Development of the SRF systems and fundamental studies to improve the dynamic losses in niobium and to develop better superconductors for “next-generation” CW SRF applications

Activities of the Institute for SRF Science and Technology in the POF3 Programme “Matter and Technology” (MAT)

Topic 1: Accelerator Research and Development (ARD)

The Institute of SRF Science and Technology (G-ISRF), participates in Helmholtz Research Program Matter and Technology in the topic Accelerator Research and Development.  The focus lies on the subtopics ST1 'Superconducting RF Science & Technology' and, indirectly, on ST3 Picosecond and Femtosecond Electron and Photon Beams.  The R&D activities concentrate on three main areas:

  1. Development of the SRF systems for the demonstration energy-recovery facility bERLinPro (ST1)
  2. Development of the SRF systems for the upgrade of BESSY-II for short, high-current bunches (BESSY VSR) (ST1 & ST3)
  3. Fundamental studies to improve the dynamic losses in niobium and to develop better superconductors for “next-generation” CW SRF applications (ST1).

ST1 – Superconducting CW RF Science and Technology

CW Superconducting radio-frequency (SRF) systems often are the enabling technology without which many next-generation accelerator concepts, both large-scale and compact,  turn-key systems, would be difficult or impossible to realize.  Examples of such systems are CW free electron lasers, x-ray laser oscillators, high-flux Compton gamma-ray sources, ERL-based light sources and electron-hadron colliders, or cavities for short photon pulse operation in storage-ring light sources. High-power ion and proton accelerators also rely on CW or long-pulse SRF systems, such as rare-isotope facilities or accelerator driven systems (ADS) for nuclear-waste transmutation.

CW operation offers many attractive features, including high-average power with very flexible beam patterns, improved stability, and the possibility of using highly sensitive detection techniques such as lock-in systems. The challenge lies in realizing the full potential of SRF, i.e., in advancing its science and technology for CW high-gradient (> 15 MV/m) and high current (> 10 mA) operation while reducing the cryogenic load significantly (at least a factor 2) to reduce the size and complexity of the cryogenic plants.  This challenge includes the generation of high-brilliance CW beams. Here too SRF units have great potential to deliver beam parameters beyond what is currently possible. 

Within ARD G-ISRF addresses each of these challenges. To this end, G-ISRF operates the HoBiCaT facility for testing SRF accelerator units and participates in the SRF photoinjector characterization facility “GunLab” at HoBiCaT. Additional infrastructure is being constructed to allow the systematic RF characterization of superconductor samples and single cavities with additional diagnostics.

Design of a Mark I SRF Photoinjector with cathode

insert and twin RF input coupler for high-power operation.

SRF Photoinjector development

  • Design and production of high-current-capable SRF photoinjector systems for high-brilliance beams (includes activities with Universität Siegen and Universität Rostock)
  • Characterization of the injector systems in GunLab and bERLinPro.
  • Long-term operation of SRF injectors.

For high-brilliance electron LINACs the electron source represents one of the most challenging parts of the machine.  SRF photoinjectors are at an early stage of development but have the greatest potential because they are able to deliver the CW operation at high voltage and accelerating gradients, which are essential to deliver the highest possible beam brightness.  The SRF injector represents a complex system where several challenges “collide”.

A normal-conducting cathode must be integrated under UHV conditions into the superconducting cavity while ensuring that

  • no particles contaminate the cavity
  • it is thermally isolated from the superconducting cavity
  • the rf field in the cavity cannot propagate along the cathode insert
  • it is electrically isolated from the cavity to electrically bias the insert for multipacting suppression.

Beyond this, the cavity design must take into account beam dynamics considerations to preserve the high emittance.  Since projects such as bERLinPro require high-currents, the power the injector transfers to the beam is large (> 200 kW) and special RF input couplers must be developed that handle the power but do not disrupt the beam.

CW SRF accelerating cavities for high-current operation

  • Development of high-current-capable CW SRF cavities for bERLinPro and BESSY VSR
  • Prototype tests on vertical test stands and in HoBiCaT
  • Beam tests in bERLinPro and BESSY VSR

Both CW high-current LINAC and storage ring applications that require high-voltages must employ SRF accelerating units. Such systems are in use, for example, in FLASH or at CEBAF.  However, present day accelerators are not designed for high-average-current operation (< 1 mA).  The challenge now lies in pushing this current limit significantly beyond the 1 mA threshold while maintaining an operating gradient in excess of 15 MV/m.  In particular, new L-band cavity designs must be developed that

  • generate little higher-order-mode power
  • allow for easy extraction of any HOM power that is generated
  • have low surface electric and magnetic fields for high-voltage operation
  • occupy as little space (length) as possible
  • are designed to cause little disruption to the beam.

