1 Development and testing of a pulsed helium ion source for probing materials and warm dense matter studies Qing Ji a, Peter Seidl a, Will Waldron a, Jeff Takakuwa a, Alex Friedman b, David Grote b, Arun Persaud a, John Barnard b, and Thomas Schenkel a a Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA b Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 16 th International Conference on Ion Sources, New York City, New York, USA August 23-28, 2015

2 Outline Overview of Neutralized Drift Compression eXperiment (NDCX-II) o Creating a high energy and high intensity ion pulse Source of ions  acceleration, compression and focusing  target sample o Probing materials and warm dense matter studies Switching from a lithium to helium ion source Testing result of the pulsed helium ion source o Short pulse beam o Beam current o Uniformity o Commissioning the helium ion source on NDCX-II Summary

3 NDCX-II is a pulsed induction linac that produces intense, pulsed ion beams N eutralized D rift C ompression e X periment Ion Pulse ParameterNDCX-II goals Pulse length (FWHM)0.6 to 1 ns Kinetic energy1.2 MeV Ions per pulse3x10 11, I peak ~ 60A Ion beam spot size (FWHM)~ mm 2 Beam energy fluence5-10 J/cm 2 Repetition rate~2 shots/minute Ion speciesH, D, He, Li, K

4 Neutralized drift compression to 1 ns pulse lengths and 1 mm beam spots using ATA induction cells Li +, K +, He + injector Modified ATA induction cells with pulsed 2.5 T solenoids HV transmission lines Final focus solenoid and target chamber Ion source and injector, 500ns Target Linac custom waveforms for rapid beam compression Neutralized drift compression and final focus P.A. Seidl et al, NIM A (in press)

5 Diagnostics include fiber coupled to streak spectrometer (~10 ps), II-CCD. Considering laser or x-ray probes. Diagnostics: Streaked optical spectrometry Ion scattering VISAR-interferometry Future: Auxiliary sup-ps probes (e. g. laser based XUV, …)

6 NDCX-II provides opportunities to probe materials response to ion bombardment (t, E) T. Schenkel et al., Towards pump–probe experiments of defect dynamics with short ion beam pulses, NIM B 315 (2013) 350 Beam Target Faraday Cup α Time [µs] 1 µm target E 0 =287 keV F-cup + target F-cup only x 0.1 Li 135 keV Si 250 nm Current [mA] Aperture Rotation, α [degree] Loss due to damage build-up time current

7 NDCX-II provides uniquely intense, short ion pulses for materials and warm-dense matter research Lower intensities: defect dynamics in materials  fusion materials isolated cascades overlapping cascades amorphization and melting warm (~1 eV), dense matter ~ nC, 1.2 MeV, ~1 mm 2, ~1 ns 1-30 nC, MeV, few mm 2, ~1-20 ns Ions deposit energy via collisions with target electrons and nuclei Ion driven heating is uniform for ion energies near the Bragg peak in stopping power Higher intensities: extreme chemistry and warm dense matter Fusion relevance Ion heating of matter (WDM) Materials studies for fusion reactors Intense beams and beam-plasma physics

8 Large area (10.9 cm) thermionic alumino-silicate Li + hotplate ion source produces <1 mA/cm 2 Li + P.A. Seidl et al, NIM A (in press), FHWM 1.4mm 0.2 – 0.5 mA/cm 1250  C >3 kW dissipated in filament

9 A helium ion source can deliver more ions/pulse and is a better match to top energy of 1.2 MeV in NDCX-II 1.2 MeV He + is close to the Bragg peak for stopping in matter  more uniform heating. More ions/pulse.  greater intensity on target and higher target temperatures. o Estimated target temperature in Au ≥ 0.5 eV (69 nC in bunched, focused beam) in our present setup. Lower cost of ownership. Other noble gas ions are possible to use (Ne, Ar) Goal: 160 mA He + ions, for 80 nC per pulse on target.  n < 2  -mm-mrad. Maintain low vacuum pressure in the injector column and accelerator. Designed for occasional swapping between helium plasma source and lithium hot-plate source

10 Adapting the helium ion source on the NDCX-II injector V source = kV V Pierce = 135 kV V extractor = 135/1.1 = 123 kV -40 kV < V accel < 0 kV V exit = 0 V (ground) V extractor V accel V exit V Pierce V source Rogowski coil current monitor First focusing solenoid

11 Vinj=150 kV emittance  -mm-mrad Good beam clearance observed WARP3D simulation of He injection WARP 3D simulations shows good beam optics with adjustment of voltages on the injector

12 A filament discharge multi-cusp plasma ion source can deliver over 80 mA/cm 2 He + Multi-cusp plasma ion source Pulsed filament discharge Multi-aperture extraction o 1-mm-diameter aperture o Emission area ~ 7cm in diameter o over 50% transparency o ~ 1900 beamlets. Current density of the 7.5-cm- dia helium ion source 80 mA/cm 1850W

13 Pulsed He + ion beam profile measurement -V pad I pad -V plt I plt Current monitor -1500V -2000V -2200V floating 6-mm aperture Sampling ~ 1% of total beam V_ext

14 Scope traces of pulsed He + ion beam show peak current over 200mA within several microsecond pulse width I pad V_ext I plt electrons He+ ~ 264 mA (including secondary electrons) Filament 5.8 sec Extraction 5.65 sec 4  sec 50 msec Arc 5.6 sec 100 msec

15 Within +/- 5% of variation, helium ion beam is uniform over an area of ~ 6 cm in diameter ~ 6 cm

16 Ion source repetition rate is currently limited by the heat load on the plasma facing grid 0.1-mm thick Invar plate (no water cooling)

17 Pulsed He + Source has been commissioned on NDCX-II and delivered more charges than the Li + source

18 Summary NDCX-II provides uniquely, intense short ion pulses for beam physics, materials and warm dense matter research. A helium plasma ion source was developed to replace prior-used lithium ion source. Pulsed He + beam current as high as 200 mA at the peak, and 4  s long was measured for a 7-cm-diameter emission area. Within +/- 5% of variation, the uniform beam area is approximately 6 cm in diameter. This work was supported by the Office of Fusion Energy Science, and performed under the auspices of the U. S. Department of Energy by Lawrence Berkeley National Laboratory under contract No. DE-AC02-05CH11231.