PROCESSES
Researchers shrink terahertz accelerator for electron beams
By A Julien Happich team of researchers at the Deutsches Elektronen-
Synchrotron (DESY) has developed a novel type of
terahertz-based accelerator that can recycle some of the
laser energy fed into it to boost the energy of the accelerated
electrons a second time.
Only 0.79 millimetres in diameter, the 1.5 centimetres long
device uses a longer pulse of narrowband terahertz radiations
at many different wavelengths. The
multicycle pulse is reported to significantly
extend the interaction section
with the particle
Dongfang Zhang and his colleagues
from the Center for Free-
Electron laser Science (CFEL) at
DESY presented their experimental
accelerator in the journal Physical
Review X.
“We feed the multicycle terahertz
pulse into a waveguide that is lined
with a dielectric material”, says
Zhang. Within the waveguide, the
pulse’s speed is reduced. A bunch of
electrons is shot into the central part
of the waveguide just in time to travel
along with the pulse. “This scheme
increases the interaction region
between the terahertz pulse and the
electron bunch to the centimetre
range—compared to a few millimetres in earlier experiments,”
reports Zhang.
The device did not produce a large acceleration in the lab.
However, the team could prove the concept by showing that
the electrons gain energy in the waveguide. “It is a proof of
concept. The electrons’ energy increased from 55 to about 56.5
kilo electron volts,” says Zhang. “A stronger acceleration can
be achieved by using a stronger laser to generate the terahertz
pulses.”
The set-up is mainly designed for the non-relativistic regime,
meaning the electrons have speeds that are not so close to the
speed of light. Interestingly, this regime enables a recycling of
the terahertz pulse for a second stage of acceleration.
“Once the terahertz pulse leaves
the waveguide and enters the vacuum,
its speed is reset to the speed of
light,” explains Zhang. “This means,
the pulse overtakes the slower electron
bunch in a couple of centimetres.
We placed a second waveguide
at just the right distance that the
electrons enter it together with the
terahertz pulse which is again slowed
down by the waveguide. In this way,
we generate a second interaction
section, boosting the electrons’ energies
further.”
In the lab experiment, only a small
fraction of the terahertz pulse could
be recycled this way. But the experiment
shows that recycling is possible
in principle, and Zhang is confident
that the recycled fraction can be
substantially increased.
Proof of concept for cascaded terahertz accelerator
using long pulses. The mini-accelerator uses terahertz
radiation that can be recycled for a second stage of
acceleration. Credit: DESY, Science Communication
Lab.
“Our cascading scheme will greatly lower the demand on
the required laser system for electron acceleration in the nonrelativistic
regime, opening new possibilities for the design of
terahertz-based accelerators”, emphasised Nicholas Mattlis,
senior scientist and the team leader of the project in the CFEL
group.
World’s highest power density through 3D printing
By Christoph Hammerschmidt In electromobility, one of the primary objectives of developers
is to accommodate the most powerful
engines in the smallest possible space.
Two UK-based engineering companies,
Equimake (Snetterton) and HiTEA (Bristol),
have therefore joined forces – their goal is
to develop the world’s most power dense
electric motor.
Codenamed Ampere, the motor will use
additive manufacturing to make it lighter,
smaller and more efficient than conventional
permanent magnet motors. Targeting a peak
power of 220kW at 30,000rpm and a weight
of less than 10kg, Ampere is aiming to be
the world’s most power dense electric motor
at 20 kW per kg - four times as power dense as conventional
electric motors. Its pioneering use of additive manufacturing –
essentially 3D metal printing – will also allow it to use the least
amount of high strength alloy and magnet material, keeping
cost as low possible. Applications are not only in electric cars,
but likewise in aerospace and marine.
The key to the motor’s performance is its
combination of an advanced motor design
with additive manufacturing, allowing its
metal structure to be 3D printed, rather than
milled from a solid billet. This brings several
advantages. Firstly, metal is only put where
it is needed. Secondly, thermally efficient
thin walls and optimised fine surface details
can be combined directly with the motor’s
structure, replacing multi- part assemblies
with a single, complex architecture that has
exceptional cooling ability, is lightweight,
has low inertia and allows for greatly increased
rotational speed.
This approach not only means that Ampere will use the least
amount of high strength alloys in its construction, but also the
least amount of expensive active materials – the magnets – too,
keeping cost as low as possible. First prototypes are set to be
up and running in 12 months’ time.
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