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Unformatted text preview: elerated species (Hegelich
et al., 2002).
A more complete spectral characterization of multicomponent ion beams can be obtained with Thomson parabola
spectrometers, based on the principle for mass spectrometry
introduced by Thomson (1911). A schematic of the device is
shown in Fig. 10(a). Ions are deﬂected by parallel E and B
ﬁelds [with E $ 104 V=m (Sakabe et al., 1980)] resulting in a
characteristic deﬂection pattern in which species with
different charge-to-mass ratio form separated parabolic
traces in the detector plane, as shown in the typical image
of Fig. 10(b). Modiﬁed magnetic and Thomson spectrometers, having imaging and angular resolution capability, have
also been developed (Ter-Avetisyan et al., 2009; Chen et al.,
2010; Jung et al., 2011a).
The detectors used in conjunction with these spectrometers
are typically either CR39, image plates (IP), scintillating
plates, or microchannel plates (MCPs). Photostimulable IPs
are ﬁlmlike radiation image sensors, developed for x-ray
medical imaging, which are composed of specially designed
phosphors that trap and store radiation energy in metastable
excited states and can be absolutely calibrated in terms of
particle ﬂux (Mancic et al., 2008; Freeman et al., 2011).
Scintillating plates (Green et al., 2010) or MCPs
(Ter-Avetisyan, Schnrer, and Nickles, 2005) are favored in
situations where online detection is required (e.g., highrepetition laser systems), as the scintillator screen or the
MCPs phosphor are imaged on a charge-coupled device
(CCD) and the detector does not require replacing after
exposure. Scintillators can also be used for beam proﬁling
(Sakaki et al., 2010), with potential for energy range selection
(Green et al., 2011). FIG. 10 (color online). Left: Schematic of a Thomson parabola
(courtesy of S. ter-Avetisyan). Right: A typical example of ion
traces obtained with the Thomson parabola.
Rev. Mod. Phys., Vol. 85, No. 2, April–June 2013 761 A different approach also allowing online beam monitoring
is the use of time-of-ﬂight (TOF) techniques, where the broadband ions are left to propagate over a given distance and then
detected employing scintillating plates coupled to a photomultiplier (Nakamura et al., 2006), Faraday cups, or semiconductor detectors (Margarone et al., 2011). The time-varying signal
produced by the detectors maps the ion energy spectrum,
although the ﬁnite response time of the detector and realistic
propagation distances limits the energy range over which these
measurements can be applied. State-of-the-art TOF-MCP detectors allow for measurements of protons with a kinetic
energy up to $20 MeV=nucleon (Fukuda et al., 2009).
III. TARGET NORMAL SHEATH ACCELERATION
A. TNSA scenario: Main experimental observations As anticipated in Sec. II.C the TNSA process (Wilks et al.,
2001) is a consequence of the large charge separation generated by hot electrons reaching the rear side of the target.
There, a cloud of relativistic electrons is formed, extendin...
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