11institutetext: Kapteyn Astronomical Insitute, University of Groningen,
Groningen, The Netherlands

# ALMA Band 9 upgrade: a feasibility study

S. Realini 11<EMAIL_ADDRESS>R. Hesper 11 J. Barkhof 11 A. Baryshev 11

###### Abstract

We present the results of a study on the feasibility of upgrading the existing
ALMA Band 9 receivers (602-720 GHz). In the current configuration, each
receiver is a dual channel heterodyne system capable of detecting orthogonally
polarized signals through the use of a wire grid and a compact arrangement of
mirrors. The main goals of the study are the upgrade of the mixer architecture
from Double-Sideband (DSB) to Sideband-separating (2SB), the extension of the
IF and RF bandwidth, and the analysis of the possibilities of improving the
polarimetric performance. We demonstrate the performance of 2SB mixers both in
the lab and on-sky with the SEPIA660 receiver at APEX, which shows image
rejection ratios exceeding 20 dB and can perform successful observations of
several spectral lines close to the band edges. The same architecture in ALMA
Band 9 would lead to an increase in the effective spectral sensitivity and a
gain of a factor two in observation time. We set up also an electromagnetic
model of the optics to simulate the polarization performance of the receivers,
which is currently limited by the cross-polar level and the beam squint, i.e.
pointing mismatch between the two polarizations. We present the results of the
simulations compared to the measurements and we conclude that the use of a
polarizing grid is the main responsible of the limitations.

##  1 Introduction

With its 66 high-precision antennas located on the Chajnantor plateau at 5000
m altitude, the Atacama Large Millimeter/submillimeter Array (ALMA) is the
largest ground-based aperture synthesis telescope for observation in the
millimeter and submillimeter regime. The broad frequency range available
(30–950 GHz) is divided into ten different receiver bands, where each receiver
unit (called âcartridgeâ) is built independently. Each receiver band detects
two orthogonal linear polarizations and down-converts the signals to an
intermediate frequency. Each of the ten bands should be optimized to provide a
good coupling with ALMA telescope as well as low sidelobe and cross-
polarization levels. In addition, the beams for the two orthogonal
polarizations on the sky should coincide and any deviation, called beam
squint, should be minimized.

Band 9 (610-720 GHz) is ideal to study warm ($T>100$ K) and dense gas
spectroscopically for several astrophysical objects: high redshift galaxies,
starbursts, black holes in AGN, star-forming regions, proto-planetary disks,
and the Solar system. It also allows one to detect high excitation lines of
CO, HCN, HNC and HCO+ across cosmic time. Since such observations provide a
unique window on galaxy evolution, we should seek the best available detector
technique for this band. The present ALMA Band 9 receivers use a double side
band (DSB) mixer scheme Baryshev2015 , while all frequency bands below 650 GHz
use dual sideband (2SB) heterodyne receivers to better to remove the
atmospheric noise in the image IF band. Moreover, the existing Band 9
receivers have a crossâpolar performance which does not meet the ALMA
specifications of -23 dB and they show a relatively large beam squint, which
makes this channel not suitable for extended-source polarimetry.

These issues lead to the definition of a project to study a possible
improvement of Band 9 performance within the framework of ESO âAdvanced Study
for Upgrades of the Atacama Large Millimeter/sub-millimeter Array (ALMA)â. The
main goals of the project are described in Section 2.

##  2 Project overview

In its current configuration, the ALMA Band 9 receiver is a dual channel
heterodyne system capable of detecting orthogonally polarized signals using a
wire grid combined with a compact arrangement of mirrors. The requirements on
the optics performance are a beam squint < 1% of FWHM and a cross-polar level
-23 dB below the co-polar total energy level. However, the crossâpolar
performance of existing receivers does not meet the requirements specified for
the ALMA channels, and they show a relatively large beam squint, which makes
this channel not suitable for extended-source polarimetry.

These requirements are very challenging and not completely fulfilled in Band
9. Due to the non-optimal performance of its receivers, we performed a
feasibility study to make an upgrade to 2SB receivers with a possible
improvement of the optical system within the cartridge. The study, titled
âFull 2SB Receiver Upgrade for ALMA Band 9: Implementation Studyâ, is founded
by ESO and the main goals are report :

  * •

extension of the IF bandwidth to $4\times 12$ GHz (2 sidebands and 2
polarizations), without compromising the other performance parameters;

  * •

extension of the RF bandwidth beyond the nominal 600-720 GHz;

  * •

analysis of the possibility to improve the optical cross-polarization
performance compared to the currently installed configuration, e.g. by using a
single-horn configuration with an orthomode transducer;

  * •

investigation of the causes of the scatter in the beam squint to understand if
it could be improved by reducing tolerances of the grid.

