A Plasma Lens for GeV Particles

At next week's American Physical Society Division of Plasma Physics/International Congress on Plasma Physics meeting in Quebec City, Hector Baldis of Livermore (925-422-0101, baldis1@llnl.gov) will present experiments showing that plasmas have focused high-density, high-energy electron and positron beams 1000 times better than magnetic quadrupoles used in conventional accelerator technology.

In the E150 experiment carried out at the SLAC Final Focus Test beam, a standard plasma--a gas of ions and electrons--could focus a 30-GeV electron beam to one-third of its original diameter in just 2 centimeters. In addition, the researchers demonstrated plasma focusing of high-energy positron beams for the first time. Technologies have long existed for focusing MeV electron beams, but not for the GeV beams used in state-of-the-art accelerator experiments.

How does the plasma focus particle beams so well? To understand this effect, it is important to realize that electrically charged particles in a beam experience two competing forces: a "Coulomb" force which tries to push like charges apart, and magnetic forces which push them together. A high-energy beam redistributes the plasma electrons as it passes through, and this serves to reduce the net Coulomb force while leaving the magnetic force unaffected; this manages to pinch the beam closer together. Conventional plasmas seem to focus the beams very well; no exotic plasmas must be prepared. (Paper BO2.002; more information on meeting here.)

Heartbeats are Highly Interrelated During REM Sleep

Heartbeats are highly interelated during REM sleep, the stage in which the brain is very active and when most dreaming takes place. This is the finding of a German-Israeli research team (Armin Bunde and Jan Kantelhardt, Justus-Liebig-University, 011-49-641-99333-60 or -73, kantelhardt@physik.uni-giessen.de; Shlomo Havlin, Bar-Ilan University).

Examining heartbeat patterns in 15 healthy individuals and 26 with a sleep disorder, the researchers looked at interbeat intervals, the times between successive heartbeats. Looking at deep sleep and light sleep, they found that interbeat intervals exhibited no interrelationships for more than 5 successive heartbeats. But during the REM (rapid eye movement) stage, which takes up about 20 percent of total sleep time, they found the interbeat intervals at one time and those at much later times were correlated over a whole range of time scales--for example, a shorter-than-average interval would more likely be followed by another shorter-than-average interval instead of a longer one, even if the researchers selected two intervals that were minutes away from each other.

Therefore, the heartbeat during REM sleep seems to rely upon a sort of "memory" that does not appear during the other phases of sleep. This finding, the researchers believe, could provide insights into the biological regulatory processes that govern heart rate variability. In addition, they say that the heartbeat information could potentially lead to a convenient "sleep phase finder" which could determine sleep stage by monitoring heartbeat. The researchers point out that the overall situation is reminiscent to DNA--in which the sequences of "letters" (base pairs) in non-coding ("junk DNA") regions have correlations, while the coding (gene-containing) regions do not have correlations. (Bunde et al., Physical Review Letters, 23 October 2000; Select Article.)

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