Formation and Propagation of a Proton Ring

Abstract

A magnetically self-confined ring of 430-keV protons with a field reversal on axis of 3% is formed in an 8-kG solenoid, as evidenced by propagation more than 1 m in 15-400 mTorr air or hydrogen without dispersion. Up to 83% of the proton kinetic energy is in rotation and inductive coupling of axial ring energy to azimuthal plasma current induced in air (but not hydrogen) gives reflection of >50% of the ring from a 23% mirror.PACS numbers: 52.55.Ke Magnetic field configurations in which an externally applied magnetic field is reversed by currents flowing in the plasma itself are expected to have favorable plasma confinement properties and have, therefore, attracted considerable interest for fusion reactors. The current may be carried by either low-energy plasma particles or by rings of high-energy particles. Synchrotron radiation losses by energetic electrons, the possibility of plasma heating by the energetic ions, 1 and theoretical predictions of a more stable configuration 2 have lead to field-reversal schemes with use of high-energy ions. Recent developments in intense ion-beam technology have made it possible to attempt single-pulse injection and trapping of a field-reversing proton ring in a mirror well on a time scale of 0.1-1.0 jmsec. Indeed, the transient reversal of an applied magnetic field by a rotating proton beam has recently been reported, 3 although the average axial velocity and its dispersion are too large for the beam to be trapped in a mirror well or confined in its own diamagnetic well.In this Letter we report experimental results on the formation and dynamics of a rotating proton ring, whose quality is such that particles are trapped in their own 3% diamagnetic well. To summarize, an annular ~430-keV proton beam from a magnetically insulated diode is injected through a cusplike magnetic field to form a rotating proton beam. With <r 15 mTorr air or ^ 50 mTorr H 2 in the experiment chamber, up to 83% of the total kinetic energy of the ring is in rotation, and the axial energy dispersion of the protons with enough energy to traverse the cusp is sufficiently small so that > 70% are held together axially by their own 3% diamagnetic well as they propagate more than 1 m in an 8-kG solenoidal magnetic field. Up to 15% of the residual axial beam energy is inductively coupled to azimuthal plasma currents induced in the beam-generated plasma in air, but such currents are not observed in hydrogen. Consequently, the proton beams are more efficiently reflected from the 1.23-mirrorratio downstream mirror in air than in hydrogen, with <r 50% achieved in air. About 1% inductive energy coupling to wall resistors was observed in both air and hydrogen experiments.A schematic drawing of the apparatus is shown in Fig. 1(a). The 2.5-m-long solenoid is operated at 8 kG in the experiments reported here. The proton source is an annular magnetically insulated diode, similar to one described previously. 4 Pulsed coils within the anode of the diode provide both closed magnetic-flux surfaces around the anode which insulat...

Field-reversed configurations with a component of energetic particles

“…A schematic of the system which approximates closely the experimental arrangement of Dreike et al [22][23][24] is shown in Fig. 2.2.…”

“…For reasons explained in Section 2.9, this was not an ion ring in equilibrium but nevertheless it demonstrated that ion sources of the requisite power and charge are available. A more modest programme at Cornell is directed to the creation of partially field-reversed equilibrium ion rings (f <0.1), Dreike et al [22][23][24].…”

Self-trapping of a rotating ion ring

“…For instance, a commonly used injection scheme for ion ring configurations is cusp injection, 13 where ions injected axially into the device from a ring diode are accelerated azimuthally by the radial component of the magnetic field near the diode. For instance, a commonly used injection scheme for ion ring configurations is cusp injection, 13 where ions injected axially into the device from a ring diode are accelerated azimuthally by the radial component of the magnetic field near the diode.…”

Ускорение Ионных Колец Сжимающимися Лайнерами

“…Образование ионных колец по схеме, представленной на рис. 1, доказано в опытах с протонами [3,4]. Возможность сжатия магнитного потока проверена, например, в эксперименте [5].…”

“…По этой причине при вводе пучка нарушается аксиальная симметрия, что является необходимым условием для образования конфигурации с обращенным полем [13,14]. И, действительно, такие кольца образовывались в опытах [3,4]. В более поздних исследованиях, представленных в обзорах [15,16], этот процесс был изучен более детально.…”