S.M. Lidia, G. Bazouin, P.A. Seidl Accelerator and Fusion Research Division Lawrence Berkeley National Laboratory Berkeley, CA USA

Transcription

1 S.M. Lidia, G. Bazouin, P.A. Seidl Accelerator and Fusion Research Division Lawrence Berkeley National Laboratory Berkeley, CA USA The Heavy Ion Fusion Sciences Virtual National Laboratory 1

2 NDCX Increased fluence with neutralized drift compression NDCX-II 1.2MeV Li + 50nC 0.5ns 0.7mm spot Online 2012 NDCX-I 300kV K + 15nC ~2ns 2mm spot Online 2009 The Heavy Ion Fusion Sciences Virtual National Laboratory 2

3 NDCX Increased fluence with neutralized drift compression NDCX-II 1.2MeV Li + 50nC 0.5ns 0.7mm spot Ion source Online 2012 NDCX-I 300kV K + 15nC ~2ns 2mm spot Online 2009 The Heavy Ion Fusion Sciences Virtual National Laboratory 3

4 NDCX Increased fluence with neutralized drift compression Acceleration and pulse shaping NDCX-II 1.2MeV Li + 50nC 0.5ns 0.7mm spot Online 2012 NDCX-I 300kV K + 15nC ~2ns 2mm spot Online 2009 The Heavy Ion Fusion Sciences Virtual National Laboratory 4

5 NDCX Increased fluence with neutralized drift compression NDCX-II 1.2MeV Li + 50nC 0.5ns 0.7mm spot Online 2012 Neutralized drift compression Target chamber NDCX-I 300kV K + 15nC ~2ns 2mm spot Online 2009 The Heavy Ion Fusion Sciences Virtual National Laboratory 5

6 NDCX-I Experiment Longitudinal compression Energy modulation Ion beam source Beam diagnostics Final Focus solenoid δv/v ~ 20% Solenoid transport Induction Bunching Module (IBM) Plasmaneutralized drift Target chamber Peak current ~2.8A Uncompressed current ~30mA Compression Ratio >90 2.8A 30mA FWHM ~3ns The Heavy Ion Fusion Sciences Virtual National Laboratory 6

7 Aberrations at the target plane reduce beam fluence Beam fluences (mj/cm 2 ) averaged over 3.5ns gate window 50% radius 1.0mm 50% radius 2.4mm Total energy 0.03 mj Uncompressed Total energy 2.0 mj Compressed Chromatic aberrations: Circle of least confusion r c ~ 5mm Peak fluence ratio ~20X while the peak current ratio ~90X The Heavy Ion Fusion Sciences Virtual National Laboratory 7

8 Measuring the modulated beam phase space The NDCX-I neutralized drift beamline was split and beam diagnostics added. Deep Faraday cups measure total current waveform. Box 1 Box 2 The Heavy Ion Fusion Sciences Virtual National Laboratory 8

9 Measuring the modulated beam phase space The NDCX-I neutralized drift beamline was split and beam diagnostics added. Deep Faraday cups measure total current waveform. Box 1 Box 2 Faraday slit-cup (reversed bias) Horizontal and vertical slits and Faraday slit-cups provide timeresolved phase space density. The Heavy Ion Fusion Sciences Virtual National Laboratory 9

10 Controlling backstreaming electron flows Box 1 Box 2 The Heavy Ion Fusion Sciences Virtual National Laboratory 10

11 Controlling backstreaming electron flows Electrons from the plasma and secondary emission can counterpropagate in the beam potential, disrupting beam transport tunes. Box 1 Box 2 The Heavy Ion Fusion Sciences Virtual National Laboratory 11

12 Controlling backstreaming electron flows Electrons from the plasma and secondary emission can counterpropagate in the beam potential, disrupting beam transport tunes. Biased-ring electron traps Permanent-magnet dipole pair (+-, 900 G peak field, Zero on axis field integral) Box 1 Box 2 The Heavy Ion Fusion Sciences Virtual National Laboratory 12

13 WARP model predictions of gap focusing and beam transport Beam head Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 13

14 WARP model predictions of gap focusing and beam transport Beam modulation Beam head beginning Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 14

