Experimental Evaluation of a Negative Ion Source for a Heavy Ion Fusion Driver

1. Experimental Evaluation of a Negative Ion Source for a Heavy Ion Fusion Driver L. R. Grisham (PPPL) S.K. Hahto, S. T. Hahto, J.W. Kwan, K. N. Leung (LBNL) HEAVY ION FUSION 2004 Conference Princeton, New Jersey June 7 - 11, 2004

2. Features of Negative Ion Drivers • Negative ion beams will not draw electrons from surfaces, so focusing properties will not be contaminated (no electron clouds). • Negative halogen beams extracted from ion source should not have the low energy exchange tails of positive ions. • If eventually want atomic beams, negative ions can be efficiently converted to atomic beams with photodetachment neutralizers. • For sufficiently low target chamber pressures, atomic driver beams would reduce average beam self perveance and spot size.

3. Critical Technical Issues • Choice of beam element (mention in this talk) • Ion source (subject of this talk) • Photodetachment neutralizer (HIF 2002) • Vacuum requirements in accelerator and beam transport system (HIF 2002) • Ionization of beam particles in target chamber (HIF 2002)

4. Choice of Beam Element • Electron affinity determines ease of producing negative ions. • The halogens F, Cl, I, and Br all havehigh electron affinities over 3 eV. • Bromine (mass 81, affinity 3.63 eV) and iodine (mass 127, affinity 3.06 eV) are probably the best mass choices for a heavy ion driver, but require heated sources to produce vapor. • Fluorine (mass 19, affinity 3.45 eV) and chlorine (mass 37, affinity 3.61 eV) are the easiest choices for a proof-of-principle source because they are diatomic gases at room temperature. • In the semiconductor industry, Cl- is the dominant negative ion in RF discharges at 10 -- 20 mtorr.

5. Ion Sources • RF- driven plasma sources should be able to produce high current densities of F, Cl, I, or Br because they all have high electron affinities. • Form negative ions by dissociative attachment of electron to vibrationally excited dimers, similar to process in volume D- sources. • With adequate negative ion density in plasma, extractable negative ion current density will be determined by the strength of the extraction electric field, which is a function of the extractor design, not the ion polarity. • Thus, the extractable current densities of negative ions of F, Cl, I, or Br should be similar to the extractable current densities of positive ions of similar masses. • Unlike hydrogen negative ion sources, halogen negative ion sources should not require the addition of cesium.

6. Co-extracted Electrons from Ion Source • In D- sources with no electron suppression, electrons extracted from the source plasma will outnumber the negative ions in the beam by the ratio of their mobilities (roughly the square root of the ratio of their masses) multiplied by their relative abundance in the near-extractor plasma and the rato of the electron/ion temperatures. • In D- ion sources, electron suppression techniques reduce co-extracted electron component by factors of order a 100 or more. • Electron suppression techniques include: • Uniform magnetic filter field in extraction plane. • Biasing extractor by a few volts relative to plasma. • Small permanent magnets in extractor to dump remaining electrons after extraction stage before further acceleration. • Addition of massive positive ions to plasma to impede electrons. • Massive negative ions are only slightly affected by the magnetic fields that deflect the electrons (relative magnetic rigidity is proportional to the square root of the ratio of their masses).

7. Experimental Setup for Negative Chlorine Experiments

8. Positive Cl Spectrum is 82% Atomic; Contains Molecular Ions and Contaminants, and Charge Exchange Tails

9. Negative Chlorine Spectrum is 99.5% Atomic, with Negligible Tails

10. System Tested With Oxygen, which has Electron Affinity of 1.46 eV

11. Negative Oxygen Current is Linear with RF Power

12. Chlorine, with Electron Affinity of 3.61 eV, Yields More Negative Ions and Fewer Electrons Than Oxygen with Affinity of 1.46 eV

13. Negative Chlorine Current Density Linear with RF

14. Smaller Effect of Bias on Electrons in Chlorine Suggests They are a Smaller Portion of the Negative Population than with Oxygen

15. Electrons Are Much More Sensitive to Pressure and Filter Position than the Negative Chlorine

16. Low Electron Current Probably due to Two Mechanisms • Near-equivalence of negative and positive ion densities leaves little room in charge space for electrons near extraction plane. • This effect should persist when use heavier halogens. • The positive component of the plasma consists of massive Cl+, which retards electron flow to extractor, as observed in H- sources containing Cs+ • This effect may become more beneficial when use heavier halogens (iodine is nearly as massive as cesium) • MFE negative ion sources need magnets in extractor grids to dump electrons because they operate in long pulses, and electrons would otherwise reach full energy; not the case for HIF, might be able to avoid extractor magets (which lower grid transparency)

17. Pepper Pot Used for Rough Emittance Comparison of Positive and Negative Chlorine • Measured apparent “temperatures” in directions parallel and perpendicular to direction of the electron analysis magnetic field. • For positive chlorine ions, parallel “T” was 0.2 eV and perpendicular “T” was 0.5 eV. • For negative chlorine ions, parallel “T” was 0.3 eV and perpendicular “T” was 0.5 eV • These values are all smaller than is common for positive ions extracted from ion-electron plasmas (2.4 eV for Ar+ from quite similar source). • Ion-ion plasmas should have lower ambipolar potentials than ion-electron plasma, as well as shorter gradient lengths, so it may be that both positive and negative ions have lower Teff than positive ions from ion-electron plasmas.

18. Achieved Current Densities • RF source produced 45 mA/cm2 of Cl- or 53 mA/cm2 of total positive ions with 2.2 kW of RF power (-/+ ratio of 0.79) • Spectrum was 99.5% negative atomic chlorine; 0.5% negative chlorine molecules; negative spectrum much purer than positive spectrum • At optimized conditions, co-extracted e- current was only 7 times greater than Cl- current, much less than to be expected from the difference in mobilities (240). Ratio was a factor of 30 - 40 more favorable with chlorine than with less-electronegative oxygen. • Negative atomic chlorine current density not very sensitive to pressure; scaled linearly with RF power; should scale to around 100 mA/cm2 at 5 kW (power supply limit was 2.2 kW)

19. Halogen Ion Source Characteristics Appear Acceptable for HIF • Current densities adequate for HIF, and similar to positive ions (so extractable currents will be limited by extractor design for either negative or positive ions • Negligible negative molecular ions or contaminants • Negligible charge exchange tails to increase longitudinal emittance • Ion-ion plasma results in few co-extracted electrons • Perhaps because of low ambipolar potentials and shorter gradient lengths in ion-ion plasmas, Teff of both negative and positive beams appears low, so halogen sources might allow smaller spot size for both positive and negative ions than would positive ions from ion-electron plasmas.

20. Future Directions for Exploration • Tests with Cl on Joe Kwan’s 100 kV Livermore test stand planned • Scale to higher RF source power and multiple beamlets • Measure Cl+, Cl-, e- currents and emittance • Tests with heated source using bromine or iodine (in the future, if the HIF community decides to pursue this approach) • Test of small proof-of-concept photodetachment neutralizer (in the future, if the HIF community is interested in the option of neutralizing the beam)

21. Best e-/Cl- Ratio Was Obtained at 40 mTorr with a Different Antenna (which Broke)