Experimental evaluation of a negative ion source for a heavy ion fusion negative ion driver - eScholarship

Experimental Evaluation of a Negative Ion Source for a Heavy Ion Fusion Negative Ion Driver L. R. Grisham 1 , S. K. Hahto 2 , S. T. Hahto 2 , J. W. Kwan 2 , K. N. Leung 2 Princeton Plasma Physics Laboratory, P. O. Box 451, Princeton, N.J. 08543 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720 Abstract Negative halogen ions have recently been proposed as a possible alternative to positive ions for heavy ion fusion drivers because electron accumulation would not be a problem in the accelerator, and if desired, the beams could be photodetached to neutrals [1,2,3]. To test the ability to make suitable quality beams, an experiment was conducted at Lawrence Berkeley National Laboratory using chlorine in an RF-driven ion source. Without introducing any cesium (which is required to enhance negative ion production in hydrogen ion sources) a negative chlorine current density of 45 mA/cm 2 was obtained under the same conditions that gave 57 mA/cm 2 of positive chlorine, suggesting the presence of nearly as many negative ions as positive ions in the plasma near the extraction plane. The negative ion spectrum was 99.5% atomic chlorine ions, with only 0.5% molecular chlorine, and essentially no impurities. Although this experiment did not incorporate the type of electron suppression technology that is used in negative hydrogen beam extraction, the ratio of co-extracted electrons to Cl - was as low as 7 to 1, many times lower than the ratio of their mobilities, suggesting that few electrons are present in the near-extractor plasma. This, along with the near-equivalence of the positive and negative ion currents, suggests that the plasma in this region was mostly an ion-ion plasma. The negative chlorine current density was relatively insensitive to pressure, and scaled linearly with RF power. If this linear scaling continues to hold at higher RF powers, it should permit current densities of 100 mA/cm 2 , sufficient for present heavy ion fusion injector concepts. The effective ion temperatures of the positive and negative ions appeared to be similar and relatively low for a plasma source. 1. Introduction The primary driver concept for heavy ion fusion (HIF) has traditionally relied upon positive ions, although Koskarev suggested charge-symmetric drivers using alternating bunches of positive and negative ions in an RF accelerator [4]. More recently, it has been suggested that negative halogen ions could be used as heavy ion fusion driver beams. Relative to positive ions, negative ions would have the advantage that they would not accumulate electrons in the deep potential wells that will characterize HIF beams [5], and positive ion accumulation by negative ion bunches is likely to be less of problem, since positive ions formed in the beam path will accelerate much more slowly than electrons, and thus will be less likely to be trapped, and positive ions, unlike electrons, will not be pulled from nearby surfaces. Moreover, because the primary formation process for negative halogen ions is dissociative attachment