Construction • Inexpensive • Voltage Gain Brau et al., RSI, Sept. 1977

Recombination Diode Ringing Suppression Spiral Generator Improvements Delay Line Patents Pending Inversion of One Channel Transformer Output SG MOSFET Switching

Longitudinal Focusing – Induction Modules D.X. Wang, UMD, 1993

Future Work • Closure • Refine work • Multiple Perturbations • Modulation (esp. simulation) • LF – HV tests, Beam tests

Conclusion All beams are sometimes Intense; Some beams are always Intense! • UMER – Intense Beams • Many interesting Longitudinal effects • Lots of work to be done