G-ISRF is exploring the possibility of using multiple waveguides at the ends of new cavities for HOM damping, since these satisfy the criteria listed above.  While the cavity frequency will be different for BESSY VSR and bERLinPro, the main requirements are similar and one can take advantage of many synergies in developing joint designs.

Electromagnetic models of high-current HOM-damped cavities being investigated for bERLinPro and BESSY VSR.

Low-loss CW SRF Systems

  • Development of a sample measurement system to characterize RF properties of SRF suitable materials and coatings
  • Study and improve surface resistance of niobium and other materials
  • Study RF losses in cavities coated with the most promising candidate materials identified with samples.

For GeV-class machines the cryogenic load dictates that present LINACs operate at or below 20 MV/m, and often even in pulsed mode. Even so, the cryogenic load is a a nearly prohibitive 1 – 10 kW at 2 K.  Similarly, smaller, 10 – 100 MeV-class “compact” accelerators are hard to realize in a cost effective manner because the cryogenic plant dominates the cost and complexity.  The achievement of high cavity quality factors thus is a vital objective of the CW SRF R&D.  The goal is to find techniques for niobium that will reduce the losses significantly below what is currently possible.  Moving beyond niobium, several new superconductor systems (including Nb3Sn) hold the promise of achieving even higher quality factors or the same value as Nb but at higher temperature.  If their promise can be realized then the cryogenic plant can be significantly simpler and perhaps even “simple” cryogenic coolers may be employed one day for “turnkey” operation of small CW accelerators.

To support systematic investigations G-ISRF is constructing sample test stands (e.g., quadrupole reasonator) that enable the full characterization of superconductors over a wide range of operating parameters.  These experiments should yield a better understanding of the underlying physics.  In parallel, cavity measurements with diagnostics (e.g., thermometry) permit the study of the technological applicability of the best treatments identified.  E.g., first measurements have already demonstrated that temperature gradients along an SRF cavity at the instance it turns superconducting can significantly increase the RF losses during subsequent cavity operation.

Schematic of a quadrupole resonator being constructed at HZB to measure the RF properties of SRF samples at a wide range of temperatures, magnetic field strengths and frequencies.

ST3 – Picosecond and Femtosecond Electron and Photon Beams

  • Beam studies with high-charge for multi-user operation in circular accelerators (achieving control and stable user operation for ultra-short bunches

To fully characterize materials and their function, and perhaps even influence their function, modern synchrotron light sources must enable the determination of matter’s structure and its dynamics. The latter requires the creation of very short photon pulses and hence, in storage rings, electron bunches.  But in standard x-ray and UV storage rings the natural pulse length is limited to some 10 ps.  To circumvent this limit HZB is developing a new scheme called BESSY VSR to shorten pulses to 1 ps and less by installing L-band SRF “overvoltage” cavities.  These systems must be capable of handling the 300 mA beam current required for high-average flux operation.  A combination of two different cavity frequencies enables both short and “standard” long pulses to be delivered to the user simultaneously.

The BESSY VSR SRF cavity system shares many similarities with those of bERLinPro.  Such units are going to be developed by G-ISRF mainly in ST1.  However, much of the information from the cavity development, including the impedance spectrum, is vital to analyze the stability and performance of BESSY VSR at high current.  The initial theoretical studies are being performed by HZB’s Institute for Accelerator Physics (G-IA) with input from G-ISRF’s cavity designs and prototype measurements.  Stability simulations have already demonstrated, precise control of the cavity fields will be required to ensure that the beam quality (emittance/length) will not suffer during BESSY VSR operation.  A combination of RF control and feedback/feed forward schemes will need to be developed by G-IA and G-ISRF to combat various “noise sources” and instabilities.

Microphonics spectrum of an SRF cavity measured in HoBiCaT.  This spectrum was used in BESSY VSR simulations to study its impact on beam stability.