15 WARP model predictions of gap focusing and beam transport Beam De-acceleration modulation Beam head beginning phase Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 15

16 WARP model predictions of gap focusing and beam transport Beam De-acceleration modulation Beam head beginning phase Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 16

17 WARP model predictions of gap focusing and beam transport Beam De-acceleration Zero-crossing modulation Beam head beginning phase Maximum dv/dt Focusing variation from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 17

18 WARP model predictions of gap focusing and beam transport Beam De-acceleration Zero-crossing modulation Beam head beginning phase Maximum dv/dt Focusing variation from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 18

19 WARP model predictions of gap focusing and beam transport Beam De-acceleration Zero-crossing modulation Beam head beginning phase Maximum dv/dt Focusing variation from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 19

20 WARP model predictions of gap focusing and beam transport Beam De-acceleration Zero-crossing modulation Beam head beginning phase Maximum dv/dt Focusing variation from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 20

21 WARP model predictions of gap focusing and beam transport Beam Compressing De-acceleration Zero-crossing modulation pulse Beam head beginning phase Maximum dv/dt Focusing variation from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 21

22 WARP model predictions of gap focusing and beam transport Beam Compressing De-acceleration Zero-crossing modulation pulse Beam head beginning phase Maximum dv/dt Beam modulation produces time-varying Focusing envelope variations from waveform Unmodulated beam Box 1 Box 2 Plasma exit Plasma entrance IBM gap The Heavy Ion Fusion Sciences Virtual National Laboratory 22

23 Unmodulated beam profiles exhibit considerable structure Box 1 Experimentally measured beam profiles (500 ns gate) Box 2 The Heavy Ion Fusion Sciences Virtual National Laboratory 23 Quantitative agreement between theory and experiment in RMS spot size.

24 Measured phase space evolution 16 µsec animation. Modulated portion ~1.5 µsec Unmodulated b Modulated b n.b.: A slight difference in horizontal and vertical scales exists. The Heavy Ion Fusion Sciences Virtual National Laboratory 24

25 Measured phase space evolution 16 µsec animation. Modulated portion ~1.5 µsec Unmodulated b Modulated b n.b.: A slight difference in horizontal and vertical scales exists. The Heavy Ion Fusion Sciences Virtual National Laboratory 25

26 Box 2 phase space measurements reveal bifurcated phase space Horizontal Unmodulated Beam Vertical The Heavy Ion Fusion Sciences Virtual National Laboratory 26

27 Box 2 phase space measurements reveal bifurcated phase space Two populations present from beam head and along entire pulse. Horizontal Unmodulated Beam Vertical Bifurcation also appears in slit-scintillator phase space measurements. Complicates analysis of beam-gap dynamics and transport. The Heavy Ion Fusion Sciences Virtual National Laboratory 27

28 Time Resolved Variations in Beam Parameters Modulated Unmodulated The Heavy Ion Fusion Sciences Virtual National Laboratory 28

29 Time Resolved Variations in Beam Parameters Modulated Unmodulated Emittance variations indicate nonlinear evolution Complicated variation and lack of azimuthal symmetry The Heavy Ion Fusion Sciences Virtual National Laboratory 29

30 Modulated beam comparison with axisymmetric WARP model Differences between modulated and unmodulated beam parameters Modulated WARP-rz Modulated WARP replicates linear optics behavior No nonlinearities No emittance variation The Heavy Ion Fusion Sciences Virtual National Laboratory 30

31 Summary Improving target beam fluence in heavy ion beam neutralized drift compression geometries may require compensation of time-varying focusing elements. We have made a series of time-resolved measurements of the beam parameters and phase space density of an intense, velocity-modulated ion beam transported through a plasma-neutralized channel. Measurements indicate significant deviations from linear behavior in axisymmetric transport channels. Possible mechanisms for variation from linear behavior may include: 3D density perturbations in the space charge dominated beam, Coupling to weak magnetic dipole chicanes, Nonlinear beam-plasma interaction, Electron trap biasing resulting in backstreaming electron flows. Upcoming studies will examine these mechanisms. The Heavy Ion Fusion Sciences Virtual National Laboratory 31