High Current Superconducting RF and RF Control

1
High Current Superconducting RFand RF Control

• Terry L. Grimm
• Niowave
• Michigan State University

2
Outline

• SRF working group charge
• Cost impediment and mass production
• Requirements
• Synergy
• Critical items and discussion
• Conclusion

3
Working Group Charge

• Challenges for the application of SRF to ERLs
• Identify critical SRF items
• Evaluate readiness of SRF
• Organize RD effort internationally

4
High Cost

• Primary impediment to the application of SRF to
  ERLs is cost
• Time
• Money
• SRF technology already shown to work
• Progress and advancements are ongoing

Assembly Line Mass Production
One of a Kind State of the Art
5
Examples of Cost Savings

• Commercialization and competition have greatly
  reduced the cost of similar high tech systems

Microwave Oven
MRI-magnetic resonance imaging (GE Medical
Systems)
Cancer Therapy (Varian Medical Systems)
6
Requirements

• Moderate gradients (10-20 MV/m)
• High current (milliamp to ampere level)
• Continuous wave (cw) operation
• Low cryogenic load
• Efficient use of RF power
• Affordable

7
Synergy

• SRF advances being made for other applications
• High energy physics (ILC, neutrino source)
• Nuclear physics
• Light sources (ring based)
• Heavy ion linacs (neutron source, radioactive
  beam facility, transmutation)
• Capitalize on those investments and advancements

8
Critical SRF items

• Affordable cost
• Cavity design
• Cryogenic load
• RF power source
• RF control

• Cryomodule design
• Assembly method
• Power coupler
• HOM damping
• Frequency tuner
• Microphonics
• Lorentz force detuning
• Cryogenic distribution

9
Affordable Cost

• Typical/rough cost of an ERL facility
• 25 Cryomodules beamlines
• 25 RF and magnet power/controls
• 25 Cryoplant distribution
• 25 Civil infrastructure
• Must consider all aspects and their implications
  on the other systems to minimize the cost

10
Cavity Design 1

• Material
• High purity bulk niobium (RRR300)
• More vendors entering the market so the price
  will continue to drop
• Fine and large grain
• Cutting material directly from the ingot may
  reduce costs
• In the future advanced materials may help
• Nb/Cu, Nb3Sn, MgB2,
• Type
• Elliptical, reentrant, half-reentrant
• Consider designs with higher Ep/Ea lower cryo
  losses
• Lower Bp/Ea
• Higher G(R/Q)

11
Cavity Design 2
Fill
Drain
Flip 180
Has the potential to achieve gt50
MV/m Initial tests reached 24.6 MV/m
Positioned to avoid gas pockets
Acid/water can drain
Electric Magnetic field contours
12
Cavity Design 3

• Frequency
• Tradeoff with many parameters
• Existing machines and typical frequencies
• Lower frequency requires less cells, has lower
  BCS losses, and has less HOM issues
• Cell to cell coupling
• 1.5 to 1.9 to maintain field flatness
• HOM requirements will likely require a larger
  value

13
Cavity Design 4

• Number of cells
• Consider more cells if higher cell to cell
  coupling low coupler power
• Accelerating gradient
• 10-20 MV/m based on cost minimization with the
  cryoplant
• Use simple chemical etching (BCP) since not
  pushing the limit
• Quality factor, Q
• Plan on residual resistance less than 10 n?
• Design value based on choice of temperature and
  safety margin

14
Cryogenic Load

• Cryoplant costs are a significant fraction of an
  ERL
• Typically 2 K (super-fluid, sub-atmospheric) or
  4.5 K (above atmospheric pressure)
• 2 K plant has lower mechanical and Carnot
  efficiency, and costs more per watt, but needed
  for high frequency
• Portable applications would benefit from
  operation at 4.5 K (above atmospheric pressure)
• Above atmospheric pressure and simpler, with the
  option to just supply liquid via a storage Dewar
• Push to lower frequency (lt500 MHz)

15
RF Power Source

• Many options
• Solid state, tetrode, IOT, klystron, magnetron
• Industrial cw frequencies (klystron magnetron)
• 500 MHz
• 900 MHz
• 2.45 GHz
• May choose the frequency based on the
  availability and cost of the RF sources

16
Cryomodule Design 1

• Assembly method
• Method of putting the ship in the bottle will
  drive the entire design
• Insert the cold mass axially into the cylindrical
  vacuum vessel
• Assemble shields (thermal and magnetic) and
  vaccuum vessel around the cold mass
• Power coupler
• Coaxial or waveguide
• Ceramic window (disc or tube typically)
• Consider better integration with the rf source
  and its output power coupler
• HOM damping
• Ferrites, waveguide and lumped circuit HOM
  couplers
• Consider using the power coupler for HOM damping

17
Cryomodule Design 2

• Frequency tuner
• cw operation does not require a rigid tuner
• Consider placing the stepper motor and piezo
  actuator at room temperature outside the vacuum
  via a mechanical linkage
• Microphonics
• Vibrations caused by rotating machinery
• Discrete frequencies so easy to cancel
• Damp source, transmission or use adaptive
  feedforward cancellation
• Already demonstrated loaded-Qs of 107- 108

18
Cryomodule Design 3

• Lorentz force detuning
• cw operation so not a major concern
• Consider simpler stiffening of the cavity and
  tuner
• Cryogenic distribution
• 2 K JT valve and heat exchanger location
• Bayonets or welded connections
• Bayonets large thermal load, double O-ring with
  guard vacuum and interlocks to avoid
  contamination of the liquid He
• Welded connections requires cutting/grinding
  for removal, single vacuum break

19
RF Controls

• Digital control systems continue to improve with
  higher speed and capability
• Feedback to control beam instabilities
• Feedback for transients
• Simplify tuning and operation
• No longer a critical item, especially for cw
  applications
• Well established
• Cost will continue to decrease

20
Conclusion

• Superconducting RF is ready to be applied to
  Energy Recovery Linacs
• Capabilities cost will continue to improve
• RD for other applications will advance ERLs
• RD specifically for ERLs is needed
• This biannual workshop is an excellent start to
  organize international effort

21
Examples of Cost Savings

• Commercialization and competition have greatly
  reduced the cost of similar high tech systems

Microwave Oven
MRI-magnetic resonance imaging (GE Medical
Systems)
Cancer Therapy (Varian Medical Systems)