Effect of off-axis beam through the buncher
Julian McKenzie
Updates:
19/05/2009
1.
Removed mode 2 plots and added GPT tracking (sections 3 and 4)
Introduction
Steering the beam through the injector beamline has proven to be tricky especially
with the influence of stray/Earth fields. Recent simulations [1] show that this effect is
especially important at our current gun voltage of 230 kV and therefore we are likely
to enter the RF cavities off-axis. For the buncher this effect could be particularly
important.
Current tracking simulations in ASTRA and GPT use a simple model of the buncher
cavity where the code takes the Ez field on axis and uses approximations to find Er
and Bφ. However, since these do not take into effect the cavity/beampipe geometry,
off-axis these fields are inaccurate. Only a 3D model of the fields would show an
effect.
2.
Buncher fields
The buncher cavity was previously modelled in CST Microwave Studio.
Fig2.1. The buncher cavity
Various modes can be calculated. In the following plots, the fields are all given at
1 Joule input energy. So will be scaled down when doing tracking.
2.1
Mode 1 E-field
Fig 2.2
This is the main accelerating mode at the peak accelerating phase. You can see that
there is only a small transverse component but this increases with radius. Also note
that we operate this at zero-crossing where this field is at a minimum (but only when
the beam is at the centre of the cavity!).
2.2
Mode 1 H-field
Fig. 2.2
At zero-crossing the E-field is a minimum, therefore the H-field is a maximum. At the
bunching phase, it looks like this, looking down the beampipe.
3.
On-axis particle tracking with GPT
The E and H fields were exported from CST Microwave Studio with a 1mm grid in all
3-dimensions. To scale this down to the 1.3 MV/m peak fields as suggested by the
Astra simulations [2] for the 230 keV gun setup, we have to scale this field down by a
factor 0.03 since CST calculates the fields for 1 Joule of stored energy [3]. These
fields can be imported into GPT for tracking using elements not included in the
standard GPT release [4].
In the following simulations, the beam profile from [1] is used at the entrance to the
buncher. This is the beam profile for 1000 macroparticles created through tracking
with GPT for the 230 keV injector setup not including stray fields into the calculation.
First a comparison was taken for the beam with tracking through 1D and 3D fields onaxis. In this simulations, the centre of the buncher cavity is at 0.369 m. The start of
the simulation is the beginning of the beampipe as shown in Fig 2.1. The buncher was
set to zero-crossing at the bunching phase.
Fig 3.1 shows the 1D field case and Fig 3.2 shows the 3D field case. These plots show
the trajectories of the macroparticles in the y-z plane.
Fig.3.1: 1D buncher fieldmap
Fig 3.2: 3D buncher fieldmaps
As can be seen, the tracking of the core beam remains the same with both 1D and 3D
maps but in the 3D case, stray particles off-centre get focussed into the core of the
beam.
4.
Off-axis particle tracking with GPT
The beam was then started with +5 and +10 mm in the y-direction and shown below:
Fig 4.1: Beam y + 5 mm.
Fig 4.1: Beam y + 10 mm.
References
[1]
[2]
[3]
[4]
<\\apsv4\astec\Projects\4gls\ERLP\Machine operations\Alice injector
modelling with stray fields v3.doc>
<\\apsv4\astec\Projects\4gls\ERLP\Machine operations\Baselines Inj 230kV,
4.8MeV v.01.pdf>
Monika Balk, CST, private communication
Bas van Geer, Pulsar Physics, private communication