We study the stability of small strangelets by employing a simple model of strange matter as a gas of non-interacting fermions confined in a bag. We solve the Dirac equation and populate the energy levels of the bag one quark at a time. Our results show that for system parameters such that strange matter is unbound in bulk, there may still exist strangelets with A < 100 that are stable and/or metastable. The lifetime of these strangelets may be too small to detect in current accelerator experiments, however.

We report on the autoresonant (nonlinear phase locking) manipulation of the diocotron mode in a non-neutral plasma. Autoresonance is a very general phenomena in driven nonlinear oscillator and wave systems, and allows us to control the amplitude of a nonlinear wave without the use of feedback. These are the first controlled laboratory studies of autoresonance in a collective plasma system. [S0031-9007(99)09167-X] PACS numbers: 52.25.Wz, 05.45.Xt, 52.35.Mw 4444 0031-9007͞99͞82(22)͞4444(4)$15.00

Longitudinal compression oC a tailored-vciocity, intcmc neutralized ion heam hns been demonstmtcd. The compression tnlm plnec in a 1-2 m driit seetion Blled with plam~n to provide spacedierge neutduntioo. An induction cell produces a head-to-tnil velocity ramp that longitudinally compresses theneutrdized beam, enhancingthe b-peak current hyafndor of 50 audprodudag a pulse duration OC about 3 us. T h i s m m e m e n t hns been confirmed indopmdsntly with two darercot diagnostic eystems. The simultaneous l m v e r a e nnd longihdinal compression oI an ion heam is reguired t o achieve the high intensities necrssnry to create high energy density matter and fusion conditions. A recent driver study for inertial fusion, for Longitudinal compression of space-chargedominated beams has been studied extensively intheory and simulations [ll-E]. The compression is initiated by imposing a linear head-to-tail velocity tilt to a driitiig beam. Longitudinal space-&urge forces limit the beam compression ratio, the ratio oI the initial to i i nmal current, to about ten in most applications. iments on NDCX. To provjde the head-ta-tail velocity tilt, aninduction module withvariablewltagewaveronn is placed immedintely downstream of the last quadrupole mnpet. This is IoUmved hy a neutralized drii section which consists of a one -meter-long plasma column produced hy an AI cathodic cm: [ZO]. A diagnostic hmc is located at the downstream end of the plwarna column Thebeampmducedfromthesourcehasa5 paflat-top. The inductiontit voltage 'c~rves' out a -300 115 segment ofthe flat-top which compresseslongitudinallyas it driits through the plasma column. The final compressed beam is m e w e d in the dormetrenm diagnostic box.The induction cell consists oi 14 independently-driven magnetic cores in a preastnizad gas @Fa) region that is separated Erom the vnouum by a conventional high voltage insulator. The rvnveforms applied t o the 14 coria inductively add at the acceleration gap. Each core is driveu by a thyratronawitched modulator. Because the modulntor for each core can be designed to produce different waveiorms and can be triggered independently, a variety or wavdorms CM bs produced nt the acceleration gap using the 14 discrete building bloclw.The plasma column is formed hy two pulsed a u m i n u m cathodic arc sources loceted at the d m s t r e a m ond. Each source is equipped with a 45O open-arcutechm

The autoresonant ͑nonlinear phase locking͒ manipulation of the diocotron mode in a non-neutral plasma is investigated. Autoresonance is a very general phenomenon in driven nonlinear oscillator and wave systems. By sweeping or chirping the drive frequency, autoresonance allows the amplitude of a nonlinear wave to be controlled without the use of feedback. The experimental results, including a novel scaling relation, are in excellent agreement with a simple theoretical model. These are the first controlled laboratory studies of autoresonance in a collective plasma system.

A newly installed Lithium Granule Injector (LGI) was used to pace edge localized modes (ELM) in DIII-D. ELM pacing efficiency was studied injecting lithium granules of nominal diameter 0.3–0.9 mm, speed of 50–120 m s−1 and average injection rates up to 100 Hz for 0.9 mm granules and up to 700 Hz for 0.3 mm granules. The efficiency of ELM triggering was found to depend strongly on size of the injected granules, with triggering efficiency close to 100% obtained with 0.9 mm diameter granules, lower with smaller sizes, and weakly depending on granule velocity. Robust ELM pacing was demonstrated in ITER-like plasmas for the entire shot length, at ELM frequencies 3–5 times larger than the ‘natural’ ELM frequency observed in reference discharges. Within the range of ELM frequencies obtained, the peak ELM heat flux at the outer strike point was reduced with increasing pacing frequency. The peak heat flux reduction at the inner strike point appears to saturate at high pacing frequency. Lithium was found in the plasma core, with a concurrent reduction of metallic impurities and carbon. Overall, high frequency ELM pacing using the lithium granule injection appears to be compatible with both H-mode energy confinement and attractive H-mode pedestal characteristics, but further assessment is needed to determine whether the projected heat flux reduction required for ITER can be met.