The technical feasibility of an ALMA Band 9 upgrade from the existing double-
sideband configuration to the sideband-separating one has already been
demonstrated in a previous ESO study 2sbUpgrade . Since the proposed
rebuilding of the Band 9 cartridges could be an opportunity to bring the
cross-polar performance in line with the original specification, we carried
out a dedicated analysis to investigate the improvement in the cross-
polarization performance by changing the way in which polarization separation
is performed, i.e. removing the grid and using an orthomode transducer (OMT)
to separate the two orthogonal polarizations. Here we present also the results
of the analysis performed to check the effect of the mounting of the grid on
the beam squint.

###  2.1 Science drivers

The proposed upgrade would give significant scientific advantages by improving
the sensitivity by a factor 2 through the elimination of the atmospheric noise
in the unwanted image sideband. It would also allow a reduction in line
confusion in spectral line surveys and a factor 4 improvement in the required
integration time. This is highly beneficial because Band 9 requires favorable
weather conditions, which only occur a modest fraction of the time.

We identified also additional science cases, especially related to the
polarization capability. Among these we find the study of magnetic fields in
very dense environments of circumstellar envelopes around evolved stars and
high mass star-forming regions through the vibrationally excited water masers
at 658 GHz. Polarization observations in Band 9 can be a powerful tool to
study also dust settling and grain growth in accretion/protoplanetary disks
around young stellar objects.

##  3 The ALMA Band 9 receiver

Each ALMA Band 9 receiver unit (Fig. 1, left) is installed in the telescope
front end and has been designed to detect and down-convert two orthogonal
linear polarization components of the light collected by the ALMA antennas
Baryshev2015 . The radiation, collected by the telescope, enters the front end
through a vacuum window and a set of infrared filters. The beam is then
refocused with a compact arrangement of elliptical mirrors, which is fully
contained within the cartridge (Fig. 1, right). The optics assembly contains a
polarizing grid to separate the two orthogonal linear polarizations and two
beam splitters to combine each resulting beam with a local oscillator signal.
The two beams are then sent to independent superconductor-insulator-
superconductor (SIS) mixers that perform the heterodyne down-conversion.
Finally, the generated intermediate frequency (IF) signals are amplified by
cryogenic and room-temperature HEMT amplifiers and exported to the telescopeâs
IF backend for further processing and correlation.

Figure 1: _Left:_ Picture of ALMA band 9 receiver cartridge. _Right:_ Layout
of the optical system for focusing the RF beam and injecting the LO signals

##  4 Results

###  4.1 Extension of the IF Bandwidth

To determine the maximum IF bandwidth achievable by the Band 9 sideband
separating receivers, we should consider the bandwidth of the mixer devices
themselves and the technological extensions needed to harness this bandwidth.
In addition, even if the mixers can deliver the expected bandwidth, the
greatest technical challenge is obtaining cryogenic low-noise amplifiers
(LNAs) and quadrature hybrids. Currently, the Observatorio Astronomico de
Yebes (Spain) is involved in the further development of this type of
amplifiers, especially towards larger bandwidth and better input matching,
supported by a dedicated ESO technology development program.

To test the performance of the sideband-separating mixers, we measured the two
most significant properties: the image rejection ratio (IRR) and the noise
temperature over the IF band. Figure 2 shows the combined IRR data for the
tested mixer pair in the IF range 4â18 GHz.

Figure 2: Combined IRR data for the tested mixer pair in the IF range 4â18
GHz.

The measurements show that the existing Band 9 SIS mixer design can achieve a
larger IF bandwidth than the original design specification and the performance
will be similar up to 18 GHz.

###  4.2 Extension of the RF Bandwidth

The band edges of ALMA Band 9 were originally chosen to be 602 and 720 GHz
because of the presence of strong water vapour absorption lines at 557 and 750
GHz, each pressure broadened of about 50 GHz. However, very dry conditions
allow useful observations even outside of this range, especially if the source
is sufficiently bright. A good example is a spectral line survey of the Orion-
KL star-forming region performed with the SEPIA660 2SB receiver at APEX sepia-
meas .

As demonstrated by the measurements performed on the SEPIA660 receiver, the
existing SIS junctions that make up most of mixer in the delivered Band 9
cartridges typically have an RF bandwidth extending significantly beyond the
602â720 GHz range of the Band 9 specification. Also the reflective optics in
the cartridge as well as the corrugated feedhorns have bands wider than the
specification. The only modification needed is the extension of the local
oscillator (LO) tuning range by the use of commercial components. Therefore,
we can extend the RF bandwidth to 580â735GHz.

