Nugget on Multi-Species Cyclotron Modes

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Nugget on Multi-Species Cyclotron Modes
Cyclotron Modes in Multispecies
mθ=2 + + fci
+ + fExB
Driscoll /Dubin/O'Neil, UCSD
Ion Plasmas
Plasmas exhibit a variety of cyclotron modes, and
electric fields shift the mode frequencies away from the
“bare” cyclotron frequencies 2πfci =qiB/mi. Recent
theory (Dubin 2013) gives precise predictions of these
shifts, for the "regular" mθ=1 (center-of-mass) mode,
and for the unusual mθ=2 (cartoon shown) and mθ=0
modes. For a species of fraction δi, the shift is given by
fExB [ mθ-2 + δi (1-R ) ], where fExB is the ExB drift
rotation frequency, and R represents image charge
corrections from Rwall.
1950
A new laser-thermal cyclotron spectroscopy technique
(Affolter 2013) gives the first quantitative measurement
of these shifts. The figure shows mθ=0, 1, 2 modes for
Mg isotopes 24, 25, 26. The novel mθ=0 mode has no
radial electric field, and thus couples only at the ends.
A theory fit to the data shown gives the three φχι to an
accuracy of 2 parts in 104. These new results may be
relevant to "space charge" and "amplitude" calibrations
for cyclotron mass spectroscopy devices widely used in
molecular chemistry and biology.
Cyclotron frequencies [kHz]
m =2
θ
1900
f
m =1
Mg
ci 24
θ
24
m =0
1850
θ
m =2
f
1800
θ
ci 25
m =1
Mg
θ
25
1750
f
m =0
θ
m =2
θ
m =1
Mg
ci 26
θ
26
1700
m =0
θ
1650
0
5
10
15
20
25
30
Plasma ExB Rotation Frequency [kHz]
M.Affolter et al, Phys. Plasma 22, 055701 (2015)