Research Thrust to Address Major Measurement Gaps

Transcription

1 Research Thrust to Address Major Measurement Gaps RENEW Workshop Presentation Theme: Plasma Material Interface Subpanel on Internal Components Presented by Tony Peebles, UCLA Physics & Astronomy Department: Plasma Science & Technology Institute

2 Overview Funding for diagnostic development has been stagnant for years Existing program focused on development for existing fusion devices US-IPO focused on delivery of ITER commitments Based on transfer of existing measurement techniques to ITER Focused on front-end design Funding has been sporadic to-date. Hopefully will soon improve No funded program to develop new burning plasma diagnostics CTF and DEMO will need far more than transfer of existing techniques Represents a major gap. Need to insert creativity into the program Need an initiative focused on development of innovative diagnostics/monitors compatible with a CTF & DEMO Optical-based diagnostics represent major risk This dictates development of creative, robust diagnostic techniques

3 Importance of measurements in a burning plasma Measurements provide the essential window to view and monitor a burning plasma ITER requires an array of measurements for Machine protection Plasma control Physics understanding of burning plasma DEMO will require radiation hardened monitors for protection and control These monitors should evolve from diagnostic systems demonstrated on ITER and a CTF They will have to be extremely reliable, long-lived and accurate Plasma interface should be straightforward We need to prepare for CTF and DEMO NOW Short-term focus on ITER has led to potential major measurement gap in preparation for CTF and DEMO

4 Challenges of a burning plasma environment On ITER there will be: High neutron and gamma radiation fluxes (up to x 5 max experienced on existing machines) Substantial heat loads from plasma radiation (~ x 10) High neutral particle fluxes from charge exchange processes (x 5) Material evaporated from the divertor and first wall (x?) Pulse lengths will be hundreds of seconds, hence high neutron fluence (> 10,000), etc. Relativistic effects (T e > 20kV), increased background & wall reflection for visible diagnostics such as MSE, CER, BES, Thomson, etc. On a CTF or DEMO the environment will be far worse than ITER Currently there is a focus to satisfy ITER measurement requirements This focus may not contribute to the longer term needs of DEMO e.g. visible diagnostics may work for ITER but not in a CTF or a DEMO Need to creatively/urgently address BOTH short (ITER) and long-term (CTF & DEMO) needs

5 A simple transfer of existing techniques will downgrade measurement capability The need for plasma control and plasma monitoring will be critical in ITER, CTF and DEMO Machine integrity must be assured Plasma control and fusion burn must be optimized ITER is also important from a physics perspective: For the first time it will be possible to study a burning plasma Essential to understand the physics for projection to DEMO, etc. Transfer of existing techniques degrade measurement capability Some existing measurement techniques have fundamental limitations A burning plasma will cause environmental damage to diagnostic components and thereby limit performance and MTBF The need for an immediate initiative to prepare diagnostics for the burning plasma era has reached a critical stage If we drift towards ITER, CTF & DEMO assuming that the essential safety, control and physics measurements will be available, then we are facing potential disaster Area has been underfunded US unprepared for burning plasma era

6 Environmental impact on components Radiation-induced effects Conductivity (RIC), electrical degradation (RIED), electromotive force (RIEMF), etc. Impacts in-vessel wiring, magnetic coils and loops, pressure gauges, bolometers, soft x-ray detectors, Langmuir probes, etc. Erosion and deposition on mirrors, windows, polarizers, etc. Severe impact on optical diagnostics such MSE, MSE, BES, Thomson Reduces MTBF major impact in a CTF or DEMO Radioluminescence, radiation induced absorption Impacts optical fibers, windows, etc. Heating Impacts all in-vessel components, mechanical stress, distortion, etc. The nuclear environment also sets stringent demands on the engineering of diagnostic systems for example on neutron shielding, Tritium containment, vacuum integrity, material structure damage, transmutation, swelling, etc.