###  4.3 Polarization Performance

To evaluate the impact of the polarization grid on the cross-polar level, we
simulated the original Band 9 optics using the software GRASP. We built the
model according to the values reported in Granada . The final layout of the
model includes two feedhorns, three ellipsoidal mirrors and the polarization
grid. The feedhorn is modelled as a hybrid-mode feedhorn based on the values
from the drawings. This is a good approximation to the actual corrugated horn.
The mirrors shape is defined by the mechanical drawings assuming the values
for the cold structure. The grid is modelled as a regular grid of conducting
parallel wires. We define polarisation 0P as the reflected component from the
grid wires, which are parallel to the direction of the horn axes, while
polarisation 1P is the component transmitted through the wire grid.

####  4.3.1 Cross-polarization

We started the analysis considering the nominal optics to determine the
reference cross-polarization level. We performed main beam simulations using
Physical Optics (PO) and Physical Theory of Diffraction (PTD) on all the
reflectors. To verify the model, we compared the results of the simulations
with the available measured values and we found compatible values of -17 dB.
Figure 3 shows the two orthogonal cuts of the 0P (reflected) and 1P
(transmitted) polarisation beams at the same measurement location, i.e. in the
focal plane.

Figure 3: Orthogonal cuts of the 0P and 1P polarisation beams in the focal
plane. The displacement of the E- and H-planes depends on the angle of the
incoming RF signal, which is 0.94∘ with respect to the cartridge axis.

Then we repeated the simulations removing the grid from the model to get an
idea of the best cross-pol obtainable with the existing optics when this is
re-used in a single-horn configuration. With this configuration we get a
cross-polarization level of $\sim$-23.8 dB, i.e. within the specification
established for ALMA.

####  4.3.2 Beam squint

Since the Band 9 orthogonally polarised beams are divided using a wire grid,
they follow separate paths from the grid to the related feedhorn. Displacement
of the optical components can cause a beam squint between the two main beams
on the sky, potentially affecting the receivers calibration procedure.
Variations of the beam squint between cartridges are very difficult to
calibrate out and must be tightly controlled if not to hamper wide-field
polarisation observations.

The beam squint for OMT-based bands is generally within 2%, while the current
configuration of Band 9 with the wire grid has a larger scatter Neil . In
addition, the on-sky beam squint shows a preferred direction, with an angle of
$\sim$39.5∘.

Since the inaccuracy in the mounting of the grid could be one of the causes of
the scatter, we tried to reproduce this effect using the GRASP model to
simulate the squint. We computed the beam squint in the focal plane for a
random tilt of the grid around its axes and we quantified the offset of the
beam in the focal plane to make a comparison with measured data. We assumed
two independent gaussian distributions for the tilt in the two perpendicular
directions because they are decoupled due to the mechanical construction. By
optimizing the values of the two independent standard deviations to match the
data, we found that a gaussian distribution with a standard deviation
$\sigma=$ 0.28∘ in one direction and $\sigma=$ 0.19∘ in the orthogonal one can
give a similar scatter of the beam squint, as shown in Fig. 4. Therefore,
inaccuracies in the grid mounting could be responsible for the observed beam
squint scatter.

Figure 4: Comparison of the measured (blue) and simulated (red) on-sky
polarization beam squint. Simulations were performed assuming a random tilt of
the grid with a gaussian distribution in the two orthogonal directions. The
line computed from the fit shows the direction of the beam squint in the sky.

##  5 Conclusions

The main conclusions of the study are that the SIS mixers allow an extension
of the upper limit of the IF band to at least 18 GHz, the RF bandwidth can be
widened to 580â735 GHz and the limitation of the current polarimetric
performance can be explained by deviations in the grid mounting angle for the
beam squint and by the presence of the grid in combination with the mirrors
for the cross-polarization level.

## References

  * (1) A. Barishev et al., Astron. Astrophys. 577, A129 (2015)
  * (2) R. Hesper et al.,ALMA document FEND- 40.02.09.00-1974-C-REP (2023)
  * (3) A. Barishev et al., ALMA document FEND-40.02.09.00-1874-A-REP (2012)
  * (4) F. Montenegro-Montes et al., The Messenger 176, 20 (2019)
  * (5) A. Barishev et al., Tech. rep., Kapteyn Astronomical Institute (2004)
  * (6) N. Phillips, Tech. rep., ESO (2020)