Feasibility of Laser Stripping via a Broad Stark State
Isao Yamane
KEK, High Energy Accelerator Research Organization
Oho 1-1, Tsukuba-shi, Ibaraki-ken, 301-0801 Japan
Abstract. Laser wave length, magnetic field and laser power density necessary for the laser stripping via a broad Stark
state are estimated for several energies of H° beam. Application of a Fabry-Perot resonator and emittance growth
accompanying stripping were studied. The laser stripping via a broad Stark state turned out to be feasible.
INTRODUCTION
BSS LASER STRIPPING
High-energy beams of excited H° atoms, which
have a principal quantum number: n no less than 3, can
be stripped in a same manner as high-energy H~
beams[l]. So, we have studied laser stripping schemes
including laser pumping of H° atoms from the ground
state to a higher excited state. But, it turned out there
are two serious problems which are inherent in the
laser pumping of high-energy H° beams in the free
space. One problem is the Doppler broadening of the
transition frequency distribution due to the momentum
spread of the H° beam. The other is that the population
of the upper state is saturated to 1/2 by competition
between pumping up and down. Therefore, we need to
take such a measure as the Rabi-oscillation.
When we use Lorentz force to cause Stark effect, a
laser stripping scheme like Fig.l (a) may be possible.
H° beams, that have been injected into a straight
section of a ring, are conducted into a magnetic field
of an undulator, then pumped up to a broad Stark state
through interaction with a laser beam. Excited H°
atoms of the beam immediately decay to proton and
electron and formed proton beam is captured by the
ring.
The concept of strip.ping process is shown in Fig. 1
(b). Adjusting the magnetic field of the undulator so
that the saddle point energy of the potential comes
near to the aimed energy level of H° atoms, a broad
Stark level as broad as the Doppler broadening of the
transition frequency distribution is provided. Then, H°
atoms are pumped up to the broad Stark level by a
laser beam. Since the level width of the upper state is
as broad as 1013Hz, lifetime of excited if atoms is
very short. Excited H° atoms immediately decay to
protons and electrons through autoionization.
Therefore, the population of the excited state cannot be
increased and pumping down of excited H° atoms to
the ground state by stimulated emission of a photon is
negligible. Thus, the reaction proceeds onedirectionally from pumping up to autoionization. As a
result, stripping efficiency can be made 100%.
In order to overcome these problems, we introduced a
method to utilize a broad Stark state[2]. For the
convenience sake, this method of laser stripping is
called here as the BSS laser stripping.
SM
N*
H*~*p*e
v
p
H?
undulator
Electric Field
Autoionization
FIGURE 1. a) New scheme of laser stripping, (b)
Conceptual picture of stripping process
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
340
TABLE 1. Photo-ionization via Broad Stark State; (n=5,k=-4,m=Q)
T (GeV)
\5LF (nm)
7Laser(W/cm2)
0.800
327.9
Interaction Length = 3m
1.000
1.300
2.000
372.2
437.4
586.2
H° kinetic energy
Laser wavelength
39.1
41.5
Laser power density
^tark ^
0.171
| 7Laser(kW/cm2) |
3.91
43.9
0.148
0.123
0.090
Interaction Length = 0.3m
| 4.15 | 4.39 | 4.69
Since the physics of the Stark effect and the resonant
photoionization is well established, numerical
estimation of parameters, such as the magnetic field to
cause necessary Stark effect and the necessary laser
power density, is possible using formulae and data
found in textbooks and data tables[3-5]. Table 1 is an
example of numerical estimation made by a method
described in the reference[2].
B'
\
»'
Magnetic field
| undulator/FP system
|
be possible. Here H° and laser beams collide with a
very small angle. The interaction region, that is about
30cm long, is covered by a magnetic field to cause
necessary Stark effect to H° atoms. Magnetic field of
outer magnets which compose an undulator with the
central one is set a little lower so that stripping does
not occur in these magnets. When diameters of H° and
laser beams are taken as 3mm and 10mm respectively,
the colliding angle is 20mrad. Since the size of the
beam duct is typically 10cm, the separation of two
mirrors of the FP resonator becomes 6m or longer.
However, this system is very simple and seems enough
practical.
When we take the Stark state with n=5, k=-4 and
m=0 and an interaction length of 3m, necessary laser
power density is about 40W/cm2. For SOOMeV and
2GeV H° beam, necessary laser wavelengths are 328
nm and 586nm, and necessary magnetic fields are 0.17
and 0.09T, respectively. For the same Stark state and
an interaction length of 30cm, necessary laser power
density increases to about 4kW/cm2.
I
46.9
EMITTANCE GROWTH DUE TO
STRIPPING
An important item to be checked is emittance
growth due to stripping. Here we consider laser
stripping in an undulator magnet as is shown in Fig.
3(a). For simplisity sake, magnetic field of outer
magnets are taken same as that of the central magnet.
Then the proton beam circulating in the ring takes an
orbit shown by a thick line. Maximum deflection :0
and maximum displacement: do are given by equations
shown in the right side.
I,
FIGURE 2. Laser stripping scheme using a Fabry-Perot
resonator and an undulator.
APPLICATION OF FABRY-PEROT
RESONATOR
Nowadays, such a Fabry-Perot resonator is
available that has a finesse near to 105, is 6m or longer
and stacks a 10kW/cm2 light beam with a diameter of
10mm, as is reported by G. Cantatore [6] in this
workshop. Using such a FP(Fabry-Perot) resonator,
the interaction length can be reduced to about 30cm or
shorter. Then, a laser stripping system like Fig. 2 may
341
REFERENCES
0 a BIm/(Bp)
(a)
1.
I. Yamane, PRST-AB Vol. 1, No 053501, 1998.
2.
I. Yamane, KEK Report 2001-20, February
2002, A.
H. A. Bethe and E. E. Salpeter, Quantum
Mechanics of One and Two Electron Atoms
(Springer, Berlin, 1957).
V. S. Letokov, Laser Photoionization
Spectroscopy, Academic Press, 1987.
K. Omidvar, Atom, and Nucl. Data Tables 28,
215-238(1983).
G. Cantatore et al., contribution to this
workshop.
3.
11 T t r
a bcd e
4.
5.
6.
FIGURE 3. (a) Deflection and displacement in the
undulator magnet, (b) Motion of formed proton in the phase
space.
Laser stripping of H° beam occurs in the central
magnet. Fig. 3(b) shows motion in the phase space of
protons stripped from H° atoms at various points of the
magnet. While protons formed at the point a in the
central magnet go through and reach to the end of the
undulator magnet, they walk along a dotted line and
reach to the point a in the phase space. In the same
manner, protons formed at the point b through e walk
along respective dotted lines and reach the point b
through e in the phase space. Therefore, protons
formed by laser stripping receive a spread of
deflection: 20, and a spread of displacement :
2(d0+61m). For example, when 2GeV H° beams are
stripped by a 532nm laser beam in an undulator
magnet with B=0.09T, lm=30cm and lg=10cm, 0 is
1.45mrad and (d0+61m) 0.58mm.
These spreads are considered to increase the
emittance of the formed proton beam by several times
from the original emittance of H° beam, that is
typically iTcmmmrad. However, such an emittance
growth is not considered to be so serious in the
situation where we must form a ring beam with an
emittance of several hundreds Tcmmmrad by a
sophisticated phase space painting. Emittance growth
accompanying laser stripping should be taken into
account as a part of the phase space painting.
CONCLUSION
Laser stripping via a broad Stark state is feasible
for GeV-energy H° beams, using a Fabry-Perot
resonator and an undulator magnet.
342