Secondary electron rf-detector is the only beam diagnostic
instrument at present for longitudinal profile measurement of
short ion bunches. The detector with longitudinal rf-modulation
of secondary electrons in capacity gap of quarter-wave coaxial
resonator with a helical inner conductor is considered for the
measurement (with phase resolution of about one degree of 148.5
MHz) of the ISTRA ion linac beam with pulsed current up to 200
mA at ion energy of 3...36 MeV. The detector description and bench
testing results of its main units will be presented.
The proton linac ISTRA-36 [1] creating at ITEP for
the first model of radwaste transmutation plant requires appropriate
beam diagnostic provision to understand and minimize beam-loss.
Detector for ion bunch phase distribution measurements with resolution
of about 1° for the power beam of the ISTRA linac is discussed
in this paper. This beam diagnostic instrument is required, first
of all, for precise beam matching and setting up rf-parameters
of the accelerator cavities. The detector is the key tool in the
longitudinal beam emittance measurement system that will be also
accomplished using the existing bending magnets in the linac channel.
Chosen method of the emittance measurement will allow to research
the beam distribution in longitudinal phase space without any
model assumption of it. First the method was proposed and successfully
realised at the I-100 linac ( IHEP ) in 1980 [2]. Figure 1 makes
clear it. The detector (9) was spaced after spectrometer (7) and
at the 10 m distance from the I-100 linac.
Narrow momentum width of the collimator (8) of 0.1 %
and small beam divergence (0.5 mrad) after collimators (5,6)
allowed, in fact, to reserve the bunch phase distribution for
separated part of the beam and, carrying out the same measurements
at different particle momentum, to determine the distribution
in the longitudinal phase space shown in Fig.2 where isolines
of the beam distribution in the longitudinal phase space at different
density levels (pointed in left column of the table) and corresponding
beam per cents and beam longitudinal emittance measured are presented
too. More in detailed of it one can find in [3,4].
| Lev. | % | *104
(eV*s) |
| .2 | 74 | 3.8 |
| .3 | 63 | 2.6 |
| .4 | 54 | 2.0 |
| .5 | 45 | 1.5 |
| .75 | 24 | .75 |
The principle of operation of the bunch phase distribution (BPD) detector (9) (Fig.1.) has been reported elsewhere [4,5]. Briefly, in the device the BPD of a high energy ion beam is isohronously transferred into the same distribution of the low energy secondary electrons which, then, is coherently transformed into transverse one through rf-modulation in the resonator gap (14) and spectrometer (15) allowing direct presentation it on a low frequency display (12).
Taking into account the beam space charge effect
the detector realizing the same principle of operation has been
chosen for the ISTRA beam.
The detector for the ISTRA linac beam proposed in
[5] is schematically shown in Fig.3.
There are some distinctions between the new detector and the mentioned above. Taking into account that the lower proton energy the higher its energy loss in the target, within considered beam energy range, the thin carbon fibre of 8 mm diameter was chosen to decrease the fibre heating under the beam. Moreover, for that diameter and high negative voltage (of about 8 kV) applied to the target (1) the phase dilution of the secondaries on the distance (10 mm) until the collimator (2), caused by their initial energy spread, will be less than 0.01 deg. of 148.5 MHz (see Fig.3 in [6]).
To decrease the detector sizes a open quarter-wave
coaxial resonator with a helical inner conductor was chosen the
photo of which ( before brazing ) is shown in Fig.4.
At pulsed rf-power consumption of about 40 W the modulating gap voltage reaches demanded value of 2 kV. To suppress multipactoring the inductance part of the resonator contains an atmospheric air. Resonator feedthroughs are not vacuum - tight.
Multichannel collector (7) of the secondary electrons installed at the exit of the magnet spectrometer (6) allows to record a bunch phase distribution for a time less than the beam pulse duration, i.e. it will allow us to investigate changes of the phase distribution along the beam pulse. Estimations of separated pulsed charges of the secondaries at the entrance of the collector show that magnification of about 104 reached with installation of microchannel plates will be enough to record the phase distribution of the ISTRA beam in 100 points of its pulse. Below, everywhere, results of consideration will be presented for the following ion beam parameters: proton energy - 3 MeV, pulsed beam current - 150 mA, rms beam radius - 2.5 mm, beam pulse duration - 150 ms, pulse frequency - 25 Hz.
It should be noted that the detector can be installed
so that the ion beam axis will be perpendicular to the plane Fig.3,
then the monitor size along the ion beam will be 40 mm [4].
The detector phase resolution (Dj) is mainly defined by the phase dilution (Dj1) of the secondaries on the distance h = 10 mm, the shutter phase resolution (Dj2) and the additional phase dilution (Djq) caused by the ion beam space charge effect. As it was mentioned above Dj1 = 0.01. In Fig.5 the phase dilution Djq is plotted as a function of the target place relatively to beam axis. Distance between the beam axis and the collimator of 10 mm is fixed.
