D.G. Koshkarev - ITEP,
I.L. Korenev, L.A. Yudin - MRTI RAS
Moscow, Russia
Linac for singly-charged (positive and negative) ions of the four
various Pt isotopes has been proposed. Eight beams of different charges and
masses of ions are accelerated in parallel RFQ channels to an energy
of 100 MeV. The beams are then brought together by a system of alternating
gradient magnet for a 
bending and matching
of the beams. The main channel which accelerats all beams together
consists of three stages. The first one (till 600 MeV) is a Wideroe
structure followed by two consecutive Alvarez channels (2.5 GeV and 10 GeV)
having different radio frequencies. Characteristics of the output beam for
each kind of ions are: average pulse current 45 mA, horizontal emittance
, vertical emittance 
,
momentum spread 
, bunch length 3.6 cm, and
spacing between bunches of each kind is 15.3 m.
In this work we describe the scheme of a special linac (see Fig. 1) for
simultaneous acceleration of eight platinum ions (four isotopes)
till maximum energy of 
. The total average pulse current is
. The linac is a beam generator for the power HIF driver
with total stored energy 
and total power 
[1].
For ITEP Heavy Ion Fusion Project (IHIFP) it is necessary to have the beams of negative and positive heavy ions with current about 50 mA. The existing now Negativ Heavy Ion Sources (NIS) provide the output current two order of magnitude less. Nevertheless, the simple theoretical consideration show the increase opportunity of NIS current up to requirement level to be realistic.
We are planing to organize our NIS investigation at the injector of the heavy ion RFQ linac (TIPr-1). We suppose to develop this investigation in two directions.
The first - a new version of NIS. As the prototype of this version we have chosen the MEVVA ion source version ITEP-90 [2].
The second, as a reserve variant, Plasma - Shutter NIS, has taken into
account. For the effective surface 140 
as sputtering target, we hope
to receive the heavy negative ion current about 100 mA. It is according to
scaling law for plasma ion sources (H.V. Smith, Jr. Paul Allison, and
J.D. Sherman - Los Alamos NL).
For IHIFP we plan to use eight ion sources of eight kinds of platinum ions
( 
)
are divided into two symmetrical arrays according to the mass and charge sign
of ion: four sources of positive ions and four sources of negative
ions [1].
All sources produce identical parallel beams: ion energy of 
,
beam radius of 
, transverse emittance
of 
in each direction. Each source must provide the entry separatrix bucket
of injector (momentum spread 
and phase length 
) with
an average pulse current of 
.
The distance between beams in each array is 
. The distance between
arrays (measured from the middle axes) is 
. The sources
of each array are positioned symmetrically with respect to the common axis
of symmetry. The higher mass isotope the source is further from the
common axis.
Table 1: The stages of injector and main channel
Each ion source is followed by its RFQ injector. All parallel
channels are located in one horizontal plane. Four injector channels of
each array may share the same vacuum system. The total length of RFQ injector
is about 
, output energy is 
. The injector consists of four
stages: RFQ buncher (RFQb) and three consequent RFQ accelerating channels
(RFQ1, RFQ2, RFQ3) with different radio frequencies (RF). RFQb is essentially
the initial part of RFQ1, but the electrodes in RFQb are modulated so that
the separatrix length is constant and equal to 
. Characteristics of
RFQ channels are presented in Table 1 where b is bunch length and
a is beam radius. Mismatching of transverse and
longitudinal oscillations due to "jumps" of RF is minimized by suitable
choice of channel characteristics at boundary points. The RFQ3 channel is
ended by special debunching cavity for reducing the momentum spread before
the bending magnet. The length of the cavity is 
that is a quarter of
the period of the longitudinal oscillation. The cavity may be a continuation
of RFQ3 with different modulation of electrodes. As a result the momentum
spread at the exit of the injector is 
and the bunch length is 
. The RF power consumed by beams in eight
injectors for each stage is (5.4 + 12.6 + 18) MW accordingly.
The eight ion beams are brought together into the main accelerating
channel by the bending magnet arrangement. Transverse matching and focusing
are achieved by suitable choice of the bending field index n in alternating
gradient magnetic sectors and inclined face at the entry of the arrangement.
The arrangement consists of 6 periods of alternating gradient magnets and of
separate 
bending magnet with uniform field. The period consists
of 3 magnetic sectors: 2 outer ones with angle spacing of 
and field index n = -15 and central sector with angle spacing 
and n = 17. The first magnet of the first period has the inclined entry
face with angle 
. According to the arrays of injector channels
there are 2 separated magnet arrangements for positive and negative ions. The
final bending magnet
with uniform field is common for all beams (both positive and negative ions).
The bending field is 
at an average radius of 
, the gap
between
magnetic poles is 
. The lengths of the beam trajectories in bending
magnets and phases of accelerating fields in injectors have been chosen so
that all ion beams are positioned in the consecutive RF buckets of the single
main accelerating channel. Because of the RF has been increased to 4 times
as large as initial one the longitudinal distance between ion bunches of the
identical kind is 4 wavelengths ( 
). Thus it is possible to place each
isotope in its own bucket. The bunches of the same isotope with opposite
charges are shifted by half a wavelength ( 
). Furthemore, in the
Wideroe channel two bunches of the same isotope with opposite charges are
placed in one RF period.
The bending magnet arrangement is followed by a transverse matching
device to reduce the oscillation amplitudes in the main accelerating
channel. The device consists of 6 quadrupole lenses with permanent magnets
(for example Sm-Co). A sequence of elements in horizontal plane is L, F1, L,
D1, L, D2, L, F2, L, F3, L, D3, L1. All drift lengths L are 
, the last
drift length L1 is 
, thickness of each lens is . Gradients
are: in F1 and D1 - 
, in D2 and F2 - 
, in F3 - 
, in
D3 - 
. The total length is 
. At the exit of the matching
device, the beam parameters are: horizontal size is 
, vertical size
is 
, length of bunch is 
, momentum spread is .
The main channel consists of 3 stages. The first one is a Wideroe
accelerating structure, the second and the third ones are Alvarez
accelerators with different RF. The design parameters are shown in Table 1.
The initial part of each stage is a quarter-wave device for matching the
longitudinal oscillation; its parameters are presented in Table 2. The total
length of main channel is 
. The focusing is realized by quadrupole
lenses with permanent magnets too. The gradient of magnetic field in the
lenses is ranged from 0.7 to 3.3 kGs/cm. The three stages of main
channel require RF power of (0.18 + 0.684 + 2.7) GW accordingly.
The exit beam has an energy of , bunch length is 
, momentum
spread is , horizontal emittance is 
,
vertical emittance is 
, radius is less than
. The distance between identical ion bunches is 
.
The total growth of the longitudinal phase space along the linac is
about 
. The horizontal phase space increases by about 5 times, the
vertical one by about 3.5 times.
The calculations were made for ideal
channels without misalignments and errors.
This research was made possible by grant INTAS-94-1713