7 First mirror impacts many optical diagnostics Optical diagnostics can be affected by reflectivity and polarization changes caused by deposition/erosion of first mirror Initial studies have suggested that erosion should not have a significant effect on reflectivity over a wide wavelength range (?) In contrast, deposition rapidly deteriorates the reflectivity as well as the polarization characteristics of the reflected radiation impacts MSE, Thomson, CER, etc. Operating mirrors at elevated temperatures improves the situation Deposition is significantly reduced, but the mitigation process is complex Ongoing research involves in-situ cleaning of deposited films Via laser ablation and the exposure of mirrors to low temperature plasma Relatively recent developments are Rh-coated mirrors and nanostructure mirrors, These are being tested for their potential application on ITER

8 Fundamental principle of a technique can degrade measurement capability in burning plasmas Electron cyclotron emission (ECE) is used routinely on existing tokamak plasmas to determine electron temperature locally Core spatial resolution typically ~1cm ITER assessment by University of Texas + collaborators concluded spatial resolution in core plasma will degrade to 7 to 10cm Relativistic effects broaden & shift emission layer determined by emissivity function Widths of Emission Layer for 1st Harmonic O-mode, Scenario 2 R_maj(cm) Freq.(GHz) Width (cm) Width calculated as distance between 5% and 95% emission levels Although spatial resolution degraded still thought adequate for ITER (~a/20) except in edge pedestal Separate issue: disagreement with Thomson in JET and TFTR not understood. - Connected to differences between techniques?

9 Density profile measurement via reflectometry affected by relativistic effects - motivates new measurement Reflectometry planned for ITER density profile measurement However, high electron temperatures (20kV) significantly modify cutoff location Largest effect for X-mode RH cutoff Innovative Approach : Use reflectometry to determine BOTH electron density and temperature Relativistic effects very different for O and X-mode Use additional information to determine BOTH temperature and density profile. - excellent spatial resolution (~1cm) - may be applicable to edge pedestal - simulations has demonstrated viability of technique - no funded effort planned to test concept

10 US Research & Development has been minimal In contrast to Europe and Japan, the US effort in addressing the many measurement challenges has been minimal Funding for diagnostic development has been stagnant for years Existing program focused on development for existing fusion devices No parallel program to develop burning plasma diagnostics The US-IPO has provided some resources to address design of US-committed ITER diagnostics Focus is ITER-specific; not a CTF or DEMO R&D resources have been sporadic to-date. Hopefully will soon change! Satisfying ITER needs will not solve longer-term DEMO concerns Need to focus on robust measurement systems Diagnostic-plasma interface MUST be certain Optical based diagnostics represent high-risk This dictates development of creative, robust diagnostic techniques

11 Initiatives necessary to address gaps First initiative focused on development of new diagnostics/ monitors specifically for burning plasmas Diagnostics should be robust and able to be extrapolated to a CTF and/or DEMO. NOVEL techniques/approaches should be strongly encouraged This should be funded directly by DoE. To-date there has been very little creative thinking re new techniques Second initiative should focus on soving the environmental challenges posed to diagnostic components. e.g. plasma facing optical components such as mirrors, optical fiber damage, electrical signal/control cables, etc. Proposed solutions will require extensive testing using long pulse, high radiation and heat load environments Funding should build on the US IPO funding related to ITER Many of these problems are critical for US ITER commitments

12 First initiative should focus on new diagnostics where plasma interface is practical Neutron and gamma ray spectroscopy Plasma interface relatively straightforward (some would disagree) However, wide angular view required Microwave diagnostics Uses corrugated metallic waveguide to couple to the plasma No windows, mirrors close to plasma Investigate broader range of capabilities e.g. ITER plans to use reflectometry as a plasma position/shape monitor Is it possible to measure q-profile using microwaves? Far-infrared, infrared diagnostics Requires front-end optical components However, required optics can be relatively poor quality Extensive polarimetry system could determine q-profile Optical diagnostics Requires high quality front-end optics Challenge for ITER Perhaps impossible for DEMO

13 Priorities for the future Currently world-wide resources are being directed primarily towards solving the environmental damage issues e.g. erosion/deposition on first mirrors Motivated by transfer of existing techniques to ITER In contrast, very little resources are being directed towards new development of measurement techniques potential major gap New techniques will be essential for a CTF or DEMO Many existing techniques impractical Some new techniques are still needed for ITER e.g. confined alpha measurement uncertain It is essential that resources be allocated to the creative development of burning plasma measurement techniques Development initiative should be highest priority for the DoE US IPO should take primary responsibility for environmental damage issues where solutions are critical for ITER

14 Extra Viewgraphs

15 Reflectometry can determine both Te and ne profiles no independent temperature information required After three iterations agreement between input and inverted profiles are very good. Further improvement with more iterations Edge pedestal Simulation assumes X-mode operation GHz, O- mode 35-95GHz Simulation indicates good inversion of both density and temperature profiles. NEEDS a demonstration!