Then, considering the above mentioned quantities
as the independences one can define Dj using known algorithm from
[5]. Figure 6 shows the dependence of Dj vs. the electron input
phase for the slit width of 1 mm of the collimator (4) and the
main radius of electron trajectories in the magnet spectrometer
of 50 mm.
One of the advantages of this technique is possibility
to calibrate the detector using thermoelectrons from the target
heated by a current. The Dj2 resolution is determined by ratio
of the relative initial momentum spread of the electrons at the
gap entrance to the maximum increment of it due to the gap action
which are measured by the magnet spectrometer when the gap is
fed and not. Relationship U and the electron energy at the gap
entrance is checked on the curve of the thermoelectron distribution
in phase.
There is important effect which limits the detector
operation. When the temperature is beyond 2000 K the thermocurrent
density can excess the magnitudes compared with the secondary
electron one. It ought to note too that for the tension of the
wire it is necessary to know a possible highest wire - target
temperature because the limit of a elasticity strongly depends
on the target temperature.
Taking into account that with decreasing proton energy
its energy loss per unit length increases rapidly and velocity
of the heat transport in the target is negligible small in comparison
with a speed of heating under the beam the carbon fibre of 8 mm
diameter was chosen. For this target and the above mentioned beam
parameters (3 MeV) but for the pulse duration of 50 ms the maximum
temperature dependence on a time is plotted in Fig.7.
With increasing the pulse duration till 100 ms the increment of the maximum temperature for the pulse can reach 2500 K already. Hence, the main problem consists in the heating for a pulse, and the known flying wire technique [7] is not solution for it.
There are several ways, proposed in [8], to solve
this problem. Shortly, these proposals, shown schematically in
Fig.8, consist in the following. First, to avoid increasing the
wire heating from pulse to pulse the wire is replaced on a length
equaled to a beam diameter for a time between two beam pulses
by means of winding up the wire (Fig.8.a) from bobbin (1) on (2)
through the area occupied by the beam (5).
To decrease the wire heating, at first, the speed
of "running" wire is brought up to demanded one and
after that it is moved in the beam. Fig.8.b explains the same
principle of operation for a wire closed on itself when the wire
speed can be very high. Lastly, Fig.8.c makes clear the proposal
of "boiling" wire. A boiling heat for any material is
known to be the most magnitude at a heating process. Then, applying
thin coating of copper (1) on a tungsten core of a wire one can
keep the core at the temperature being not more than the boiling-point
for a copper equaled to 2300°C. If we take this type of the
wire for the RFQ2 linac beam at CERN [9] (with proton energy 750
keV) the copper layer of 5 mm will be enough to reserve the tungsten
core of 100 mm diameter because a proton range for a copper is
not more it.
Besides the mentioned, to extend the temperature
range we could use the delta - electrons [10]. In the case it
is not necessary to apply voltage to the target.
One can conclude that there is no limit of principle
for using the secondary electron detector for bunch phase distribution
measurements of a ion pulsed beam with average beam current up
to 10 mA now. Proposed new target technique needs its experimental
researches under intense ion beam.
Authors thank A.M.Kozodaev for support in part of
this work.
[1] A.M.Kozodaev, I.V.Chuvilo, A.A.Kolomiets et. al., "Proton 36 MeV, 0.5 mA Linac ISTRA-36 as a Driven Multipurpose Irradiation Test Facility", Abstracts of EPAC 96, Barcelona, p.157 (1996)
[2] A.M.Tron, V.N.Glushenko, V.A.Konov et.al., "Longitudinal Emittance Measurement of Proton Linac Beam", Abstracts of Conf. on Linacs, Kharkov, p.24 (1980)
[3] A.M.Tron, "Secondary Electron Methods for Ion Phase Motion Research", Proc. 1990 Int. Linac Conf., Albuquerque, p.477 (1990)
[4] A.M.Tron, "Secondary Electron Monitors in Linacs for Intense Neutron Sources", Proc. 1994 Int. Linac Conf., Tsukuba, p.917 (1994)
[5] A.M.Tron, "Secondary Electron Monitors for Ion Bunch Phase Distribution Measurements", Proc. of the 2nd European Workshop on Beam Diagnostics, Truvemunde, p.60 (1995)
[6] A.M.Tron, I.G.Merinov, "Secondary Electron Monitor for Electron Bunch Phase Distribution Measurement with Subpicosecond Resolution", Proc. of this Conf.
[7] J.D.Gilpatrick," On-line Diagnoses of High Current-Density Beams", Proc. EPAC 94, London, p.273 (1994)
[8] A.M.Tron, "Detectors for Bunch Phase Distribution Measurements of Ion Linac Beam", will be published in Proc. of Accel. Conf., Protvino, Oct.1996.
[9] M.Weiss, "The RFQ2 Complex: the Future Injector to CERN Linac 2", Proc. of EPAC 92, Berlin, p.539 (1992)
[10] A.M.Tron, V.V.Smirnov, "Method of Bunch
Phase Distribution Measurement Based on a Moller Scattering",
Proc. of this Conf.